OAuth 2.0 Client Instance Assertion

OAuth 2.0 Client Instance Assertion Independent

public@karlmcguinness.com

Security Web Authorization Protocol OAuth CIMD Workload Identity Client Instance SPIFFE This specification defines the Client Instance Assertion: a signed JWT identifying a concrete runtime instance of an OAuth 2.0 client. It registers the client_instance_assertion request parameter for carrying the assertion at the OAuth 2.0 token endpoint on the authorization_code, client_credentials, refresh_token, and JWT bearer (RFC 7523) grants; on the token-exchange grant (RFC 8693), the same assertion is presented as actor_token with actor_token_type set to urn:ietf:params:oauth:token-type:client-instance-jwt, also registered by this specification. This specification does not introduce a new client_instance identifier in protocol messages. Instead, it defines client metadata parameters (applicable to clients identified by a Client ID Metadata Document (CIMD) or registered via OAuth Dynamic Client Registration (RFC 7591)) that let a client_id identify a logical client whose concrete runtime instances are authenticated by one or more trusted instance issuers (for example, workload identity systems). The Authorization Server validates the instance assertion and represents the instance either as an act claim, when another principal is present (e.g., a user delegating to the instance), or as the access token's sub, when the instance itself is the principal (e.g., a client credentials grant). The issued access token is sender-constrained to a key the instance possesses. About This Document The latest revision of this draft can be found at . Status information for this document may be found at . Discussion of this document takes place on the Web Authorization Protocol Working Group mailing list (), which is archived at . Subscribe at . Source for this draft and an issue tracker can be found at .

Introduction OAuth 2.0 defines client_id as the identifier of a client. In deployments where a single OAuth client identifier represents many short-lived runtime instances, resource servers and authorization servers need to know not only which client made a request but which instance of that client made it. Instances may be acting on a user's behalf or as the principal themselves; this specification covers both. OAuth 2.0 Token Exchange defines the actor_token and actor_token_type token request parameters and the act claim for representing an actor in an issued token, scoped to the token-exchange grant. The OAuth Actor Profile further constrains the act claim and registers actor-related claims, but explicitly leaves out a token request parameter for proving an actor in flows other than token exchange. This document defines a profile for representing client instance identity at the OAuth 2.0 token endpoint. It:

Recognizes that an OAuth client_id commonly abstracts over many concrete runtime instances (a relationship already implicit in deployed OAuth practice; see ), and defines client metadata describing the instance issuers trusted to attest those instances. The metadata applies whether the client is identified by a Client ID Metadata Document or registered via ; see .
Defines the Client Instance Assertion, a JSON Web Token (JWT) signed by an instance issuer published in the client's metadata, and registers the client_instance_assertion request parameter for carrying it at the OAuth 2.0 token endpoint on the grants listed in . On the token-exchange grant (), the same assertion is presented as actor_token with actor_token_type set to urn:ietf:params:oauth:token-type:client-instance-jwt, also registered by this profile.
Requires issued access tokens to be sender-constrained to a key the instance possesses, and specifies how the instance assertion's cnf claim drives that binding in the interoperable re-minted assertion format.
Registers client_instance_assertion as a token_endpoint_auth_method value, allowing deployments without an online client-controlled credential to authenticate the client via the instance assertion alone.
Defines first-class support for SPIFFE workload identity, including optional direct presentation of JWT-SVIDs without re-minting.
Defines authorization server metadata so that clients can discover support.

What this document does not do:

It does not introduce a client_instance identifier parameter flowing through the authorization request, token request, introspection, or access token (see ).
It does not change the syntax or processing of the act claim beyond what already defines.
It does not define authorization endpoint interactions for conveying actor identity; like , this is left for future work.

Conventions and Definitions The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 when, and only when, they appear in all capitals, as shown here. This document uses the following terms:

OAuth Client:: As defined in , identified by a client_id (a CIMD URL or a registered client_id under ; see ). In this profile, the OAuth client publishes the set of instance issuers permitted to authenticate its runtime instances ().
Client Instance:: A concrete runtime of an OAuth client (for example, a particular process, container, function invocation, or session). See for the class-and-instance relationship between an OAuth client and its instances.
Instance Issuer:: An authority trusted by the client to authenticate client instances and issue instance assertions for those instances. Examples include workload identity providers (e.g., a SPIFFE control plane ) and platform-managed identity services.
Client Instance Assertion:: A JWT issued by an instance issuer asserting the identity of a client instance, presented at the OAuth 2.0 token endpoint as the client_instance_assertion request parameter () or, on the token-exchange grant, as the actor_token parameter with actor_token_type set to urn:ietf:params:oauth:token-type:client-instance-jwt (). The terms "Client Instance Assertion" and "instance assertion" are used interchangeably in prose; "the presented assertion" denotes the JWT carried by either parameter, with processing identical in both cases.
Delegation Case:: A token request whose grant produces a principal distinct from the instance presenting the Client Instance Assertion (for example, a user under authorization_code or jwt-bearer). The issued access token's sub is the principal and the instance appears in act per .
Self-Acting Case:: A token request whose grant produces no principal distinct from the instance (notably client_credentials). The issued access token's sub is the instance and act is omitted per .

Relationship to Other Specifications RFC 8693 (Token Exchange). On the token-exchange grant the assertion is presented as actor_token with actor_token_type set to urn:ietf:params:oauth:token-type:client-instance-jwt, per 's conventions (); on the other grants in the assertion is the client_instance_assertion request parameter (). The parameter to use is determined by the grant; the assertion format, validation, sender-constraint, and access-token surfacing are identical across both paths (). OAuth Actor Profile. defines the structure of the act claim, the sub_profile claim, and nested actor representation. This document does not redefine those constructs; it defines how a client instance proves itself at the token endpoint and how the Authorization Server (AS) represents the validated assertion in issued access tokens (act for delegation, top-level sub for self-acting). Implementations of this document MUST also implement . SPIFFE Client Authentication. (an OAuth Working Group document) defines how a SPIFFE workload authenticates as the OAuth client itself using a JWT-SVID or X.509-SVID in place of a client secret. This document operates at a different layer (actor / instance identity, not client authentication) and on different OAuth parameters and trust sources:

Layer	SPIFFE Client Auth	This document
What is authenticated	The OAuth client	An actor (instance) acting under an OAuth client
Token request parameter	`client_assertion` / `client_assertion_type`	`client_instance_assertion` (or `actor_token` on token-exchange grants)
Trust source (client metadata)	SPIFFE bundle endpoint and `spiffe_id`	`instance_issuers`
Where the SPIFFE ID surfaces	Validated against `spiffe_id`; not propagated	Surfaced in `act.sub` or top-level `sub` of issued access tokens

The two specifications are orthogonal and MAY be combined: a typical combined deployment uses (with a wildcard spiffe_id) to authenticate the OAuth client and this profile to surface and bind the specific instance. The same SVID MAY be presented as both client_assertion and client_instance_assertion in a single request when both profiles' audience, client-binding, and sender-constraint requirements are satisfied. This document does not require SPIFFE; instance issuers may use any subject_syntax, and the client may authenticate via any registered method. SPIFFE deployments get first-class support (, ). OAuth Attestation-Based Client Authentication. (an OAuth Working Group document) defines how a Client Attester issues a Client Attestation JWT that an OAuth client uses to authenticate the client instance. That specification authenticates the client instance for the purposes of client authentication and does not specify how that instance identity surfaces to resource servers. This profile addresses that surfacing gap.

Concern	Attestation-Based Client Auth	This document
Authenticates the client instance	Yes	Not by itself; consumes a separately registered client authentication method
Surfaces instance identity to the access token	Out of scope	`act.sub` (delegation) or top-level `sub` (self-acting)
Wire-level presentation	`OAuth-Client-Attestation` plus either `OAuth-Client-Attestation-PoP` or `DPoP`	`client_instance_assertion` form parameter (or `actor_token` on token-exchange grants)
Trust anchor	AS-to-Attester trust (deployment-defined)	Per-client `instance_issuers` in client metadata
Sender-constraint binding	`cnf` per ; PoP via dedicated `-PoP` JWT or DPoP combined mode	`cnf.jkt` or `cnf.x5t#S256` thumbprint
Primary motivating context	Mobile/native apps; wallet ecosystems; workload deployments	Agentic workloads, autoscaled services, ephemeral functions, sub-agents; workload identity

The two specifications are orthogonal: this profile does not require and does not redefine or extend it. Deployments may combine them by presenting a Client Instance Assertion alongside a Client Attestation, in which case the Client Attestation continues to authenticate the client instance for client-authentication purposes while the Client Instance Assertion provides this profile's resource-server-visible instance identity. additionally defines a Client Attestation presentation path directly to a resource server (Client Attestation PoP aud = resource identifier); that path is independent of this profile's access-token surfacing model. WIMSE Workload Credentials. The IETF WIMSE working group is defining specifications for workload identity (, ); WIMSE work is in progress. This profile's Client Instance Assertion is the OAuth-aware projection of the same workload identity model, carrying OAuth-specific bindings (client_id, aud) needed at the OAuth token endpoint. Deployments holding a WIMSE workload credential, SPIFFE JWT-SVID, Kubernetes projected service-account token, or other workload credential SHOULD use the OAuth-aware adapter pattern () to mint a Client Instance Assertion. For sender-constraint, this profile pins the binding member of cnf to jkt () or x5t#S256 (); WIMSE-defined binding mechanisms (for example, a future Workload Proof Token) can be added by a future specification when those mechanisms reach deployment maturity.

Client Instance Assertion Request Parameter This section defines the client_instance_assertion request parameter, the wire-level mechanism this profile uses to carry a Client Instance Assertion at the OAuth 2.0 token endpoint on the grant types listed in . The parameter carries the Client Instance Assertion JWT defined in . The assertion's purpose is to identify the client instance and bind subsequent proof-of-possession to the instance's key, independent of how the client itself is authenticated. Validation, sender-constraint, and access-token representation rules are defined in . On token-exchange grants (), the same assertion is presented as actor_token with actor_token_type set to urn:ietf:params:oauth:token-type:client-instance-jwt, per 's conventions. See . The two presentation paths produce identical processing outcomes; the parameter to use is determined by the grant.

Permitted Grant Types The client_instance_assertion parameter MAY be presented on the following token endpoint grant types:

authorization_code ()
client_credentials ()
refresh_token ()
urn:ietf:params:oauth:grant-type:jwt-bearer ()

The client_instance_assertion parameter MUST NOT be presented on the token-exchange grant (); on token-exchange grants the assertion is presented as actor_token per . An AS MUST reject a token-exchange request that includes client_instance_assertion. This document does not define behavior for the implicit grant or for the device authorization grant; specifying those is left to future work.

Token-Exchange Presentation When the grant is token-exchange (), the Client Instance Assertion is presented as the actor_token parameter with actor_token_type set to urn:ietf:params:oauth:token-type:client-instance-jwt. This is the conventional path; the AS validates the assertion under the same rules as for the client_instance_assertion parameter (). The two parameter names are wire-syntax siblings, not distinct artifacts: the assertion format (), validation procedure, sender-constraint binding, and access-token representation are identical. The grant determines which parameter name carries the assertion. In the delegation case under token-exchange, the assertion represents the actor distinct from the subject named in subject_token, which is the conventional use of actor_token. The self-acting case () does not arise on token-exchange, where requires a distinct subject.

Client Instance Model A registered OAuth client commonly abstracts over many concrete runtimes (for example, a single OAuth client identifier representing an application across iOS, Android, web, and server-side runtimes; or an agent platform across each running agent or session). This profile makes that class-and-instance relationship explicit so each runtime can be named, attested, and bound to access tokens individually. For agent platforms, a sub-agent spawned by an agent is represented as a nested actor via token-exchange (). The remainder of this section covers the architecture (), registration models (), and trust delegation model ().

Architecture Three roles cooperate to authenticate a client instance:

Role	Responsibility
OAuth Client	Logical OAuth client identified by a `client_id` (CIMD URL or RFC 7591-registered identifier). Publishes the list of trusted instance issuers in its registered client metadata.
Instance Issuer	Authenticates concrete runtime instances and issues short-lived JWT instance assertions describing them.
Authorization Server (AS)	Authenticates the client per its registered client authentication method; resolves the client metadata (via CIMD dereference or local registration storage); verifies the instance assertion against a trusted instance issuer; mints an access token whose `act` claim or top-level `sub` represents the instance.

A high-level flow: resolves client metadata | Server | -> validates the assertion | | -> issues access token with act or sub +--------------------+ ]]> When the client registers token_endpoint_auth_method = client_instance_assertion (), the two trust anchors (client credential and instance issuer) collapse onto a single artifact: the instance assertion both authenticates the client (via the client metadata endorsement of its issuer) and identifies the instance, and the request carries no separate client_assertion.

Client Registration Models This profile applies to OAuth clients regardless of how their metadata is registered with the AS. For most existing OAuth deployments, this means RFC 7591-style static registration administered by the AS operator (whether via the dynamic registration endpoint or pre-registered out of band); CIMD is the more recent option that adds public discovery and cross-organization auditability where those are required. The descriptor format and processing rules are identical in both cases.

Static registration ():: The client_id is opaque or AS-assigned. Metadata is registered via the dynamic registration endpoint or out of band, and stays internal to the AS. Updates take effect when the registration store is updated. This model integrates with existing RFC 7591-based registration toolchains and is the typical mode for closed or managed deployments where the trust relationship does not need to be publicly discoverable.
Client ID Metadata Document ():: The client_id is an HTTPS URL that dereferences to a metadata document. The AS fetches the document on demand and caches it; updates take effect after the cache window expires (or upon explicit re-fetch). This model suits cross-organization deployments where the trust relationship between the OAuth client and its instance issuers must be auditable in a publicly resolvable document, and is operationally easier for cross-organization SPIFFE federation () because the foreign organization can publicly publish its bundle endpoint and instance descriptors. Under static registration, equivalent federation requires manual coordination of registration between organizations.

The descriptor format (), the instance-assertion auth mode (), the SPIFFE compatibility features (), and the AS processing rules () all apply uniformly to both models. The only deployment-time differences are how metadata updates propagate (cache TTL versus admin update) and whether the trust relationship is publicly discoverable.

Trust Delegation Model This profile defines a three-party trust delegation between the client, the instance issuer, and the AS. The client delegates attestation of its runtime instances to one or more instance issuers; the AS relies on that delegation as expressed in the client's registered metadata.

Delegation by the OAuth Client By listing an instance issuer in its instance_issuers metadata (), a client delegates to that issuer the authority to attest that a concrete runtime is an instance of the client. The descriptor bounds the delegation: trust_domain, subject_syntax, spiffe_id, and signing_alg_values_supported constrain what the issuer may assert and what the AS will accept (). The issuer's per-client minting obligations are in ; client-side guidance on choosing issuers is in . The per-client minting requirement (an issuer mints assertions naming a given client_id only for runtimes authorized as instances of that client) prevents cross-client instance impersonation when the same instance issuer is listed by multiple OAuth clients. Because the instance_issuers listing endorses the issuer to mint tokens naming this client_id, an instance assertion signed by such an issuer is itself attributable to the client. This makes the assertion usable as the client credential under .

Authority of the Authorization Server The AS treats the registered instance_issuers list as authoritative: it derives its trust in an instance assertion solely from the descriptor whose issuer member matches the instance assertion's iss claim. AS-side configuration that augments or overrides the registered list (for example, an AS-operator-managed allow-list of additional instance issuers) is out of scope for this document. Deployments that introduce such configuration SHOULD document the resulting trust model and ensure it is consistent with the per-client trust the registered metadata expresses; in particular, AS-side issuer additions weaken the client's ability to audit who can act on its behalf.

Trust Lifecycle The trust relationship between client and instance issuer is mutable. When the client's metadata changes (for example, an instance issuer is removed, its jwks_uri or jwks rotates, its trust_domain or spiffe_id is replaced, or its signing_alg_values_supported narrows), updates take effect according to the registration model: for CIMD, the AS applies the same freshness and re-fetch rules it applies to other CIMD-published trust material such as jwks_uri (see ); for static registration, updates take effect when the AS's registration store is updated and re-read. While the AS may continue to honor a stale descriptor within the propagation window, this profile imposes no additional revocation requirement on previously issued access tokens. AS treatment of access tokens whose validated instance identity is no longer endorsed after the update is governed by ; sizing the resulting trust-withdrawal latency is in .

Cross-Organization Federation A client MAY list instance issuers operated by a different organization, including cases where SPIFFE trust domains differ. The per-client minting rule () applies unchanged. For SPIFFE cross-trust-domain deployments, the descriptor's spiffe_bundle_endpoint MUST be operated by the foreign organization or its delegate.

Metadata and Discovery This section defines client metadata used to delegate instance attestation and authorization server metadata used by clients to discover support.

Client Metadata Extensions This document defines client metadata parameters describing the trust relationship between a client and the instance issuers that authenticate its runtime instances. These parameters are registered in the OAuth Dynamic Client Registration Metadata registry () and apply to clients regardless of how their metadata reaches the AS:

For clients identified by a Client ID Metadata Document , these parameters appear in the CIMD document and the AS resolves them by dereferencing the CIMD URL.
For clients registered via OAuth Dynamic Client Registration (or admin-registered with RFC 7591-shaped metadata), these parameters appear in the registered client metadata stored by the AS.

The descriptor format and processing rules are identical in both cases. discusses the trade-offs between the two registration models.

instance_issuers OPTIONAL. A non-empty JSON array of instance issuer descriptor objects. Each descriptor declares an issuer that the client trusts to authenticate its instances. If this parameter is absent, or is present as an empty array, the AS MUST NOT accept instance assertions of type urn:ietf:params:oauth:token-type:client-instance-jwt for this client; the AS SHOULD treat an empty array as a metadata error and log it for the client operator. The set of accepted instance issuers for a given client_id is a trust boundary: any listed issuer can mint assertions that the AS accepts under this client. Deployments that place workloads belonging to distinct organizations or tenants under a single client_id should consult for the trust-aggregation implications. An instance issuer descriptor has the following members:

issuer (REQUIRED):: A StringOrURI identifying the instance issuer. This value MUST exactly match the iss claim of accepted instance assertions and MUST be unique within the instance_issuers array. For raw JWT-SVID compatibility (), this value is the SPIFFE JWT-SVID issuer for the trust domain. For re-minted Client Instance Assertions, this value identifies the OAuth-aware adapter or instance issuer that signed the assertion; trust_domain and spiffe_id then bound the SPIFFE subject space that issuer is allowed to assert.

A descriptor MUST contain exactly one of jwks_uri, jwks, and spiffe_bundle_endpoint. If two or more are present, or all are absent, the AS MUST reject the descriptor as invalid client metadata.

jwks_uri:: An HTTPS URL of a JWK Set containing the public keys used to verify signatures of instance assertions issued by this issuer.
jwks:: An inline JWK Set serving the same purpose as jwks_uri.
spiffe_bundle_endpoint:: An HTTPS URL of a SPIFFE trust bundle endpoint from which the AS resolves verification keys for instance assertions issued by this issuer. When present, subject_syntax MUST be "spiffe", and the AS MUST validate assertions from this issuer under SPIFFE semantics: it MUST treat the assertion as a SPIFFE JWT-SVID (or a re-minted Client Instance Assertion signed with a key distributed in the SPIFFE bundle) and apply the validation rules of . This descriptor field is intended for JWT-SVID validation and for other assertions signed with keys distributed in the SPIFFE bundle for the relevant trust domain. OAuth-aware adapters that sign re-minted Client Instance Assertions with separate OAuth signing keys use jwks_uri or jwks instead. Bundle endpoint format and resolution rules are governed by SPIFFE; see for the analogous use in client authentication.
signing_alg_values_supported (OPTIONAL):: A JSON array of JSON Web Signature (JWS) alg values the AS accepts for instance assertions issued by this issuer. If present, the AS MUST reject instance assertions whose alg is not listed. Issuers SHOULD publish only algorithms they actually use.
subject_syntax (OPTIONAL):: A short identifier indicating the syntactic profile of the sub claim used by this issuer. This document defines two values: "uri" (default, arbitrary StringOrURI) and "spiffe" (a SPIFFE ID ; see also for the related SPIFFE-based client authentication profile). An AS that does not understand the value MUST reject instance assertions for that descriptor with invalid_grant.
trust_domain (OPTIONAL):: When subject_syntax is "spiffe", a SPIFFE trust domain that the sub claim MUST belong to. The AS MUST reject any instance assertion whose sub does not lie within this trust domain. A SPIFFE ID lies within a trust domain only when it parses as a valid SPIFFE ID whose trust-domain component exactly equals trust_domain; ASes MUST NOT use case folding, Unicode normalization, or percent-decoding to make a non-matching trust domain match. trust_domain is meaningful only when subject_syntax is "spiffe"; an AS MUST ignore trust_domain in descriptors whose subject_syntax is any other value. A descriptor's trust_domain is independent of any SPIFFE trust domain associated with the client itself under ; the two MAY differ.
spiffe_id (OPTIONAL):: When subject_syntax is "spiffe", a SPIFFE ID that further bounds which workloads this issuer may attest as instances of this client. The value is a SPIFFE ID, optionally with a trailing "/*" wildcard. Without "/", the instance assertion's sub MUST equal this value exactly; with "/", matching follows the spiffe_id matching rule of . If both spiffe_id and trust_domain are present, the trust domain in spiffe_id MUST equal trust_domain. When present, this member structurally binds a workload subtree to this client; see . If subject_syntax is "spiffe" and spiffe_id is absent, trust_domain MUST be present and the descriptor delegates the entire trust domain to this instance issuer. Clients SHOULD include spiffe_id; omitting it is appropriate only when every workload in the SPIFFE trust domain is authorized to act as an instance of the client.

Example client metadata document with a SPIFFE instance issuer:

Authorization Server Metadata This document defines the following AS metadata parameters for (see ):

client_instance_assertion_supported:: A boolean indicating whether the AS supports the client_instance_assertion request parameter () on the grants listed in . An AS implementing this profile MUST publish this parameter set to true. Clients use it to decide whether to assemble token requests carrying a Client Instance Assertion on non-token-exchange grants. This signal is intentionally coarse: it does not describe grant-specific enablement, raw JWT-SVID support, accepted sender-constraint methods, refresh-token behavior, or client-specific registration policy. Clients may still need registration-time or deployment agreement with the AS for those details.
actor_token_types_supported:: A JSON array of actor_token_type values supported by the AS on the token-exchange grant (). An AS implementing this profile that supports the token-exchange presentation () MUST publish this parameter and include urn:ietf:params:oauth:token-type:client-instance-jwt in it. Other values MAY appear and are processed under their own specifications; their trust resolution is not via instance_issuers.

In addition, an AS that supports MUST advertise client_instance_assertion in token_endpoint_auth_methods_supported (). Example AS metadata document (abridged):

Client Instance Assertion Format This section defines the format of the Client Instance Assertion, the signed JWT this profile uses to identify a client instance at the OAuth 2.0 token endpoint. A Client Instance Assertion is a JWT that asserts the identity of a client instance. It is presented as the client_instance_assertion request parameter on the grants listed in (), or as the actor_token parameter with actor_token_type set to urn:ietf:params:oauth:token-type:client-instance-jwt (see ) on the token-exchange grant (). The assertion serves two purposes: it authenticates the runtime instance (workload identity), and it asserts that the instance is a member of the named OAuth client. This document defines a single JWT carrying both, signed by the instance issuer. This matches the prevailing pattern in workload identity systems, which already issue audience-scoped, signed assertions of runtime identity (e.g., JWT-SVIDs in ).

Issuer Obligations The instance issuer is the trust authority for the assertion and MUST, before minting an instance assertion under this profile:

Authenticate the runtime instance (e.g., via attestation, platform-level identity, or possession of an instance key); and
Verify, under issuer-side policy, that the runtime is permitted to claim the client_id named in the token. This typically means the runtime is operationally part of the client's deployment. An instance issuer MUST refuse to mint an instance assertion whose client_id claim names a client for which the runtime has not been authorized, by issuer-side policy, as a member.

An instance issuer MUST NOT reassign an active or audit-relevant sub value to a different runtime. Issuers SHOULD use stable, non-reassigned subjects, or include sufficient generation or session uniqueness in sub to distinguish runtime incarnations. If subject reassignment is unavoidable, the client, issuer, and AS audit logs need enough lifecycle metadata to distinguish the old and new runtimes. How the issuer internally authenticates the runtime is out of scope. Common deployment patterns (adapter, raw JWT-SVID compatibility, X.509-SVID binding) are described in .

JWT Claims The following claims are defined for client instance assertions.

iss (REQUIRED):: The instance issuer identifier. MUST exactly match an issuer member of an instance_issuers descriptor in the client's registered metadata.
sub (REQUIRED):: The identifier of the client instance, in the syntax declared by the descriptor's subject_syntax (default: arbitrary StringOrURI). Sub-uniqueness considerations for self-acting tokens are addressed in .
aud (REQUIRED):: The intended audience, identifying the AS. The AS validates aud per Section 3, accepting its own issuer identifier or token endpoint URL; if multiple values are present, at least one MUST match. Each AS SHOULD specify a single canonical aud format (typically its issuer identifier) and document it; instance issuers SHOULD use that canonical form. Where instance assertions are scoped per AS, instance issuers SHOULD mint an AS-specific instance assertion rather than a multi-aud JWT, to limit the replay surface.
client_id (REQUIRED unless the SPIFFE compatibility conditions of are met):: The client_id of the client to which this instance belongs. This claim uses the JWT client_id claim defined in Section 4.3. The claim binds the actor token to a specific client and is not part of the actor's identity (per , client_id identifies an OAuth client, not an actor). When present, the AS MUST reject the token if this value does not exactly equal the client_id of the authenticated client. When omitted under , the binding is established structurally by the matched descriptor's SPIFFE scope (spiffe_id when present, otherwise trust_domain) rather than by a JWT claim, and a SPIFFE JWT-SVID may be presented as the Client Instance Assertion directly without re-minting.
exp (REQUIRED):: Expiration time. Issuers SHOULD set short lifetimes (e.g., five minutes or less); see .
iat (REQUIRED):: Issued-at time.
jti (REQUIRED):: A unique identifier used for replay prevention; see .
sub_profile (RECOMMENDED):: One or more OAuth Entity Profile names classifying the actor. defines this claim as OPTIONAL; this profile elevates it to RECOMMENDED so resource servers can apply actor-class-aware policy without bespoke configuration. Its syntax (a space-delimited string of profile names) is the one defined by . This document registers the value client_instance (). Issuers MAY include additional values registered with the "Actor Profile" usage location in the OAuth Entity Profiles registry, or privately defined collision-resistant values, per .
cnf (REQUIRED unless the token is a raw JWT-SVID accepted under ):: A confirmation claim carrying a key bound to this instance. The cnf value MUST contain exactly one of jkt (a JWK SHA-256 thumbprint per Section 3.1) or x5t#S256 (an X.509 certificate SHA-256 thumbprint per Section 3.1) as the binding member; other confirmation methods registered under MAY appear alongside but are not the binding and do not change this profile's sender-constraint verification requirement. The instance issuer MUST mint cnf from a key the named runtime instance demonstrably possesses (e.g., an instance-attested key, a per-instance workload key, or a Demonstration of Proof-of-Possession (DPoP, ) public key presented to the issuer at attestation time). Binding rules and AS verification are defined in . Raw JWT-SVID compatibility is the only exception to this claim requirement, because the AS validates the SVID without re-minting; see and .
nbf (OPTIONAL):: Not-before time. If present, the AS MUST reject the token before this time.

When validating exp, nbf, and iat, ASes SHOULD permit a small clock skew tolerance, typically no more than 60 seconds, applied symmetrically. This bound is consistent with the short-lifetime recommendation in and prevents brittle inter-clock failures across deployments. A Client Instance Assertion MUST NOT contain an act claim. The assertion is a direct identity assertion of a single party (the instance); per , an assertion that carries an act claim represents a delegation chain rather than a direct identity, and the AS MUST reject such a token with invalid_grant (, ). Additional claims MAY be present and MUST be ignored if not understood, except where this document or specifies processing rules. Future profiles requiring AS understanding of a new claim SHOULD use the JWS crit header parameter ( Section 4.1.11) to mark it must-understand; ASes MUST reject assertions whose crit header is malformed or includes claims they do not implement, per Section 4.1.11.

Signing and JOSE Header A Client Instance Assertion MUST be signed using an asymmetric JWS algorithm; none and symmetric (HMAC-based) algorithms (HS256, HS384, HS512) MUST NOT be used and ASes MUST reject assertions signed with them. The descriptor's signing_alg_values_supported (), when present, MUST contain only asymmetric algorithm identifiers. Implementations SHOULD follow the JWT BCP guidance in . ASes and instance issuers implementing this profile MUST support ES256 (). This is the mandatory-to-implement baseline for interoperability. ASes and instance issuers SHOULD additionally support RS256 and MAY support other asymmetric JWS algorithms (, ) as deployment requirements dictate. Issuers SHOULD include a kid in the JWS protected header; ASes SHOULD use kid for key selection. Issuers minting a Client Instance Assertion under this profile MUST set the JWS typ (type) protected header parameter to client-instance+jwt per Section 3.11, and ASes MUST reject such assertions whose typ is anything else. Explicit typing prevents JWT confusion attacks where a token of a different type (for example, a WIMSE workload identity credential , a JWT-SVID outside the SPIFFE compatibility mode, or an OAuth JWT access token ) is mistaken for a Client Instance Assertion. The only exception is the SPIFFE compatibility mode in , where a raw JWT-SVID is intentionally presented without re-minting. In that mode, the AS MUST validate the token as a JWT-SVID according to and , and MUST NOT require the JWS typ header to be client-instance+jwt. Verification keys are obtained from the descriptor's jwks_uri, jwks, or spiffe_bundle_endpoint for the issuer that matches the iss claim; the AS MUST verify alg against signing_alg_values_supported when present.

Example Assertion A decoded re-minted Client Instance Assertion (JWS protected header and JWT payload):

Token Endpoint Processing This section specifies AS-side processing for token requests that provide a Client Instance Assertion. The assertion is presented either as the client_instance_assertion request parameter on the grants in () or as the actor_token parameter with actor_token_type = urn:ietf:params:oauth:token-type:client-instance-jwt on the token-exchange grant (). This section defines the validation, authorization-time consistency, sender-constraint, representation, refresh, client authentication, SPIFFE compatibility, and error rules. This profile applies whether the AS issues JWT access tokens () or opaque (reference) access tokens. The representation rules in describe the claims an issued access token carries (act, sub, client_id, cnf, sub_profile); for JWT access tokens these appear directly in the token payload, while for opaque access tokens they MUST be reflected in introspection responses (, ). The sender-constraint binding in applies to both formats; the binding key is verified at presentation regardless of whether the access token is self-contained or requires introspection.

Token Request A client presents a Client Instance Assertion at the token endpoint using the parameter appropriate to the grant:

On the grants listed in , the assertion is presented as the client_instance_assertion request parameter ().
On the token-exchange grant (), the assertion is presented as the actor_token parameter with actor_token_type set to urn:ietf:params:oauth:token-type:client-instance-jwt ().

The following example shows a client credentials grant carrying a Client Instance Assertion. The client authenticates with private_key_jwt; line breaks are for readability: grant_type=client_credentials &scope=repo.write &client_id=https%3A%2F%2Fapp.example.com%2Fagent &client_assertion_type= urn%3Aietf%3Aparams%3Aoauth%3Aclient-assertion-type%3Ajwt-bearer &client_assertion=eyJhbGciOiJFUzI1NiIsImtpZCI6... &client_instance_assertion=eyJhbGciOiJFUzI1NiIsImtpZCI6... ]]>

Authorization Server Processing When evaluating a token request for this profile, an AS implementing this document MUST perform the following checks and steps in addition to grant-type-specific processing. In this section the term "presented assertion" means the client_instance_assertion parameter () on the grants in , or the actor_token parameter (with actor_token_type set to urn:ietf:params:oauth:token-type:client-instance-jwt) on the token-exchange grant (). Processing is identical regardless of which parameter carried the assertion. Before the steps below, the AS MUST reject the request with invalid_request if any of the following pre-conditions hold:

Parameter on the wrong grant. The Client Instance Assertion is carried as client_instance_assertion on grants in and as actor_token (with actor_token_type = urn:ietf:params:oauth:token-type:client-instance-jwt) on the token-exchange grant. The AS MUST reject a request that carries client_instance_assertion on the token-exchange grant, carries client_instance_assertion on a grant not listed in , or carries actor_token with the client-instance-jwt token type on any grant other than token-exchange.
Token-exchange actor_token / actor_token_type mismatch. The AS MUST reject a token-exchange request in which exactly one of actor_token and actor_token_type is present, or in which actor_token_type is urn:ietf:params:oauth:token-type:client-instance-jwt but actor_token is absent.
Malformed assertion. The AS MUST reject a request whose presented assertion is not a syntactically valid JWT.

Authenticate the client. Authenticate the client using its registered token_endpoint_auth_method per and, if applicable, . The client_id identifies the OAuth client. When the registered method is client_instance_assertion, follow instead of presenting a separate client-controlled credential.
Match the token type. On a token-exchange request, if actor_token_type is not urn:ietf:params:oauth:token-type:client-instance-jwt, processing under this document does not apply; the AS processes the request per , including handling other registered actor_token_type values under their own specifications. If the AS does not support urn:ietf:params:oauth:token-type:client-instance-jwt for the requested grant, it MUST reject the request with unsupported_token_type (). On the grants in , presence of client_instance_assertion selects processing under this document.
Resolve client metadata. Retrieve the client metadata for the authenticated client_id. For clients identified by , dereference the CIMD document subject to its caching rules. For clients registered via or pre-registered with the AS, read the stored metadata. The remaining steps operate on the resolved metadata regardless of source.
Locate the instance issuer descriptor. Parse the presented assertion as a JWT and read its iss claim. Find the descriptor in instance_issuers whose issuer member exactly equals iss. If no descriptor is found, or instance_issuers is absent, reject the request with invalid_grant ().
Verify the signature. Using the descriptor's jwks_uri, jwks, or spiffe_bundle_endpoint, verify the JWS signature per , , and, when applicable, .
Validate JWT claims. Validate iss, sub, aud, exp, iat, nbf, and jti per and Section 3, subject to the raw JWT-SVID exceptions in . Enforce subject_syntax, trust_domain, spiffe_id, and signing_alg_values_supported when present in the descriptor. If subject_syntax is "spiffe" and spiffe_id is absent, require trust_domain and treat the descriptor as delegating the whole trust domain. Validate the JWS typ per and reject unrecognized crit header parameters per .
Verify client_id binding. If the instance assertion contains a client_id claim, it MUST exactly equal the authenticated client_id; reject with invalid_grant otherwise. "Exactly equal" means octet-for-octet equality on the UTF-8 encoding of the two values: ASes MUST NOT apply case folding, Unicode normalization, percent-decoding, URI canonicalization, or other string-equivalence transformations before comparison. The same octet-equality rule applies wherever this document requires iss, sub, client_id, aud, or spiffe_id values to "match" or "exactly equal" another value, except where a specifically cited matching rule (for example, the SPIFFE "/*" wildcard rule in ) defines a different comparison. If the instance assertion has no client_id claim, the AS MUST verify that the matched descriptor satisfies the SPIFFE compatibility conditions (); if not, reject with invalid_grant. When the descriptor satisfies those conditions, the AS MUST verify that the presented assertion's sub falls under the descriptor's SPIFFE scope: spiffe_id when present, otherwise the descriptor's trust_domain; if not, reject with invalid_grant.
Verify proof-of-possession of the cnf key. Verify possession of the assertion's cnf key per ; reject with invalid_request if verification fails. For raw JWT-SVIDs accepted under , establish the binding key per instead.
Apply replay checking. After client_id binding and PoP verification succeed, apply the replay check per ; reject with invalid_grant if a previously seen (iss, jti) tuple is found. For raw JWT-SVIDs, this replay check applies only when jti is present. The replay check follows client_id binding so that an attacker cannot burn a legitimate client's jti by presenting the assertion under a mismatched client_id. The replay check follows PoP verification so that the cnf-bound reusable-mode optimization in can be applied.
Enforce delegation policy. Apply the AS's local maximum delegation depth per .
Check authorization-time consistency. For grants that originate from a prior authorization step (notably authorization_code), apply the rules of .
Bind the instance to the issued access token. If issuance succeeds, represent the instance in the access token per and apply the sender-constraint binding per , using the key whose possession was verified in step 8. Reflect any prior actor chain present in input tokens by nesting per ; chain merging rules are given in .

If validation succeeds, the AS issues an access token (and optionally a refresh token) per the requested grant.

Client Authentication via Instance Assertion A client MAY register the token_endpoint_auth_method value client_instance_assertion in its registered metadata (whether published as a CIMD document or stored at the AS) to indicate that the AS authenticates the client implicitly from a presented Client Instance Assertion, without requiring a separate client_assertion or other credential controlled by the client itself. This mode is the natural choice for workload-only deployments (for example, agentic services, autoscaled microservices, or ephemeral functions) where there is no human user authorizing operations and the team operating the runtime is also the natural owner of the workload identity provider. For these deployments, requiring a separate client-level credential typically means provisioning a private key into every pod alongside the instance-attested assertion, which (per ) does not meaningfully improve defense against runtime compromise. The mode is also appropriate where the client identifier is a logical CIMD URL with client-key custody centralized away from the runtime, or where the workload identity provider trusted to attest instances is the only authority the client wishes to publish. The trust chain to the client is preserved: the client's listing of the instance issuer in its registered metadata is itself the endorsement, and a token signed by such an issuer naming this client_id is attributable to the client. When token_endpoint_auth_method is client_instance_assertion, every accepted instance issuer for the client is also a client authentication trust root. Clients MUST NOT enable this method unless each listed issuer is authorized for that role. client_instance_assertion is a confidential-client token-endpoint authentication method: the AS authenticates the request through the client's registered endorsement of the presenting instance issuer. Clients without any registered trust relationship for the AS to evaluate cannot use it. This exclusion is scoped to this auth method only; deployments using for client authentication follow that specification's client-authentication model, independent of this section.

Request Format A request using this auth method MUST include the client_id form parameter and the assertion under the parameter appropriate to the grant (client_instance_assertion for grants in , or actor_token with actor_token_type for the token-exchange grant). It MUST NOT carry client_assertion or any other client authentication credential. The client_id form parameter is required so the AS can resolve client metadata before validating the assertion; the assertion's client_id claim () is then matched against this value. Example, using the client_credentials grant:

Validation Procedure When the registered token_endpoint_auth_method for the client_id is client_instance_assertion, the pre-conditions of (rejecting malformed or misplaced presented assertions) still apply, and the AS replaces step 1 of ("Authenticate the client") with the following procedure:

Resolve client metadata for client_id (per the registration model: dereference the CIMD URL or read stored registration data).
Validate the presented assertion using the token-type check (where applicable), instance issuer descriptor lookup, signature verification, and JWT claim validation rules in .
Verify that the presented assertion's client_id claim exactly equals the request's client_id parameter.
Verify that the presented assertion contains a cnf claim; this mode does not permit a cnf-less assertion, because the assertion serves as the sole client authentication credential and the bearer-replay considerations in apply with no fallback credential.
Verify proof-of-possession of the assertion's cnf key per .
Apply the replay check in after client_id binding and PoP verification succeed.
Reject the request with invalid_client if any of steps 1-6 fails. This re-code applies to failures that would otherwise be returned as invalid_grant (under ) or invalid_request (under ); pre-condition failures of (malformed JWT, misplaced parameter) continue to return invalid_request.
Treat the client as authenticated. The validated assertion also satisfies this profile's assertion requirement and is used for instance representation per .

The presented assertion's aud claim serves both purposes (the client-assertion audience and this profile's assertion audience). A single value identifying the AS satisfies both. The SPIFFE client_id claim omission mode () does not apply to client_instance_assertion client authentication. Because the same JWT is the sole client authentication credential, the assertion MUST contain the client_id claim and the AS MUST verify it exactly as described above. After this procedure completes, processing continues with the "Enforce delegation policy" step of and onward, reusing the validated instance assertion. The AS MUST NOT re-apply the token-type check, descriptor lookup, signature verification, claim validation, client_id binding, PoP, or replay checks to the same assertion in a way that would cause the request to fail replay detection for its own presentation.

Authorization-Time Consistency When a token request is made under the authorization_code grant ( Section 4.1), the user has authorized the client identified by client_id, not any specific client instance. The AS MUST ensure that the instance introduced at the token endpoint is consistent with that authorization:

The client_id authenticated at the token endpoint MUST match the client_id that received the authorization_code ( Section 4.1.3). Combined with the client_id-binding requirement in , this prevents an instance assertion from another client from being attached to a code.
The AS MUST NOT permit the instance identity to bypass standard authorization-code controls (single-use redemption, redirect URI matching, and any code challenge bound to the original authorization request).
If the AS has any authorization-time policy that depends on the instance (for example, a per-instance allow-list), the AS MUST evaluate that policy against the instance assertion presented at /token and reject inconsistent requests with invalid_grant.

When the issued access token is to be DPoP-bound, clients SHOULD include the dpop_jkt parameter ( Section 10) on the authorization request, naming the same public key whose thumbprint will appear in the instance assertion's cnf.jkt at the token endpoint. When dpop_jkt is present, the AS binds the authorization code to that key per , and at the token endpoint MUST verify that the DPoP proof, the authorization code's dpop_jkt, and the instance assertion's cnf.jkt all reference the same key. This provides cryptographic continuity from /authorize to /token bound to a specific instance: only the instance that holds the named DPoP private key can redeem the resulting code, even if the code is intercepted or transferred to another runtime under the same client. If dpop_jkt is absent, DPoP still sender-constrains the issued access token at the token endpoint, but this profile does not provide cryptographic continuity between the authorization endpoint and the token endpoint for that authorization code. For Mutual-TLS-bound access tokens (), authorization-code continuity is deployment-specific. If the AS binds the authorization request or authorization code to a client certificate seen at the authorization endpoint, then at the token endpoint the AS MUST verify that the certificate used to redeem the code and the instance assertion's cnf.x5t#S256 match the certificate bound at authorization time. Otherwise, mTLS sender-constraint is established at the token endpoint and does not by itself provide authorization- to-token endpoint continuity. User consent under this profile applies to the client as a whole; consent thereby covers all instances attested by listed instance issuers. The key-bound continuity above adds cryptographic guarantees about which instance redeems a code, but does not by itself constitute per-instance consent. An AS MAY require per-instance or per-key authorization policy when the authorization request includes a sender-constraining key such as dpop_jkt. Such policy is deployment-specific: dpop_jkt identifies a key, not an instance, unless the AS has an authorization-time mapping from that key to an instance identity. In those deployments, the AS can require user or administrator approval for the specific instance or key and then verify at the token endpoint that the DPoP proof, authorization code binding, and instance assertion cnf.jkt all reference the approved key. ASes that record consent SHOULD record the descriptor scope under which consent was granted (in particular, the descriptor's issuer and trust_domain), and MAY refuse access tokens for the same client issued under a different descriptor scope than the one consented. This matters for clients deployed across multiple trust domains (for example, "production" vs. "staging" SPIFFE trust domains, or distinct PaaS environments) where the user's consent to one is not necessarily consent to another. This document does not define a general authorization endpoint mechanism for presenting instance identity. Deployments requiring standardized per-instance consent without an authorization-time key mapping need a separate extension.

Sender-Constrained Access Tokens When the AS issues an access token under this profile, whether the instance is represented in act (delegation case; ) or in sub (self-acting case; ), the AS MUST issue a sender-constrained access token bound to a key the instance possesses. Established mechanisms include DPoP and Mutual-TLS-bound access tokens . The AS MUST NOT issue a bearer access token under this profile. Sender-constraint is a structural prerequisite, not a preference: per-instance non-repudiation depends on binding the access token to a key the validated instance possesses. Deployments adopting this profile therefore require AS and RS support for DPoP (), Mutual-TLS-bound access tokens (), or both, and SHOULD verify implementation support before committing. Adoption guidance for DPoP or mTLS rollout is outside the scope of this document; deployments unable to deploy either mechanism within their adoption timeline should defer adopting this profile. If the instance assertion includes a cnf claim (), the AS MUST:

bind the issued access token to the same key by setting the access token's top-level cnf to the instance assertion's cnf value;
verify possession of the cnf key at the token endpoint, matching the binding member used in cnf per . For cnf.jkt, the JWK thumbprint of the DPoP proof's public key MUST equal cnf.jkt. For cnf.x5t#S256, the certificate authenticated at the TLS layer MUST match cnf.x5t#S256. Other confirmation methods present in cnf are not binding members for this profile and MAY be ignored unless local policy or their defining specifications require additional processing;
reject the request with invalid_request if verification fails.

This protects the instance assertion from bearer-style replay within its validity window (); without it, the instance assertion would be a bearer credential whose replay is bounded only by exp and the jti cache. The binding key MUST be specific to the validated client instance. A credential shared by the client as a whole, such as the client-level mTLS certificate authenticated under , the client's private_key_jwt key, or any other client-controlled key not provisioned per-instance, is not sufficient. A re-minted Client Instance Assertion MUST contain cnf so that the binding key is supplied by the same authority that named the instance. The only profile-defined case where cnf can be absent is raw-JWT-SVID compatibility, where the AS establishes an instance-specific binding through a channel independent of the SVID; rules are in . Deployments combining client-level Mutual-TLS-bound client authentication () with this profile MUST establish instance binding through a separate, instance-specific key. The typical configuration uses the client's mTLS certificate at the TLS layer for client authentication and a cnf.jkt in the instance assertion paired with DPoP at the token endpoint for instance binding. Per-instance mTLS certificates issued by the instance issuer (or otherwise bound to instance attestation) are an alternative; in that case the same TLS certificate satisfies both client authentication and instance binding only if the AS treats it as belonging to the instance for binding purposes.

Access Token Representation This section defines how a validated Client Instance Assertion surfaces in the issued access token. It does not restate the generic access-token claim set: JWT access tokens issued under this profile follow ; opaque (reference) access tokens carry the same set of claims through introspection per . The profile-specific surfacing rules are:

The access token MUST be sender-constrained per (i.e., cnf is bound to the instance's key, not bearer).
The validated instance identity surfaces in act (delegation case) or top-level sub (self-acting case), per the classification and per-case rules below; sub_profile () signals the kind of subject in either case.
Any upstream actor chain MUST be preserved by nesting per ; merge rules are in .

A client instance may be acting on behalf of another principal (delegation case; e.g., a user authorized the request through an authorization_code grant) or acting as itself with no other principal involved (self-acting case; e.g., a client_credentials grant). The AS MUST classify each request as delegation or self-acting before populating the issued access token's claims; the classification rules are in .

Classification The AS classifies the request based on whether the grant produces a principal distinct from the client instance presenting the Client Instance Assertion:

Grant	Principal	Classification
authorization_code ()	the user who authorized the code	delegation
`client_credentials` ()	none	self-acting
`refresh_token` ()	inherited from the original grant	inherited
jwt-bearer ()	the assertion's `sub`	delegation
token-exchange ()	the `subject_token`'s subject	delegation

The jwt-bearer and token-exchange rows always classify as delegation under this profile. requires a JWT-bearer assertion that identifies a principal, and Section 2.1 requires a subject_token; in both cases another party is present and named, so the issued access token's sub is that party and the actor appears in act. ASes MUST NOT classify these grants as self-acting based on heuristic matching of subject identifiers; see . This rule applies even when the subject_token was itself a self-acting access token whose sub named the same instance now presenting the assertion (e.g., a client-credentials token from an upstream AS exchanged at a downstream AS): the resulting access token has sub and act.sub naming the same instance. This is benign chain self-reference and is not an error; the AS MUST NOT collapse the two into a self- acting representation. When neither delegation nor self-acting cleanly applies (for example, custom or experimental grants), the AS MUST refuse to issue the access token rather than guess; reject with invalid_grant ().

Delegation Case When the request is classified as delegation, the AS MUST populate the issued access token's act claim per from the validated client instance assertion:

act.iss = the assertion's iss
act.sub = the assertion's sub
act.sub_profile = the assertion's sub_profile (if present); the value client_instance SHOULD be included.
act.cnf = the assertion's cnf, if present.

The access token's sub MUST be the principal identified by the grant (e.g., the authenticated user). Sender-constraint binding (top-level cnf and PoP verification) is governed by . Note that the instance assertion's aud, client_id, exp, iat, and jti are validated and consumed by the AS but do not appear in the issued access token (client_id appears at the top level as a property of the token, not as an actor claim). For a worked example see ; for a nested actor chain (token-exchange whose subject_token already carries an act chain), see .

Self-Acting Case When the request is classified as self-acting, the instance is the principal and there is no other party on whose behalf it acts. The AS MUST populate the issued access token from the validated instance assertion:

sub = the assertion's sub (optionally with AS-applied namespacing, see below)
sub_profile = the assertion's sub_profile (if present); the value client_instance SHOULD be included
cnf is set per
act MUST be omitted

The instance issuer's identifier (the assertion's iss) is not represented as a standard access-token claim in the self-acting case; trust in the issuer is structural via the descriptor (). The AS MUST nevertheless retain the validated issuer with its token state when needed for revocation, introspection, audit, or issuer-aware resource-server policy. For JWT access tokens consumed without introspection, if resource servers need issuer context and the client lists multiple issuers with potentially colliding subject spaces, the AS SHOULD either apply AS-scoped namespacing to sub as described below or expose issuer context using a deployment-specific claim understood by the resource server. For raw JWT-SVIDs that do not carry sub_profile, the AS SHOULD set the access token's sub_profile to client_instance after successful validation, unless local policy intentionally suppresses that signal. A client that lists multiple instance issuers MUST ensure those issuers' sub spaces do not collide (for example, by using disjoint naming conventions, prefixes, or a SPIFFE trust-domain split); when the client cannot guarantee disjointness, the AS SHOULD apply AS-scoped namespacing that incorporates both the matched descriptor's issuer and the original sub value, to prevent a compromised issuer from spoofing another's sub. The specific encoding is a deployment choice. AS-applied namespacing produces an AS-scoped subject identifier; resource-server policy and audit tooling need to treat it as AS-issued rather than issuer-native. For a worked example see .

Actor Chain Merging The AS constructs the issued access token's act chain per 's Delegation Chain Validation and Construction algorithm: the validated client instance assertion is the new outermost actor, and any subject_token act chain (only applicable to token-exchange) is preserved verbatim under it. Depth limits and rejection on overflow follow . In the self-acting case () the act claim is omitted.

Refresh Tokens When an access token is refreshed ( Section 6), the AS reuses the classification () of the original grant to shape the refreshed access token; the original classification is inherited and is not re-derived from the refresh request itself. Refresh tokens issued under this profile MUST be sender-constrained to the originating instance's cnf key, by the same mechanism used to sender-constrain the access token (). Only the originating instance can present the refresh token. A refresh request MUST NOT introduce a client instance identity if the refresh token was not originally issued under this profile. A client MAY include a fresh Client Instance Assertion on a refresh request (for example, to rotate the underlying assertion before its exp) via the client_instance_assertion parameter (). When present, the assertion MUST:

be bound to the same cnf key as the refresh token;
have (iss, sub) matching those recorded at the refresh token's original issuance;
pass the instance issuer descriptor lookup, signature verification, JWT claim validation, and client_id binding checks defined in ; and
pass the replay check defined in against its own (iss, jti) tuple. The bound cnf of the refresh token prevents off-instance replay, but does not prevent an attacker with the same refresh token from replaying a captured fresh assertion across successive refreshes; the replay check closes that window.

If the presented assertion is a raw JWT-SVID without cnf, the AS MUST establish the binding key per and verify that the established binding key matches the refresh token's binding. The AS MUST reject with invalid_grant any refresh request whose presented assertion is not bound to the same cnf key, or whose (iss, sub) differ from those recorded at issuance. Because the refresh token is bound to the originating instance, it is implicitly invalidated when that instance terminates. This keeps act.sub (delegation) or sub (self-acting) stable across the refresh chain, matching the expectation of audit pipelines that a token's actor identity does not change after issuance. For SPIFFE deployments, the cnf binding key SHOULD outlive the JWT-SVID rotation cycle (typically a few minutes in default SPIFFE implementations) when refresh tokens are issued. Deployments that bind cnf to a per-instance DPoP or mTLS key held by the workload satisfy this naturally; deployments that attempt to bind cnf to the SVID's signing key directly will lose refresh-token continuity at every rotation and SHOULD NOT use that pattern. This profile does not extend refresh-token semantics to cross-instance succession; doing so would break the per-instance audit-stability invariant the profile is designed to provide. Deployments requiring cross-instance session continuity address it outside refresh-token semantics (the mechanisms are deployment choices and out of scope for this document).

SPIFFE Compatibility A SPIFFE workload typically obtains a JWT-SVID from the SPIFFE Workload API. JWT-SVIDs carry iss (the trust domain), sub (the SPIFFE ID), aud, exp, and a signature, and may carry additional registered claims such as iat and jti; they do not carry an OAuth client_id claim. To allow such SVIDs to be presented as Client Instance Assertions without re-minting, this profile defines an optional SPIFFE compatibility mode driven entirely by descriptor configuration. An AS is not required to support raw JWT-SVID compatibility in order to support re-minted Client Instance Assertions with subject_syntax = "spiffe". This profile uses JWT-format Client Instance Assertions. X.509-SVIDs are not presented as Client Instance Assertions; SPIFFE deployments using X.509-SVIDs authenticate at the TLS layer (per ) and obtain a JWT-SVID separately for presentation. The X.509-SVID certificate thumbprint MAY serve as cnf.x5t#S256 in either the re-minted assertion or the issued access token's binding. The AS MUST select exactly one validation mode before accepting the assertion and MUST apply that mode's rules exclusively. Selection is determined by whether the assertion contains a client_id claim: presence selects re-minted mode unconditionally, even when the descriptor's SPIFFE conditions would otherwise have permitted raw mode. This means an OAuth-aware adapter that mints a Client Instance Assertion with a SPIFFE-formatted sub together with a client_id claim is always processed as re-minted, and the client_id value is verified per .

Mode	Selected when	Key source	Claim requirements	Binding
Raw JWT-SVID mode	The token has no `client_id` claim and satisfies	`spiffe_bundle_endpoint`	SPIFFE JWT-SVID claims; `client_id`, `typ`, `cnf`, and `jti` are not required	Established separately per
Re-minted assertion mode	The token contains `client_id`, or raw JWT-SVID mode does not apply	`jwks_uri`, `jwks`, or `spiffe_bundle_endpoint` when the signing key is distributed in the SPIFFE bundle	Full Client Instance Assertion claims and `typ` per and	The assertion's `cnf` drives binding per

In raw JWT-SVID mode, the AS MUST:

match the descriptor by exact comparison of the JWT-SVID iss to the descriptor's issuer;
require subject_syntax = "spiffe";
validate the token as a JWT-SVID using SPIFFE JWT-SVID validation rules and the descriptor's spiffe_bundle_endpoint;
validate the SPIFFE JWT-SVID claims required by SPIFFE and ignore unrecognized claims unless local policy rejects them;
validate aud and exp;
validate iat if present;
validate nbf if present;
enforce the descriptor's signing_alg_values_supported when present;
apply the replay-cache rule in if jti is present;
validate sub as a SPIFFE ID and enforce the descriptor's spiffe_id, or trust_domain when spiffe_id is absent; and
establish an instance-specific sender-constraint binding per .

In raw JWT-SVID mode, the JWT-SVID's iss claim MUST identify the SPIFFE JWT-SVID issuer for the trust domain and MUST exactly match the descriptor's issuer member. Because raw JWT-SVIDs do not require jti, an AS that accepts a raw JWT-SVID without jti MUST rely on sender-constraint and short SVID lifetimes for replay protection. In re-minted assertion mode, the AS MUST:

match the descriptor by exact comparison of the assertion's iss to the descriptor's issuer;
verify the JWS signature using the descriptor key source;
validate typ = client-instance+jwt per ;
require and validate the claims defined in , including client_id, exp, iat, jti, and cnf;
verify that client_id equals the authenticated client;
apply the replay-cache rule in ;
if subject_syntax is "spiffe", validate sub as a SPIFFE ID and enforce spiffe_id, or trust_domain when spiffe_id is absent; and
verify possession of the cnf key per .

A deployment that re-mints an SVID into a Client Instance Assertion MUST include the claims required by and MUST use typ = client-instance+jwt per .

Client ID Claim Omission Raw JWT-SVID mode applies when the presented assertion has no client_id claim and all of the following hold for the descriptor that matches the assertion's iss:

subject_syntax is "spiffe";
either (a) a spiffe_id member is present and the assertion's sub satisfies the spiffe_id matching rule of (exact match, or with "/*", path-segment prefix match per ), or (b) spiffe_id is absent, trust_domain is present, and the assertion's sub falls within that trust domain;

the AS MUST treat the descriptor as the per-client binding for the raw JWT-SVID. In this mode:

The AS MUST verify that the assertion's sub satisfies the descriptor's SPIFFE scope: spiffe_id when present, otherwise the descriptor's whole trust_domain.
All other JWT claims and validation rules of continue to apply unchanged, except that a raw JWT-SVID is not required to carry iat or jti; if either claim is present, the AS MUST validate it per and .

A token that contains a client_id claim is processed as a re-minted Client Instance Assertion, not under this omission mode; that claim MUST equal the request's client_id parameter (). The security rationale is that the descriptor's SPIFFE scope, present in the client's registered metadata (whether published as a CIMD document or stored at the AS), is itself the per-client binding: a workload's SPIFFE ID is bound to a client by the client explicitly listing the prefix that contains it, or by omitting spiffe_id and thereby delegating the whole trust domain. This is the same model uses for client authentication, applied here to actor identity. Clients SHOULD include spiffe_id unless whole-domain delegation is intentional.

SPIFFE Trust Bundle Resolution When a descriptor specifies spiffe_bundle_endpoint instead of jwks_uri or jwks, the AS resolves verification keys via the SPIFFE trust bundle endpoint. The AS MUST validate the bundle's freshness and applicability to the trust domain in the descriptor's trust_domain (or the trust domain implied by spiffe_id). The AS MUST verify JWT signatures with JWT authority keys from the bundle for the relevant trust domain, and MUST separately require the assertion's iss and sub to satisfy the descriptor's issuer and SPIFFE scope constraints. The bundle endpoint format, freshness, rotation rules, and TLS authentication (WebPKI) follow , the same handling used for client authentication. When the AS uses an X.509-SVID at the TLS layer for sender-constraint binding under raw-JWT-SVID compatibility (), the X.509-SVID is validated against the X.509 trust anchors served by the same SPIFFE bundle.

Sender-Constraint Binding for Raw JWT-SVIDs A raw JWT-SVID accepted under does not include a cnf claim. The AS MUST establish an instance-specific binding through some other means whose key is attributable to the validated instance. The AS MUST reject the request unless the presented DPoP key or mTLS certificate is bound, by the AS's local policy, to the same runtime named by the JWT-SVID's sub. The binding mechanism MUST establish key custody through a channel independent of the JWT-SVID itself (for example, issuer-provisioned per-instance credentials or a workload attestation channel that names the same sub); accepting a DPoP key solely because it accompanied a valid JWT-SVID is not a binding and reduces this mode to bearer-with-aud. Acceptable binding mechanisms include:

a per-instance mTLS client certificate provisioned by the instance issuer (or otherwise tied to instance attestation) and presented under ; when the certificate is an X.509-SVID, the AS MUST verify that its SAN URI exactly equals the JWT-SVID's sub; or
a DPoP key that the AS confirms, through deployment-specific attestation or out-of-band binding to the instance issuer, represents the same runtime named by the instance assertion's sub.

In raw-JWT-SVID mode, the AS MUST set the issued access token's top-level cnf to a confirmation member identifying the binding key established above (cnf.x5t#S256 for an X.509-SVID, cnf.jkt for a DPoP key). Access tokens bound via cnf.x5t#S256 to a rotating X.509-SVID are usable only while the workload holds that specific certificate; deployments SHOULD size access-token TTL with the SVID rotation cycle in mind. The AS SHOULD record which mechanism established the binding and which key or certificate was bound to the instance, to support incident response and per-instance revocation (). If the AS cannot establish an instance-specific binding, it MUST reject the request with invalid_request ().

Error Responses Errors are returned per Section 5.2 and Section 2.2.2. This profile uses the existing OAuth error codes:

invalid_request: pre-condition and request-shape failures, including missing or mismatched client_instance_assertion (or, on token-exchange grants, actor_token/actor_token_type), presence of client_instance_assertion on the token-exchange grant, malformed JWT, JWS typ mismatch (), sender-constraint binding failures (), and chain depth exceeding the AS local maximum ().
invalid_grant: failures of instance-assertion validation, including signature, JWT claim validation, descriptor lookup or shape, client_id binding, SPIFFE compatibility conditions, classification ambiguity, and a presented assertion carrying an act claim.
unsupported_token_type (): on a token-exchange grant, an unrecognized actor_token_type.
invalid_client: when the presented assertion is the client authentication credential under , validation failures that would otherwise be returned as invalid_grant (assertion-validation failures) or invalid_request (sender-constraint failures, including a cnf-less assertion) are returned as invalid_client. The pre-condition failures of (malformed JWT, misplaced parameter, mismatched grant/parameter) continue to be returned as invalid_request even in this mode.

The AS MAY return additional information via the error_description parameter; deployments MUST NOT include sensitive instance details (e.g., raw SPIFFE IDs of unrelated workloads) in error responses. To help client-developer debugging, AS implementations SHOULD include non-sensitive diagnostic context such as which validation step failed (for example, "issuer not in instance_issuers" or "cnf possession failed").

Resource Server Processing Resource servers consuming access tokens issued under this profile follow the resource server processing rules defined in for delegated access tokens, including actor authorization, JWT access token validation, sender-constraint validation against the top-level cnf, and introspection. This section adds three considerations specific to this profile. cnf is the instance's key, not the principal's. Under this profile the access token's top-level cnf is bound to the instance that presents the token. In the delegation case the principal in sub (typically a user) does not present the token; the instance named in act.sub does, and sender-constraint validation authenticates that instance. Self-acting access tokens () carry no act. sub names the client instance (typically a SPIFFE ID or other workload identifier), sub_profile = client_instance signals that the subject is a runtime instance, and client_id continues to name the OAuth client. Resource servers MUST NOT treat client_id as the actor identifier in the self-acting case; the actor identifier is sub. Resource servers that distinguish workload-self-acting from human- delegated requests SHOULD make the determination based on the presence or absence of act, not on the format of sub. Authorization policy SHOULD evaluate instance identity. Policies that authorize solely on client_id lose the instance-level distinction this profile is designed to provide.

Introspection Responses When the AS issues opaque (reference) access tokens under this profile, the set of profile-defined claims in the introspection response () MUST be identical to the set that would appear in the payload of a JWT access token for the same grant, including act (when applicable), sub, sub_profile, client_id, cnf, and any actor-chain members defined by . Resource servers that consume introspection apply the same processing as for JWT access tokens described above; the representational difference is only in where the claims arrive (token payload versus introspection response). The following example shows an introspection response for an opaque access token issued under the authorization_code grant (). The corresponding self-acting case () and token-exchange case () follow the same pattern: the top-level claims and any act chain appear in the response exactly as they would in a JWT access token payload.

Adoption and Migration This profile is designed for incremental adoption. Existing client metadata (CIMD documents or static registrations) that does not declare instance_issuers continues to work unchanged, and existing access tokens in circulation when a client adds instance_issuers remain valid for their original lifetime. An AS MAY implement this profile while continuing to serve clients that do not use it. Token requests are dispatched on the presence of client_instance_assertion (on the grants in ) or on actor_token_type = urn:ietf:params:oauth:token-type:client-instance-jwt (on the token-exchange grant). Other (or absent) values are processed under their own specifications. ASes implementing this profile MUST advertise support via the client_instance_assertion_supported AS metadata parameter and, for token-exchange use, via actor_token_types_supported (). Clients SHOULD verify the AS's advertised support before sending a Client Instance Assertion on a token request, since RFC 6749 permits ASes that do not implement this extension to silently ignore unrecognized parameters and issue an unbound access token. The metadata values are coarse capability signals; clients may still need registration-time or deployment agreement for grant- specific use, raw JWT-SVID compatibility, accepted sender-constraint methods, and refresh-token behavior. A client MAY add instance_issuers at any time. A client that wants to mandate Client Instance Assertions for every issued access token can register token_endpoint_auth_method = client_instance_assertion (), which intrinsically requires the assertion. Re-minted Client Instance Assertions require cnf (). A deployment whose workload identity system does not yet emit per-instance keys has two options:

Adapter pattern: an OAuth-aware adapter wraps an existing workload identity system (cluster-issued projected service-account tokens, cloud-instance metadata services, a SPIFFE control plane, etc.) and re-mints a Client Instance Assertion with cnf from the underlying credential. From the AS's perspective the adapter is the instance issuer () and inherits the obligations and trust model of that role. The adapter holds the workload-identifier to client_id mapping and the OAuth signing keys, since underlying workload-identity systems typically do not know about OAuth clients. Recommended for non-SPIFFE deployments and for SPIFFE deployments that can run an adapter.
Raw JWT-SVID compatibility: the SVID is presented as the Client Instance Assertion without re-minting; the AS establishes sender-constraint binding through a channel independent of the SVID per and (the X.509-SVID at TLS under is the common pattern). See for a worked example.

ASes and OAuth client operators SHOULD NOT enable the client_instance_assertion authentication method () without cnf: that mode has no fallback client credential, so a cnf-less assertion is fully bearer at presentation ().

Conformance An AS conforms to this document by implementing the client_instance_assertion request parameter () and its token-exchange presentation as actor_token with actor_token_type = urn:ietf:params:oauth:token-type:client-instance-jwt (), together with the validation, representation, and error processing rules in , , and , plus . Raw JWT-SVID compatibility () and the client_instance_assertion authentication method () are optional capabilities; an AS that supports either MUST conform to the respective section. A client using this profile with a Client Instance Assertion MUST publish or register instance_issuers metadata () and MUST ensure each listed issuer is authorized to attest its instances. An instance issuer MUST mint assertions per , , and . A resource server MUST process delegated tokens per and apply the self-acting semantics in when act is absent.

Security Considerations This document inherits the security considerations of , , , , , , and .

Trust Model The normative trust model for this profile is in . This subsection summarizes the security implications. A client delegates the authentication of its instances to one or more instance issuers. A compromised or misconfigured instance issuer can mint instance assertions that the AS will accept as legitimate instances of the named client. Clients SHOULD list only instance issuers under their own administrative control (or contractually equivalent), and SHOULD set spiffe_id, trust_domain, and signing_alg_values_supported to bound what each issuer is allowed to assert. Deployments hosting workloads from multiple tenants under a single client_id aggregate those tenants' trust roots into a single instance_issuers list; see for the resulting blast-radius considerations and the recommendation to issue per-tenant client_id values where tenants are independent trust boundaries. Clients using SPIFFE SHOULD include spiffe_id; omitting it delegates the whole SPIFFE trust domain and is appropriate only when every workload in that trust domain is authorized to act as an instance of the client. After a client detaches a compromised issuer, tokens minted under the prior trust may continue to validate up to the trust-withdrawal latency bound in ; operators SHOULD plan incident response around this window. The per-client minting requirement of is an operational obligation on the instance issuer, not a check the AS performs in-band. The AS verifies that a presented assertion's client_id claim matches the authenticated client, but it cannot detect an issuer that violates its obligation by minting for runtimes outside the authorized set; such assertions are accepted as valid. Clients SHOULD list only issuers whose minting policy they have audited and whose operational practice they trust to honor . Client metadata is itself trust-affecting: an attacker who can modify it can add a new instance issuer under their control. Adding an instance_issuers entry, widening a descriptor's spiffe_id or trust_domain, changing a descriptor's key source, or enabling client_instance_assertion (as a token_endpoint_auth_method) is equivalent to adding or expanding a credential issuer for the client and SHOULD require high-assurance change control by the client operator and AS. Clients publishing CIMD metadata MUST protect the publication channel (per 's requirement of HTTPS) and the storage backing it; deployments using static registration MUST protect the registration store and any administrative API used to update it. ASes resolving CIMD documents inherit 's security considerations covering transport, intermediary caches, and DNS.

Trust-Withdrawal Latency The trust-withdrawal latency, that is, the worst-case time from a client metadata change to all derived access tokens having expired, is approximately the sum of the metadata refresh interval (for CIMD, the cache TTL; for static registration, the expected lag between an admin update and AS-side propagation), the instance assertion's exp window, the AS's JWKS or SPIFFE-bundle cache TTL for the issuer, and the access token TTL. ASes SHOULD size these components together so that the resulting latency matches their incident-response target. Deployments with longer latencies SHOULD support active revocation () and introspection-based status checks at the resource server.

Instance Lifecycle Client instances are short-lived in many deployments (containers, function invocations, agent sessions). This profile relies on three mechanisms to keep actor identity current:

Rotation:: Instance issuers SHOULD mint short-lived instance assertions (). New tokens are issued continuously as instances start, restart, or rotate keys.
Revocation within the validity window:: Within an instance assertion's exp window, the AS prevents reuse via the jti replay rule (). A specific issued access token, or all tokens associated with an instance, can be revoked only via the AS's own revocation mechanisms (); this profile does not define a standardized revocation endpoint or instance revocation list format.
Trust withdrawal:: To stop accepting instance assertions from an issuer (e.g., after a workload identity compromise), the client removes the issuer from instance_issuers, replaces or removes trust_domain, or rotates jwks at the issuer level. The AS's response is governed by : subsequent uses of access tokens whose act references the withdrawn scope are treated as no longer endorsed.

Refresh windows are a particular concern: an access token refreshed without a new instance assertion may carry stale instance identity long after the original instance has terminated. ASes SHOULD prefer requiring a fresh instance assertion on refresh (), or set short refresh intervals when instance identity is present. Compromise-response latency under this profile is bounded by the shortest of: the access token TTL, the introspection cache TTL at the resource server, and the propagation time of whichever revocation signal a deployment uses (). None of these are defined by this document; deployments with strict detection-to-revocation latency requirements SHOULD tune them together and document the resulting end-to-end bound as part of operational security posture.

Token Revocation This profile supports two complementary revocation models for access tokens issued under it. Both build on ; deployments MAY support either, both, or neither. After the AS has adopted updated client metadata (), the AS SHOULD treat further use of access tokens whose validated instance identity is no longer endorsed by the client as invalid:

for delegation tokens, when the act claim names a removed instance issuer or falls outside the descriptor's updated scope;
for self-acting tokens, when the instance issuer recorded by the AS at issuance time has been removed, or when the access token's sub falls outside the descriptor's updated scope.

Where the deployment supports it, this is naturally enforced by introspection () and short access-token lifetimes; AS implementations MAY additionally revoke such tokens per , including via the per-instance mechanism in . ASes MAY apply the same policy to changes in a descriptor's jwks_uri, jwks, or spiffe_bundle_endpoint keys that permits for changes in client-level keys.

Per-Token Revocation An AS that supports revocation MAY accept the access token (or its associated refresh token) as the token parameter and revoke that specific issued token. This works unchanged for tokens issued under this profile; no profile-specific extensions to the revocation endpoint are required.

Per-Instance Revocation When an instance is compromised or otherwise needs to be quarantined, a deployment may need to invalidate all access tokens whose validated instance issuer and instance subject identify that instance, without enumerating every issued token. ASes implementing this profile SHOULD support a per-instance revocation mode keyed by the pair (instance issuer, instance subject):

for delegation tokens, the key is (act.iss, act.sub);
for self-acting tokens, the key is the instance issuer recorded by the AS at issuance time together with the access token's sub;
invalidates all currently-active access tokens matching that key, with descriptor scope as an optional additional filter;
prevents issuance of new access tokens with that instance issuer-and-subject pair as actor or principal until a follow-up condition is met (for example, expiration of an internal blocklist entry, or removal of the instance from the workload identity system).

The mechanism for triggering per-instance revocation is deployment-specific and out of scope for this document. The compromise-to-all-tokens-invalid latency is bounded by the shortest of: the access token TTL, the introspection cache TTL at any consuming resource server (), and the propagation time of whichever revocation signal the deployment uses.

Introspection Behavior on Revocation When an AS supports introspection (), introspection responses for access tokens issued under this profile MUST honor both per-token revocation and per-instance revocation: an introspection response MUST return active = false for any access token that has been revoked under either model. Introspection MUST also honor the trust update rules in : when the AS has adopted updated client metadata that removes an instance issuer or narrows a descriptor's scope so that an issued access token's act (or, for self-acting tokens, sub) is no longer endorsed, introspection responses for that access token MUST return active = false once the AS has applied the update. The claim set returned in active introspection responses is specified by .

Revocation and Refresh Tokens When a refresh token is sender-constrained to the originating instance (), per-instance revocation MUST also revoke the refresh token (and prevent any further access tokens it would mint). This profile does not define successor-instance refresh; deployments that need cross-instance session continuity use the separate mechanisms described in .

Replay Actor tokens MUST include jti, exp, and iat (), except for raw JWT-SVIDs accepted under . After identifying the issuer and validating the signature, the AS MUST reject a token whose (iss, jti) pair has already been seen within the token's validity window, and MUST retain replay-cache entries at least until the token's exp plus any allowed clock skew. For raw JWT-SVIDs, this check applies only when jti is present; otherwise replay is bounded by sender-constraint, short SVID lifetimes, and audience restriction. The replay-cache key is (iss, jti); ASes MUST NOT widen the key to include client_id. jti uniqueness is the responsibility of the instance issuer within its own iss namespace, so two different OAuth clients that list the same instance issuer share the same replay state for that issuer. An attacker controlling traffic under one OAuth client_id cannot replay assertions under another, but high-volume traffic under one client can cause replay-cache pressure for other clients sharing the same instance issuer; ASes SHOULD apply per-issuer rate limits and bounded cache caps. The cache MUST be scoped at least to a single AS instance; distributed AS deployments share the cache or coordinate as specified below. An AS MAY skip the replay check for cnf-bound assertions that have been PoP-verified at presentation, treating them as reusable within their exp window, provided the deployment documents this behavior and applies rate limits, monitoring, and audit logging. This shifts the blast radius of cnf-key compromise from one access token per assertion to the rate-limit ceiling within exp; the rate limit is therefore the bound on that threat. The MUST applies unconditionally to assertions without a verified cnf. Issuers SHOULD use short lifetimes (five minutes or less). On refresh (), AS implementations SHOULD prefer requiring a fresh instance assertion; when a fresh assertion is presented, the AS MUST apply the replay check to its (iss, jti) per . Distributed AS deployments MUST share the replay cache or coordinate to prevent cross-replica replay, except on cnf-bound paths in reusable mode where cnf+PoP verification is correctness-preserving across replicas. A compromised instance-issuer signing key creates a denial-of-service surface: an attacker can mint validly-signed assertions with arbitrary jti values. ASes SHOULD apply per-issuer rate limits and bounded cache caps; a sustained high rate of distinct jti values from a single issuer is a signal of compromise.

Audience and Confused Deputy The aud claim binds the instance assertion to a specific AS, preventing one AS from replaying it against another ( Section 3). The client_id claim, which this document treats as a binding (not as actor identity), prevents an instance assertion issued for one client from being presented under a different client's authentication.

Trust-Root Collapse The client_instance_assertion authentication method () collapses two trust roots (client credential and instance issuer) into one. Compromise of any listed instance issuer is sufficient to mint tokens that authenticate as the client. Modes such as private_key_jwt require an attacker to possess both the instance issuer's signing key and the client's private key; that two-key property holds only when the two keys live in different security domains. Where genuine custody separation between the client credential and the instance issuer is not available, the security property of private_key_jwt paired with a Client Instance Assertion reduces to that of client_instance_assertion alone. When client_instance_assertion is used, clients SHOULD constrain each instance issuer's authority through spiffe_id, trust_domain, and signing_alg_values_supported, and SHOULD list only the minimum set of instance_issuers necessary. Trust withdrawal under this auth method has immediate consequences: removing an instance issuer from instance_issuers (or narrowing its descriptor scope) invalidates client authentications that depended on that issuer's endorsement, and the AS MUST stop accepting client_instance_assertion authentications via the removed or narrowed issuer once it has applied the metadata update ().

Multi-Tenancy Under a Single Client The class-and-instance model in is a two-level model: the OAuth client (class) and its runtime instances. It does not model tenancy as a third dimension. Deployments that aggregate workloads from distinct trust domains under a single OAuth client_id concentrate those tenants' trust roots into one instance_issuers list. Compromise of any listed issuer extends to every tenant whose workloads it authenticates. This profile RECOMMENDS issuing a separate client_id per tenant when tenants need to be isolated as trust boundaries. Per-tenant client_ids give each tenant its own instance_issuers list, its own descriptor scope, and its own revocation surface, and align this profile's trust model with the deployment's tenancy boundaries. Where per-tenant client_ids are not practical, deployments SHOULD use tight per-issuer constraints in instance_issuers (per-tenant spiffe_id paths, per-tenant trust_domain values, or descriptor-level scope members that constrain accepted sub values to a specific tenant) and treat the cross-tenant blast radius as a known operational risk. Tenant isolation in such deployments is enforced by the breadth of these per-issuer constraints, not by the OAuth client identity. This profile does not define a tenant identifier as a first-class claim. Future profiles MAY introduce one if cross-deployment interoperability of tenant scoping becomes necessary.

Mode-Switch Between Delegation and Self-Acting Whether an issued access token represents delegation or self-acting () determines whether the instance is exposed to resource servers as act or as sub. An adversary that can influence classification could escalate privileges, for example by inducing the AS to drop a sub belonging to a user and re-anchor the token on the instance's sub. The classification rule in is determined by the grant type, not by comparison of attacker-influenceable subject strings; ASes MUST NOT employ heuristic or fuzzy matching of assertion contents to override the table. In particular, ASes MUST NOT normalize either side of any comparison they perform on subject identifiers (no Unicode normalization, no case folding, no percent-decoding beyond what requires for JSON parsing). When classification is ambiguous (for example, custom grants not listed in the table), the AS MUST refuse rather than guess.

Sender-Constraint Requirement Without sender-constraint, an act claim is an assertion about who acted, not a binding enforced at the resource server: any party in possession of the access token can present it as the named actor. therefore requires sender-constrained access tokens and forbids bearer issuance under this profile. Per , the resource server validates proof of possession against the access token's top-level cnf only; confirmation members inside an act object are actor context for audit and correlation, not a binding the RS independently verifies.

Delegation Control Unbounded delegation chains permit privilege amplification across boundaries. AS implementations MUST enforce a local maximum delegation depth (). recommends supporting at least depth 4 for cross-domain interop; deployments imposing lower ceilings should weigh interoperability against the privilege-amplification surface they are willing to allow.

Privacy A client instance assertion reveals fine-grained workload identity to the AS and, after issuance, to resource servers via the act claim (delegation case) or the access token's top-level sub (self-acting case). Exposing per-instance identity to resource servers is the deliberate purpose of this profile (it is what enables instance-level audit, authorization, and binding downstream), but it has privacy and operational consequences:

Resource servers gain visibility into the deploying organization's internal workload structure, including (depending on sub) cluster names, namespaces, function instance IDs, or session identifiers. Resource server operators SHOULD treat this information with the same care as any other identity attribute received from an AS, and SHOULD NOT log or propagate it more broadly than necessary.
Naming conventions in sub may inadvertently encode sensitive details. Issuers and clients SHOULD avoid encoding identifiers of human users, secret material, or internal infrastructure topology in sub, and SHOULD prefer opaque or hierarchical identifiers (e.g., a SPIFFE path) whose minimum granularity matches the auditing need.

The error response guidance in extends to logs and audit trails: instance assertion contents SHOULD be logged at a level commensurate with the sensitivity of the workload identity they convey.

IANA Considerations

OAuth Token Type IANA is requested to register the following value in the "OAuth URI" registry established by (and used by for actor_token_type values on the token-exchange grant):

URN:: urn:ietf:params:oauth:token-type:client-instance-jwt
Common Name:: OAuth 2.0 Client Instance Assertion
Change Controller:: IETF
Specification Document(s):: This document

OAuth Client Instance Subject Syntaxes IANA is requested to create a new sub-registry titled "OAuth Client Instance Subject Syntaxes" under the "OAuth Parameters" registry group established by . Registration policy is Specification Required . A Client Instance Subject Syntax is a short identifier appearing in the subject_syntax member of an instance issuer descriptor (). It declares the syntactic form of the sub claim used by the issuer and selects validation rules the AS applies to that claim. Registry fields:

Syntax Identifier:: A short label used as the subject_syntax value. ABNF: 1*( ALPHA / DIGIT / "-" ).
Description:: A short description of the subject form.
Change Controller:: As required by Specification Required policy.
Specification Document(s):: The defining specification.

IANA is requested to register the following initial values:

Syntax Identifier	Description	Change Controller	Specification
`uri`	Arbitrary StringOrURI subject (the default when `subject_syntax` is absent).	IETF	of this document
`spiffe`	SPIFFE ID . Triggers SPIFFE-specific descriptor members (`trust_domain`, `spiffe_id`) and SPIFFE compatibility ().	IETF	of this document

OAuth Parameters Registration IANA is requested to register the following value in the "OAuth Parameters" registry established by :

Parameter name:: client_instance_assertion
Parameter usage location:: token request
Change Controller:: IETF
Specification Document(s):: of this document

OAuth Dynamic Client Registration Metadata IANA is requested to register the following parameters in the "OAuth Dynamic Client Registration Metadata" registry established by . The Change Controller for each entry is IETF.

instance_issuers

Client Metadata Name:: instance_issuers
Client Metadata Description:: Trusted issuers of client instance assertions for this client.
Specification Document(s):: of this document

OAuth Token Endpoint Authentication Method IANA is requested to register the following value in the "OAuth Token Endpoint Authentication Methods" registry established by :

Token Endpoint Authentication Method Name:: client_instance_assertion
Change Controller:: IETF
Specification Document(s):: of this document

OAuth Authorization Server Metadata IANA is requested to register the following parameters in the "OAuth Authorization Server Metadata" registry established by . The Change Controller for each entry is IETF.

client_instance_assertion_supported

Metadata Name:: client_instance_assertion_supported
Metadata Description:: Boolean indicating whether the AS accepts the client_instance_assertion request parameter on the grants listed in .
Specification Document(s):: of this document

actor_token_types_supported

Metadata Name:: actor_token_types_supported
Metadata Description:: JSON array of actor_token_type values supported on the token-exchange grant.
Specification Document(s):: of this document

Media Type IANA is requested to register the following media type in the "Media Types" registry:

Type name:: application
Subtype name:: client-instance+jwt
Required parameters:: N/A
Optional parameters:: N/A
Encoding considerations:: binary; A Client Instance Assertion is a JWT; JWT values are encoded as a series of base64url-encoded values separated by period characters.
Security considerations:: See of this document.
Interoperability considerations:: N/A
Published specification:: This document.
Applications that use this media type:: OAuth 2.0 authorization servers and clients implementing this profile.
Fragment identifier considerations:: N/A
Additional information:: Magic number(s): N/A; File extension(s): N/A; Macintosh file type code(s): N/A
Person & email address to contact for further information:: Karl McGuinness public@karlmcguinness.com
Intended usage:: COMMON
Restrictions on usage:: none
Author:: Karl McGuinness
Change controller:: IETF

This media type, when used as a value of the typ JWS protected header parameter ( Section 4.1.9), MUST be client-instance+jwt (per Section 3.11; the application/ prefix is omitted).

OAuth Entity Profile IANA is requested to register the following value in the "OAuth Entity Profiles" registry established by . This registration is contingent on the establishment of that registry.

Profile Name:: client_instance
Profile Description:: A concrete runtime instance of an OAuth client identified by a client_id.
Profile Usage Location:: Actor Profile
Change Controller:: IETF
Specification Document(s):: This document

References Normative References The OAuth 2.0 Authorization Framework The OAuth 2.0 authorization framework enables a third-party application to obtain limited access to an HTTP service, either on behalf of a resource owner by orchestrating an approval interaction between the resource owner and the HTTP service, or by allowing the third-party application to obtain access on its own behalf. This specification replaces and obsoletes the OAuth 1.0 protocol described in RFC 5849. [STANDARDS-TRACK] JSON Web Signature (JWS) JSON Web Signature (JWS) represents content secured with digital signatures or Message Authentication Codes (MACs) using JSON-based data structures. Cryptographic algorithms and identifiers for use with this specification are described in the separate JSON Web Algorithms (JWA) specification and an IANA registry defined by that specification. Related encryption capabilities are described in the separate JSON Web Encryption (JWE) specification. JSON Web Key (JWK) A JSON Web Key (JWK) is a JavaScript Object Notation (JSON) data structure that represents a cryptographic key. This specification also defines a JWK Set JSON data structure that represents a set of JWKs. Cryptographic algorithms and identifiers for use with this specification are described in the separate JSON Web Algorithms (JWA) specification and IANA registries established by that specification. JSON Web Algorithms (JWA) This specification registers cryptographic algorithms and identifiers to be used with the JSON Web Signature (JWS), JSON Web Encryption (JWE), and JSON Web Key (JWK) specifications. It defines several IANA registries for these identifiers. JSON Web Token (JWT) JSON Web Token (JWT) is a compact, URL-safe means of representing claims to be transferred between two parties. The claims in a JWT are encoded as a JSON object that is used as the payload of a JSON Web Signature (JWS) structure or as the plaintext of a JSON Web Encryption (JWE) structure, enabling the claims to be digitally signed or integrity protected with a Message Authentication Code (MAC) and/or encrypted. JSON Web Token (JWT) Profile for OAuth 2.0 Client Authentication and Authorization Grants This specification defines the use of a JSON Web Token (JWT) Bearer Token as a means for requesting an OAuth 2.0 access token as well as for client authentication. OAuth 2.0 Dynamic Client Registration Protocol This specification defines mechanisms for dynamically registering OAuth 2.0 clients with authorization servers. Registration requests send a set of desired client metadata values to the authorization server. The resulting registration responses return a client identifier to use at the authorization server and the client metadata values registered for the client. The client can then use this registration information to communicate with the authorization server using the OAuth 2.0 protocol. This specification also defines a set of common client metadata fields and values for clients to use during registration. OAuth 2.0 Token Introspection This specification defines a method for a protected resource to query an OAuth 2.0 authorization server to determine the active state of an OAuth 2.0 token and to determine meta-information about this token. OAuth 2.0 deployments can use this method to convey information about the authorization context of the token from the authorization server to the protected resource. Proof-of-Possession Key Semantics for JSON Web Tokens (JWTs) This specification describes how to declare in a JSON Web Token (JWT) that the presenter of the JWT possesses a particular proof-of- possession key and how the recipient can cryptographically confirm proof of possession of the key by the presenter. Being able to prove possession of a key is also sometimes described as the presenter being a holder-of-key. Guidelines for Writing an IANA Considerations Section in RFCs Many protocols make use of points of extensibility that use constants to identify various protocol parameters. To ensure that the values in these fields do not have conflicting uses and to promote interoperability, their allocations are often coordinated by a central record keeper. For IETF protocols, that role is filled by the Internet Assigned Numbers Authority (IANA). To make assignments in a given registry prudently, guidance describing the conditions under which new values should be assigned, as well as when and how modifications to existing values can be made, is needed. This document defines a framework for the documentation of these guidelines by specification authors, in order to assure that the provided guidance for the IANA Considerations is clear and addresses the various issues that are likely in the operation of a registry. This is the third edition of this document; it obsoletes RFC 5226. OAuth 2.0 Authorization Server Metadata This specification defines a metadata format that an OAuth 2.0 client can use to obtain the information needed to interact with an OAuth 2.0 authorization server, including its endpoint locations and authorization server capabilities. OAuth 2.0 Token Exchange This specification defines a protocol for an HTTP- and JSON-based Security Token Service (STS) by defining how to request and obtain security tokens from OAuth 2.0 authorization servers, including security tokens employing impersonation and delegation. OAuth 2.0 Mutual-TLS Client Authentication and Certificate-Bound Access Tokens This document describes OAuth client authentication and certificate-bound access and refresh tokens using mutual Transport Layer Security (TLS) authentication with X.509 certificates. OAuth clients are provided a mechanism for authentication to the authorization server using mutual TLS, based on either self-signed certificates or public key infrastructure (PKI). OAuth authorization servers are provided a mechanism for binding access tokens to a client's mutual-TLS certificate, and OAuth protected resources are provided a method for ensuring that such an access token presented to it was issued to the client presenting the token. JSON Web Token Best Current Practices JSON Web Tokens, also known as JWTs, are URL-safe JSON-based security tokens that contain a set of claims that can be signed and/or encrypted. JWTs are being widely used and deployed as a simple security token format in numerous protocols and applications, both in the area of digital identity and in other application areas. This Best Current Practices document updates RFC 7519 to provide actionable guidance leading to secure implementation and deployment of JWTs. JSON Web Token (JWT) Profile for OAuth 2.0 Access Tokens This specification defines a profile for issuing OAuth 2.0 access tokens in JSON Web Token (JWT) format. Authorization servers and resource servers from different vendors can leverage this profile to issue and consume access tokens in an interoperable manner. OAuth 2.0 Demonstrating Proof of Possession (DPoP) This document describes a mechanism for sender-constraining OAuth 2.0 tokens via a proof-of-possession mechanism on the application level. This mechanism allows for the detection of replay attacks with access and refresh tokens. OAuth Actor Profile for Delegation Independent OAuth deployments increasingly involve agents and workloads acting on behalf of human users across organizational boundaries. Existing specifications provide relevant building blocks (notably the act claim from RFC 8693 Token Exchange) but do not define a consistent profile for representing delegated actor relationships across JWT assertion grants (RFC 7523), JWT access tokens (RFC 9068), and Transaction Tokens, nor for classifying actor entity types or signaling support between authorization servers and resource servers. The result is inconsistent actor representation and actor- representation interoperability gaps that force deployments to rely on proprietary conventions. This document defines the OAuth Actor Profile for Delegation. It specifies a common act claim structure extended with sub_profile for entity-type classification, processing rules for authorization servers and resource servers across the three token families and their Token Exchange inputs, and OAuth discovery metadata parameters for advertising actor-profile support. The profile applies uniformly across token types and integrates with existing sender-constraint mechanisms (DPoP, mTLS). It does not standardize the policies by which systems determine whether a given actor is permitted to act for a subject; those decisions remain deployment-specific. OAuth Client ID Metadata Document Okta This specification defines a mechanism through which an OAuth client can identify itself to authorization servers, without prior dynamic client registration or other existing registration. This is through the usage of a URL as a client_id in an OAuth flow, where the URL refers to a document containing the necessary client metadata, enabling the authorization server to fetch the metadata about the client as needed. OAuth 2.0 Entity Profiles Microsoft Corporation Microsoft Corporation Independent This specification introduces Entity Profiles as a mechanism to categorize OAuth 2.0 entities—clients and subjects—based on their operational context. Entity Profiles provide structured descriptors for the client initiating the OAuth flow and the subject represented in tokens. This document defines new JWT Claim names and metadata parameters for use in JWTs issued or consumed in OAuth flows, including but not limited to access tokens, ID tokens, JWT authorization grant assertions, and transaction tokens, as well as in token introspection responses, dynamic client registration, and Authorization Server metadata. It also defines vocabulary for classifying acting entities within delegation chains. OAuth SPIFFE Client Authentication Defakto Security Defakto Security NIST IBM Cisco Systems This specification profiles the Assertion Framework for OAuth 2.0 Client Authentication and Authorization Grants [RFC7521], the JWT Profile for OAuth 2.0 Client Authentication and Authorization Grants [RFC7523], and OAuth 2.0 Attestation-Based Client Authentication [I-D.draft-ietf-oauth-attestation-based-client-auth] to enable the use of SPIFFE Verifiable Identity Documents (SVIDs) as client credentials in OAuth 2.0. It defines how OAuth clients with SPIFFE credentials can authenticate to OAuth authorization servers using their JWT-SVIDs, WIT-SVIDs, or X.509-SVIDs without the need for client secrets. This approach enhances security by enabling seamless integration between SPIFFE-enabled workloads and OAuth authorization servers while eliminating the need to distribute and manage shared secrets such as static client secrets. Key words for use in RFCs to Indicate Requirement Levels In many standards track documents several words are used to signify the requirements in the specification. These words are often capitalized. This document defines these words as they should be interpreted in IETF documents. This document specifies an Internet Best Current Practices for the Internet Community, and requests discussion and suggestions for improvements. Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words RFC 2119 specifies common key words that may be used in protocol specifications. This document aims to reduce the ambiguity by clarifying that only UPPERCASE usage of the key words have the defined special meanings. Informative References An IETF URN Sub-Namespace for OAuth This document establishes an IETF URN Sub-namespace for use with OAuth-related specifications. This document is not an Internet Standards Track specification; it is published for informational purposes. OAuth 2.0 Token Revocation This document proposes an additional endpoint for OAuth authorization servers, which allows clients to notify the authorization server that a previously obtained refresh or access token is no longer needed. This allows the authorization server to clean up security credentials. A revocation request will invalidate the actual token and, if applicable, other tokens based on the same authorization grant. CFRG Elliptic Curve Diffie-Hellman (ECDH) and Signatures in JSON Object Signing and Encryption (JOSE) This document defines how to use the Diffie-Hellman algorithms "X25519" and "X448" as well as the signature algorithms "Ed25519" and "Ed448" from the IRTF CFRG elliptic curves work in JSON Object Signing and Encryption (JOSE). OAuth 2.0 Attestation-Based Client Authentication MATTR Bundesdruckerei SPRIND This specification defines an extension to the OAuth 2.0 protocol [RFC6749] that enables a client instance to include a key-bound attestation when interacting with an Authorization Server or Resource Server. This mechanism allows a client instance to prove its authenticity verified by a client attester without revealing its target audience to that attester. It may also serve as a mechanism for client authentication as per OAuth 2.0. Workload Identity in a Multi System Environment (WIMSE) Architecture CyberArk Zscaler University of Applied Sciences Bonn-Rhein-Sieg The increasing prevalence of cloud computing and micro service architectures has led to the rise of complex software functions being built and deployed as workloads, where a workload is defined as software executing for a specific purpose, potentially comprising one or more running instances. This document discusses an architecture for designing and standardizing protocols and payloads for conveying workload identity and security context information. WIMSE Workload Credentials Ping Identity CyberArk Defakto Security Intuit Zscaler The WIMSE architecture defines authentication and authorization for software workloads in a variety of runtime environments, from the most basic ones up to complex multi-service, multi-cloud, multi- tenant deployments. This document defines the credentials that workloads use to represent their identity. They can be used in various protocols to authenticate workloads to each other. To use these credentials, workloads must provide proof of possession of the associated private key material, which is covered in other documents. This document focuses on the credentials alone, independent of the proof-of- possession mechanism. SPIFFE: Secure Production Identity Framework For Everyone SPIFFE

Design Rationale This appendix records design choices that motivated the normative text.

Why not a client_instance identifier parameter? A new top-level client_instance identifier would have to flow through the authorization request, the token request, introspection, the access token, and several existing extensions. Each is a separate specification touch-point and a deployment cliff. The validated instance identity surfaces in the issued access token's act.sub (delegation) or top-level sub (self-acting) per , which is the only place a resource server needs it; downstream profiles consume that representation rather than a parallel identifier.

Why a dedicated client_instance_assertion request parameter? This profile defines a dedicated request parameter for presenting a Client Instance Assertion on the four non-token-exchange grants listed in , rather than reusing 's actor_token / actor_token_type parameters on those grants. defines actor_token and actor_token_type only on the token-exchange grant; this profile uses actor_token only on that grant (). The reasons for a dedicated parameter on the other grants: First, the parameter name matches what it carries. A request body with client_instance_assertion=... declares its purpose; a request with actor_token=...&actor_token_type=urn:...:client-instance-jwt requires the AS to inspect the type URN to recognize the same artifact. The dedicated parameter is more discoverable and easier to implement against. Second, the self-acting case (notably client_credentials) does not fit 's "actor" framing, in which the actor is a party distinct from the subject. In this profile's self-acting case the instance is the subject (top-level sub) rather than the actor (act); see . Naming the parameter actor_token for that case would require readers to mentally translate "actor token" to "validated instance identity assertion." The dedicated parameter removes that translation; the grant determines whether the result is delegation or self-acting. Third, the dedicated parameter parallels the wire conventions of , which uses purpose-named headers (OAuth-Client-Attestation and -PoP) rather than a typed general-purpose carrier. The two specifications use purpose-named wire artifacts that an AS can dispatch by parameter name rather than by URN inspection. The URN urn:ietf:params:oauth:token-type:client-instance-jwt remains registered to identify the assertion when carried as actor_token on a token-exchange grant per ; see . The two parameter names are wire-syntax siblings carrying the same assertion under identical validation rules.

Why client metadata as the trust anchor for instance issuers? The trust relationship between a client and its instance issuers is published in the client's registered metadata (either in a CIMD document or in the AS's registration store). This keeps the trust relationship auditable alongside other client metadata and reuses existing freshness, caching, and key-rotation mechanisms. Locating it elsewhere (in AS-side static configuration unrelated to the client, or in a separate trust-relationship registry) would have fragmented the surface and reduced the auditability of who trusts whom.

Why a token_endpoint_auth_method rather than a client_assertion_type? models its workload-identity-as-client-auth mechanism as a client_assertion_type. The natural question is why does not. The two cases differ in what the JWT names. A SPIFFE JWT-SVID presented as a client_assertion under names the client (its sub is the spiffe_id of the workload acting as the client). The client-metadata listing of spiffe_id, including the permitted "/*" path-segment wildcard, turns the SVID into a credential for the client. There is no separate notion of "instance" on the wire. A client instance assertion under this profile names the instance: its sub is the instance identifier and its client_id claim names the client. The same JWT is required to do double duty only when the client chooses token_endpoint_auth_method = client_instance_assertion; in every other auth method, a separate client credential authenticates the client and the instance assertion names the instance. Modeling the dual-use case as a client_assertion_type would have required either (a) inventing a second token type identical to the Client Instance Assertion to serve as the client assertion, doubling the wire surface, or (b) overloading client_assertion_type with the actor-token URN, which conflicts with that URN's role on token-exchange grants. Modeling it as a token_endpoint_auth_method captures what is actually happening, namely that the AS authenticates the client implicitly from its client-metadata endorsement of the instance assertion's issuer, while keeping client_assertion and client_instance_assertion semantically distinct.

Worked Examples This appendix gives end-to-end worked examples for each grant type that interacts with this profile. Examples are non-normative and omit unrelated headers or grant-specific details that do not affect actor processing. Decoded assertion blocks show only the JWT payload; re-minted Client Instance Assertions also carry a JWS protected header with typ set to client-instance+jwt per (see the full example in ). Timestamps and lifetimes in these examples are illustrative and do not override the lifetime guidance in and . The examples use a CIMD-style client_id for clarity; the same flows apply identically to static-registration deployments (), where the client_id is opaque or AS-assigned and the metadata is read from the AS's registration store rather than dereferenced. The examples share a common deployment:

OAuth client: https://app.example.com/agent
Client metadata declares one instance issuer https://workload.app.example.com (subject_syntax "uri", signing_alg_values_supported containing ES256).
AS: https://as.example.com.
Resource server: https://api.example.com.
All access tokens are DPoP-bound; the instance assertion's cnf carries the instance's DPoP key thumbprint.

Authorization Code with User Delegation Alice authorizes the agent (client) at the AS through a standard authorization_code flow. The agent runs as instance inst-01, which presents an instance assertion at the token endpoint to identify itself. Authorization request from the agent (abridged). dpop_jkt carries the thumbprint of inst-01's DPoP key (matching the instance assertion's cnf.jkt) to establish key-bound continuity per : The AS authenticates Alice, displays consent for the client (per consent applies to the client as a whole, not per-instance), binds the authorization code to the dpop_jkt value per , and redirects with an authorization_code. Token request from instance inst-01: grant_type=authorization_code &code=SplxlOBeZQQYbYS6WxSbIA &redirect_uri=https%3A%2F%2Fapp.example.com%2Fcb &client_id=https%3A%2F%2Fapp.example.com%2Fagent &code_verifier=... &client_assertion_type= urn%3Aietf%3Aparams%3Aoauth%3Aclient-assertion-type%3Ajwt-bearer &client_assertion=eyJhbGciOiJFUzI1NiIs... (private_key_jwt) &client_instance_assertion=eyJhbGciOiJFUzI1NiIs... ]]> Decoded client_instance_assertion: AS validation:

Authenticates the client (private_key_jwt).
Recognizes the client_instance_assertion parameter for this grant.
Resolves CIMD for the agent.
Locates the descriptor by matching iss.
Verifies signature via descriptor's jwks_uri.
Validates JWT claims.
Verifies the assertion's client_id == request client_id and applies the (iss, jti) replay check.
Applies AS-local maximum delegation depth.
: the request's client_id matches the client_id that received the code, and the DPoP proof's thumbprint matches both the code's dpop_jkt and the assertion's cnf.jkt.
Classifies as delegation; issues sender-constrained access token.

Issued access token: The RS authorizes the request based on (alice, inst-01) per .

Client Credentials (Self-Acting) A workload makes a client_credentials request with no human user involved. Token request: grant_type=client_credentials &scope=repo.read &client_id=https%3A%2F%2Fapp.example.com%2Fagent &client_assertion_type= urn%3Aietf%3Aparams%3Aoauth%3Aclient-assertion-type%3Ajwt-bearer &client_assertion=eyJhbGciOiJFUzI1NiIs... &client_instance_assertion=eyJhbGciOiJFUzI1NiIs... ]]> Decoded client_instance_assertion: Issued access token (self-acting; no user, instance is the principal): The RS treats sub as the workload identity (not a human user) per .

Token Exchange with Prior Delegation Chain (Agent Spawns Sub-Agent) A parent agent's user-delegated access token is exchanged at the AS for a sub-agent's downstream-resource-scoped token. The sub-agent runtime presents an instance assertion; the inbound subject_token already carries an act chain naming the parent agent. The subject_token was issued by upstream.example.com, which as.example.com trusts as a token issuer under local policy. Inbound subject_token (decoded; issued earlier when a parent agent "agent-orchestrator-alpha" obtained access on the user's behalf): Token request: grant_type=urn%3Aietf%3Aparams%3Aoauth%3Agrant-type%3Atoken-exchange &audience=https%3A%2F%2Fapi.example.com &subject_token=eyJhbGciOiJFUzI1NiIs... &subject_token_type= urn%3Aietf%3Aparams%3Aoauth%3Atoken-type%3Aaccess_token &client_id=https%3A%2F%2Fapp.example.com%2Fagent &client_assertion_type= urn%3Aietf%3Aparams%3Aoauth%3Aclient-assertion-type%3Ajwt-bearer &client_assertion=eyJhbGciOiJFUzI1NiIs... &actor_token=eyJhbGciOiJFUzI1NiIs... &actor_token_type= urn%3Aietf%3Aparams%3Aoauth%3Atoken-type%3Aclient-instance-jwt ]]> The actor_token carries the Client Instance Assertion for the sub-agent runtime inst-03, with no act of its own (per , an actor_token MUST NOT carry act). Decoded actor_token: Chain construction per :

Outermost: the validated Client Instance Assertion (sub-agent inst-03).
Inner: the subject_token's act chain, preserved (agent-orchestrator-alpha).

Issued access token: Resulting chain depth is 2, well within typical AS-local maximums. The chain reads outward-in as: sub-agent inst-03 acted on behalf of the parent agent agent-orchestrator-alpha, which acted on behalf of user alice. Each act layer names a distinct runtime; resource servers and audit pipelines can attribute the request to the specific sub-agent that performed it.

Refresh with a Fresh Instance Assertion Some time after the authorization-code example (), instance inst-01's original access token approaches expiry. The original Client Instance Assertion has also expired, so the client mints a fresh assertion (same iss, sub, and cnf) and presents it on the refresh request. The refresh token is sender-constrained to the same DPoP key per . Token request: grant_type=refresh_token &refresh_token=tGzv3JOkF0XG5Qx2TlKWIA &client_id=https%3A%2F%2Fapp.example.com%2Fagent &client_assertion_type= urn%3Aietf%3Aparams%3Aoauth%3Aclient-assertion-type%3Ajwt-bearer &client_assertion=eyJhbGciOiJFUzI1NiIs... (private_key_jwt) &client_instance_assertion=eyJhbGciOiJFUzI1NiIs... ]]> Decoded fresh client_instance_assertion: AS validation per :

Authenticate the client (private_key_jwt).
Verify the refresh token belongs to this client and is bound to the presented DPoP key.
Validate the fresh assertion per ; in particular verify that its (iss, sub) match those recorded at original issuance and its cnf matches the refresh-token binding key.
Inherit the original grant's classification (delegation), per .
Issue a new sender-constrained access token with the same act chain.

The refreshed access token is identical in shape to the original (), with updated iat and exp. A second variant of this flow omits the fresh assertion entirely: grant_type=refresh_token &refresh_token=tGzv3JOkF0XG5Qx2TlKWIA &client_id=https%3A%2F%2Fapp.example.com%2Fagent &client_assertion_type= urn%3Aietf%3Aparams%3Aoauth%3Aclient-assertion-type%3Ajwt-bearer &client_assertion=eyJhbGciOiJFUzI1NiIs... ]]> Without a fresh client_instance_assertion, the AS still inherits the original delegation classification and the original act chain (sender-constrained to the same DPoP key, since the refresh token was bound to it). This is the common case when instance identity has not changed since issuance; the issued access token's act claim is unchanged from (with updated iat and exp). Refresh does not introduce a new instance identity.

Client Authenticated via Instance Assertion A workload's OAuth client is registered with token_endpoint_auth_method = client_instance_assertion (). The workload presents no separate client credential; the Client Instance Assertion authenticates both the client (through the registered endorsement of its issuer) and the instance. Token request: grant_type=client_credentials &scope=repo.write &client_id=https%3A%2F%2Fapp.example.com%2Fagent &client_instance_assertion=eyJhbGciOiJFUzI1NiIs... ]]> Decoded client_instance_assertion: AS validation per :

Resolve the registered metadata for client_id (token_endpoint_auth_method is client_instance_assertion, so no separate client credential is expected and none is accepted on this request).
Validate the assertion using the descriptor lookup, signature verification, and JWT claim validation rules in (token-type matching does not apply on client_credentials).
Verify the assertion's client_id claim equals the request's client_id parameter.
Verify the assertion contains a cnf claim.
Verify possession of the cnf key against the DPoP proof per .
Apply the replay check ().
Treat the client as authenticated. The validated assertion represents the instance for .

Any failure in steps 1-6 above is returned as invalid_client per ; the assertion is the sole credential, so failures that would otherwise be returned as invalid_grant or invalid_request are reclassified. Self-acting access token issued by the AS:

SPIFFE Workload (Self-Acting, JWT-SVID Reuse with X.509-SVID Binding) A SPIFFE workload makes a client_credentials request. The workload holds both a JWT-SVID and an X.509-SVID issued by its SPIFFE trust domain. The workload presents the JWT-SVID directly as the client_instance_assertion under raw JWT-SVID compatibility () and presents the X.509-SVID at TLS, so the certificate supplies the sender-constraint binding (). Because the presented assertion is a raw JWT-SVID, its JOSE header follows SPIFFE conventions and is not required to carry typ=client-instance+jwt. The descriptor uses subject_syntax="spiffe" and the access token is mTLS-bound, in place of the preamble's URI-syntax descriptor and DPoP binding. Instance issuer descriptor: Token request (TLS presents the workload's X.509-SVID; client authentication uses X.509 mode, so client_assertion is omitted): Decoded client_instance_assertion: The AS verifies the JWT-SVID signature against the trust bundle, that sub falls under the descriptor's spiffe_id prefix, that the TLS-presented X.509-SVID has SAN URI exactly equal to the JWT-SVID's sub, and binds the issued access token via cnf.x5t#S256 set to the certificate's SHA-256 thumbprint. Issued access token (self-acting; TTL kept short relative to the X.509-SVID rotation cycle): At the resource server, the workload connects over mTLS with the same X.509-SVID; the RS verifies that the certificate thumbprint matches the access token's cnf.x5t#S256.

Document History RFC EDITOR: please remove this section before publication.

-01

Replaced the generic actor-token-grant-extension framing with a dedicated client_instance_assertion request parameter on the four non-token-exchange grants (). Token-exchange continues to use actor_token.
Added §spiffe-compatibility (raw JWT-SVID presentation), the optional client_instance_assertion token-endpoint authentication method, §security-multi-tenancy, and the "OAuth Client Instance Subject Syntaxes" IANA sub-registry.
Specified the validation procedure () including octet-equality comparisons and PoP-before-replay ordering, MTI signing (ES256), and access-token surfacing aligned with .
Added worked examples covering all five grants, the auth method, refresh, and SPIFFE JWT-SVID reuse.
Retitled to "OAuth 2.0 Client Instance Assertion".

-00

Initial version.

Acknowledgments The author thanks participants in the OAuth Working Group for discussions on client instance identity, workload identity, and actor-based delegation that informed this document.