Self-Issued Consulting

michael_b_jones@hotmail.com https://self-issued.info/

Ping Identity

bcampbell@pingidentity.com

Disney

charliemortimore@gmail.com

Okta

panva.ip@gmail.com

Security OAuth Working Group OAuth JWT Assertion Token Security Token This document updates RFC7521, RFC7522, RFC7523 and RFC9126 with respect to the treatment of audience values in OAuth 2.0 Client Assertion Authentication and Assertion-based Authorization Grants to address a security vulnerability identified in the previous requirements for those audience values in multiple OAuth 2.0 specifications.

Multiple OAuth 2.0 specifications use tokens (also known as "assertions") that are sent to authorization servers. These tokens contain audience value(s) that identify the intended recipient(s). When the token is a JSON Web Token (JWT) , the audience value(s) are contained in the aud (audience) claim. When performing a security analysis of a pre-final version of the OpenID Federation specification , University of Stuttgart security researchers Pedram Hosseyni, Dr. Ralf Küsters, and Tim Würtele discovered a vulnerability affecting multiple OpenID and OAuth specifications caused by ambiguities in the audience values of tokens sent to authorization servers. The vulnerability was disclosed to the OAuth working group in an interim meeting in January 2025 called for that purpose, including providing a description of the vulnerability . A paper they published describing the attack is . This specification updates the affected OAuth specifications to address the security vulnerability identified. Specifically, it eliminates former choices in the audience values of tokens sent to OAuth 2.0 authorization servers. section 3.3 requires that the client having retrieved the metadata validates the returned issuer value. Other endpoint values of the metadata are not directly validated and, if used as audience when sent to a different endpoint, can open an attack vector. A general description of the updates made is to require that the issuer identifier URL of the authorization server, as defined in , be used as the sole value of the audience of the JWT client authentication assertion. Furthermore, the authorization server rejects any JWT client authentication assertion that does not contain its own issuer identifier as the sole audience value. An explicit type for each affected kind of token, as defined in , is also defined to facilitate distinguishing between tokens produced in accordance with specifications published prior to these updates and those incorporating them. Specific updates made to each affected specification follow.

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.

All terms are as defined in the following specifications: "The OAuth 2.0 Authorization Framework" , "Assertion Framework for OAuth 2.0 Client Authentication and Authorization Grants" , and "JSON Web Token (JWT)" .

This section updates "Assertion Framework for OAuth 2.0 Client Authentication and Authorization Grants" to tighten its audience requirements. The description of the Audience parameter in Section 5.1 of (Assertion Metamodel) is replaced by: A value that identifies the party intended to process the assertion. The issuer identifier of the authorization server, as defined in , can be used to indicate that the authorization server is a valid intended audience of the assertion.

This section updates "Security Assertion Markup Language (SAML) 2.0 Profile for OAuth 2.0 Client Authentication and Authorization Grants" . It tightens its audience requirements for SAML authorization grants and it deprecates the use of SAML assertions for client authentication. The text and example in Section 2.2 of (Using SAML Assertions for Client Authentication) is replaced by: SAML Bearer Assertions MUST NOT be used for client authentication for any new applications. Should any applications already be doing this in the manner described in Section 2.2 of , it is left to the discretion of their implementers and deployers whether to migrate away from this feature and/or potentially tighten the audience values used in a manner parallel to the changes being made to RFC 7523 by this specification. The description of the Audience element in Item 2 of Section 3 of (Assertion Format and Processing Requirements) is replaced by: The Assertion MUST contain a <Conditions> element with an <AudienceRestriction> element with an <Audience> element that identifies the authorization server as the intended audience. The client is responsible for ensuring that the audience of the Assertion is appropriate for the authorization server to which it is sent. This MAY be the issuer identifier of the authorization server, the token endpoint URL of the authorization server, or a SAML Entity ID. Section 2.5.1.4 of "Assertions and Protocols for the OASIS Security Assertion Markup Language (SAML) V2.0" defines the <AudienceRestriction> and <Audience> elements. The authorization server MUST reject any assertion that does not contain its own identity as the intended audience.

This section updates "JSON Web Token (JWT) Profile for OAuth 2.0 Client Authentication and Authorization Grants" to tighten its audience requirements. In Section 3 of (JWT Format and Processing Requirements), Item 3, which describes the audience value, is replaced by: The JWT MUST contain an aud (audience) claim containing a value that identifies the authorization server as the intended audience. Two cases are differentiated: For the authorization grant, the client is responsible for ensuring that the audience of the JWT assertion is appropriate for the authorization server to which it is sent. An authorization server MAY be identified by either its issuer identifier or its token endpoint URL. For client authentication, the aud (audience) claim value MUST use the issuer identifier of the authorization server as its sole value. The authorization server MUST have an issuer identifier to be used with this specification. Unlike the aud value specified in , there MUST be no value other than the issuer identifier of the intended authorization server used as the audience of the JWT; this includes that the token endpoint URL of the authorization server MUST NOT be used as an audience value. The authorization server MUST reject any JWT that does not contain its issuer identifier as its sole audience value. In the absence of an application profile specifying otherwise, applications MUST compare the audience values using the Simple String Comparison method defined in Section 6.2.1 of RFC 3986 . In Section 3.2 of (Client Authentication Processing), the following requirement is added: Client authentication JWTs SHOULD be explicitly typed by using the typ header parameter value client-authentication+jwt or another more specific explicit type value defined by a specification profiling this specification. The introduction of strong typing for JWTs (using explicit typ values) serves as a signal to distinguish between tokens produced in accordance with specifications published prior to these updates and those incorporating them. However, the primary security protection comes from the tightened audience requirements. Since strong typing alone does not prevent the attacks described in and , the use of explicit typing is RECOMMENDED for clients, enabling them to signal their intention of sending a JWT conforming to the requirements herein. However, it is NOT RECOMMENDED for servers to reject JWTs that do not have explicit types, as doing so would cause interoperability issues with clients that already conform to the tightened audience requirements but have not yet adopted explicit typing. This approach balances security signaling with practical deployment considerations, avoiding disruption to client deployments that already conform to the tightened audience requirements but have not yet adopted explicit typing. In Section 4 of (Authorization Grant Example), the sentence: The intended audience of the JWT is https://jwt-rp.example.net, which is an identifier with which the authorization server identifies itself. is replaced by: The intended audience of the JWT is https://authz.example.net, which is the authorization server's issuer identifier.

In the same section, the JWT Claims Set example is replaced by:

In the list of agreements required by participants in Section 5 of (Interoperability Considerations), an agreement on "audience identifiers" is no longer needed for client authentication JWTs. The additional example in the following subsection is added after Section 4 of

The following example illustrates what a client authentication JWT and token request using it would look like. The example shows a JWT issued and signed by the OAuth client identified as https://client.example/. The intended audience of the JWT is https://authz.example.net, which is the authorization server's issuer identifier. The JWT is sent as part of a token request to the authorization server's token endpoint at https://authz.example.net/token.oauth2.

Below is an example JSON object that could be encoded to produce the JWT Claims Set for a client authentication JWT:

The following example JSON object, used as the header parameters of a JWT, declares that the JWT is a client authentication JWT, is signed with the Elliptic Curve Digital Signature Algorithm (ECDSA) P-256 with SHA-256, and was signed with a key identified by the kid value 16.

To present the JWT with the claims and header parameters shown above as part of an access token request, for example, the client might make the following HTTPS request (with extra line breaks for display purposes only):

This section updates "OAuth 2.0 Pushed Authorization Requests" to tighten its audience requirements. The last paragraph in Section 2 of (Pushed Authorization Request Endpoint), which describes the audience value is replaced by: This update resolves the potential ambiguity regarding the appropriate audience value to use when employing JWT client assertion-based authentication (as defined in Section 2.2 of and as updated by with the private_key_jwt or client_secret_jwt authentication method names per Section 9 of ) that was described in . To address that ambiguity, the issuer identifier URL of the authorization server according to MUST be used as the sole value of the audience. The authorization server MUST reject any such JWT that does not contain its own issuer identifier as the sole audience value. Client authentication JWTs SHOULD be explicitly typed by using the typ header parameter value client-authentication+jwt or another more specific explicit type value defined by a specification profiling this specification. The introduction of strong typing for JWTs (using explicit typ values) serves as a signal to distinguish between tokens produced in accordance with specifications published prior to these updates and those incorporating them. However, the primary security protection comes from the tightened audience requirements. Since strong typing alone does not prevent the attacks described in and , the use of explicit typing is RECOMMENDED for clients, enabling them to signal their intention of sending a JWT conforming to the requirements herein. However, it is NOT RECOMMENDED for servers to reject JWTs that do not have explicit types, as doing so would cause interoperability issues with clients that already conform to the tightened audience requirements but have not yet adopted explicit typing. This approach balances security signaling with practical deployment considerations, avoiding disruption to client deployments that already conform to the tightened audience requirements but have not yet adopted explicit typing.

The security considerations described within the following specifications are all applicable to this document: "Assertion Framework for OAuth 2.0 Client Authentication and Authorization Grants" , "Security Assertion Markup Language (SAML) 2.0 Profile for OAuth 2.0 Client Authentication and Authorization Grants" , "JSON Web Token (JWT) Profile for OAuth 2.0 Client Authentication and Authorization Grants" , "OAuth 2.0 Pushed Authorization Requests" , "The OAuth 2.0 Authorization Framework" , and "JSON Web Token (JWT)" . This specification tightens token audience requirements to prevent attacks that could result from exploiting audience ambiguities previously allowed by , , , and . These attacks are described in and .

This section registers the following media type in the "Media Types" registry in the manner described in .

Type name: application Subtype name: client-authentication+jwt Required parameters: n/a Optional parameters: n/a Encoding considerations: binary; A client authentication JWT is a JWT; JWT values are encoded as a series of base64url-encoded values (some of which may be the empty string) separated by period ('.') characters. Security considerations: See of this specification Interoperability considerations: n/a Published specification: of this specification Applications that use this media type: Applications that use this specification 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: IETF OAuth Working Group <oauth@ietf.org> Intended usage: COMMON Restrictions on usage: none Author: Michael B. Jones, michael_b_jones@hotmail.com Change controller: IETF Provisional registration? No

This section updates entries in the "OAuth Token Endpoint Authentication Methods" registry of

Token Endpoint Authentication Method Name: private_key_jwt Change Controller: IETF Reference: Section 9 of , [[this specification]] Token Endpoint Authentication Method Name: client_secret_jwt Change Controller: IETF Reference: Section 9 of , [[this specification]]

This section requests updates to the following entries in the "OAuth URI" registry of to add [[this specification]] as an additional reference. urn:ietf:params:oauth:grant-type:jwt-bearer urn:ietf:params:oauth:client-assertion-type:jwt-bearer urn:ietf:params:oauth:grant-type:saml2-bearer urn:ietf:params:oauth:client-assertion-type:saml2-bearer

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. Internet2

cantor.2@osu.edu

Nokia

John.Kemp@nokia.com

RSA Security

rphilpott@rsasecurity.com

Sun Microsystems

eve.maler@sun.com

NAT.Consulting Yubico Self-Issued Consulting Google Disney University of Stuttgart University of Stuttgart University of Stuttgart OpenID Foundation independent Self-Issued Consulting Sikt Yubico independent Connect2id IANA IANA

[[ to be removed by the RFC Editor before publication as an RFC ]] -07 Address comments from AD review by Deb Cooley: RFCs to be updated are now in the Abstract (per idnits's nits). Improved clarity of the audience description in Section 1. Removed parenthetical remark in Section 3 about SAML client authentication usage. Changed IANA media type registration contact to the OAuth Working Group. -06 Applied shepherd review comments by Rifaat Shekh-Yusef. -05 Applied editorial suggestions by Axel Nennker. -04 Applied editorial suggestions by Jamshid Khosravian. -03 Update OAuth Token Endpoint Authentication Methods IANA entries with reference to this specification Relaxed client requirement to use strong typed JWTs. SHOULD instead of MUST. Do not restrict the "aud" claim's type. Allow it to be an array with a single member. Advise the client to ensure that the audience of an assertion authorization grant makes sense with respect to where it’s being sent. Updates to the abstract and introduction to (hopefully) better reflect the more targeted scope of the work. Remove JWTs for Client Authentication example replacement (not worth it for including typ in the encoded JWT header). Add request to update existing OAuth URI registrations to add reference to this specification for the four relevant URNs. Fixup the new Client Authentication JWT Example. -02 Added Filip Skokan as an author. Applied Brian Campbell's suggestions made at IETF 122. Specifically: Focused RFC 7523 updates on JWT client authentication case. Described client responsibilities for the audience value of authorization grants. No longer mandate that the audience for authorization grants be the issuer identifier, so as to make a minimum of breaking changes. Deprecated the use of SAML assertions for client authentication. -01 Reworked to make updates to RFC 7523, rather than replacing it. Removed updates to RFC 9101. Added reference to the University of Stuttgart paper . -00 Initial working group draft, replacing draft-jones-oauth-rfc7523bis-00.

We would like to acknowledge the contributions of the following people to this specification: Brock Allen, John Bradley, Ralph Bragg, Joseph Heenan, Pedram Hosseyni, Pieter Kasselman, Jamshid Khosravian, Ralf Küsters, Martin Lindström, Axel Nennker, Aaron Parecki, Dean H. Saxe, Arndt Schwenkschuster, Rifaat Shekh-Yusef, and Tim Würtele.