Updates to OAuth 2.0 Security Best Current Practice

Attacks and Mitigations This section gives a detailed description of new attacks on OAuth implementations, along with potential countermeasures. Attacks and mitigations already covered in are not listed here, except where clarifications or new recommendations are made.

Audience Injection Attacks When using signature-based client authentication methods such as private_key_jwt as defined in or signed JWTs as defined in and , a malicious authorization server may be able to obtain and use a client's authentication credential, enabling them to impersonate a client towards another honest authorization server.

Attack Description The descriptions here follow , where additional details of the attack are laid out. Audience injection attacks require a client to interact with at least two authorization servers, one of which is malicious, and to authenticate to both with a signature-based authentication method using the same key pair. The following description uses the jwt-bearer client authentication from , see for other affected client authentication methods. Furthermore, the client needs to be willing to authenticate at an endpoint other than the token endpoint at the attacker authorization server (see ).

Core Attack Steps In the following, let H-AS be an honest authorization server and let A-AS be an attacker-controlled authorization server. Assume that the authorization servers publish the following URIs for their token endpoints, for example via mechanisms such as authorization server metadata or OpenID Discovery . The exact publication mechanism is not relevant, as audience injection attacks are also possible on clients with manually configured authorization server metadata. Excerpt from H-AS' metadata: Excerpt from A-AS' metadata: Therefore, the attacker authorization server claims to use the honest authorization server's token endpoint. Note that the attacker authorization server does not control this endpoint. The attack then commences as follows:

Client registers at H-AS, and gets assigned a client ID cid.
Client registers at A-AS, and gets assigned the same client ID cid. Note that the client ID is not a secret ().

Now, whenever the client creates a client assertion for authentication to A-AS, the assertion consists of a JSON Web Token (JWT) that is signed by the client and contains, among others, the following claims: Due to the malicious use of H-AS' token endpoint in A-AS' authorization server metadata, the aud claim contains H-AS' token endpoint (see ). Recall that both A-AS and H-AS registered the client with client ID cid, and that the client uses the same key pair for authentication at both authorization servers. Hence, this client assertion is a valid authentication credential for the client at H-AS. Once the attacker obtained such a client assertion, it can impersonate the client towards H-AS. This enables multiple attacks. For example, the attacker may obtain access tokens via a client credentials grant. Another example is the attacker initiating a regular authorization code grant where a victim user grants access to the honest client at H-AS, but the access token ends up with the attacker (since the attacker fully impersonates the client in this case, mechanisms like DPoP cannot protect against the attack). Further attack scenarios and additional details are given in .

Token Endpoint as Client Assertion Audience The use of the token endpoint to identify the authorization server as a client assertion's audience (even for client assertions that are not sent to the token endpoint) is encouraged, or at least allowed by many standards, including , , , , , , and all standards referencing the IANA registry for OAuth Token Endpoint Authentication Methods for available client authentication methods.

Endpoints Requiring Client Authentication As mentioned above, the attack is only possible if the client authenticates to an endpoint other than the token endpoint at A-AS. This is because if the client sends a token request to A-AS, it will use A-AS' token endpoint as published by A-AS and hence, send the token request to H-AS, i.e., the attacker cannot obtain the client assertion. As detailed in , the attack is confirmed to be possible if the client authenticates with such client assertions at the following endpoints of A-AS:

Pushed Authorization Endpoint (see )
Token Revocation Endpoint (see )
CIBA Backchannel Authentication Endpoint (see )
Device Authorization Endpoint (see )

Note that this list of examples is not exhaustive. Hence, any client that might authenticate at any endpoint other than the token endpoint SHOULD employ countermeasures as described in .

Affected Client Authentication Methods The same attacks are possible for the private_key_jwt client authentication method defined in , as well as instantiations of client authentication assertions defined in , including the SAML assertions defined in . Furthermore, a similar attack is possible for jwt-bearer authorization grants as defined in , albeit under additional assumptions (see for details).

Countermeasures At its core, audience injection attacks exploit the fact that, from the client's point of view, an authorization server's token endpoint is a mostly opaque value and does not uniquely identify an authorization server. Therefore, an attacker authorization server may claim any URI as its token endpoint, including, for example, an honest authorization server's issuer identifier. Hence, as long as a client uses the token endpoint as an audience value when authenticating to the attacker authorization server, audience injection attacks are possible. Therefore, audience injection attacks need to be prevented by the client. Clients that interact with more than one authorization server and authenticate with signature-based client authentication methods MUST employ one of the following countermeasures, unless audience injection attacks are mitigated by other means, such as using fresh key material for each authorization server. It is RECOMMENDED to prefer the countermeasure described in . The countermeasures described below mandate the use of single audience value (as opposed to multiple audiences in an array). This is because allows the receiver of an audience-restricted JWT to accept the JWT even if the receiver identifies with only one of the values in such an array. Since the countermeasures rely on the client using an unambiguous audience value, there is no value in including additional ones (that would need to be unambiguous as well).

Authorization Server Issuer Identifier Clients MUST use the authorization server's issuer identifier as defined in / as the sole audience value in client assertions. Clients MUST retrieve and validate this value as described in /Section 4.3 of . For jwt-bearer client assertions as defined by , this mechanism is also described in . Note that "issuer identifier" here does not refer to the term "issuer" as defined in , but to the issuer identifier defined in and . In particular, the issuer identifier is not just "an abstract identifier for the combination of the authorization endpoint and token endpoint".

Exact Target Endpoint URI Clients MUST use the exact endpoint URI to which a client assertion is sent as that client assertion's sole audience value. This countermeasure can be used for authorization servers that do not use authorization server metadata or OpenID Discovery .

Implementation Considerations Technically, the countermeasures described in and do not imply any normative changes to the authorization server: requires the authorization server to only accept a (client assertion) JWT if the authorization server can identify itself with (at least one of the elements in) the JWT's audience value. Client assertions produced by a client implementing one of these countermeasures meet this condition. However, some existing authorization server implementations only accept their token endpoint as the audience value in client assertions, including for authentication at endpoints other than the token endpoint. Clients interacting with such authorization servers MUST employ alternative countermeasures. In this case, clients SHOULD use one of the following countermeasures, in decreasing order of preference:

Use distinct authentication key material with each authorization server.
Ensure the use of unique client identifiers across all authorization servers (note that this might not be feasible in some ecosystems, for example, in connection with OpenID Federation ).
Ensure uniqueness of all (client identifier, token endpoint) pairs and use the token endpoint as the sole audience value in all client assertions.

While an authorization server that already accepts (among other values) its issuer identifier or the exact endpoint as client assertion audience values does not need to change anything, such an authorization server MAY still decide to restrict acceptance to its issuer identifier () or the endpoint that received the client assertion () as an audience value, for example, to force its clients to adopt the respective countermeasure.

Cross-toolkit OAuth Account Takeover It is increasingly common for a single OAuth client to access protected resources of multiple tools on behalf of users. Each set of tools, referred to as a toolkit in the following, is mapped to an OAuth provider configuration, which includes at least the authorization server (AS) endpoints and the client registration. A successful OAuth connection is established when the OAuth client obtains an access token for a tool based on its corresponding OAuth provider configuration. The client can then use the access token to access the user's protected resources at the tool's resource server (RS). Multiple OAuth connections can be linked to some form of user identity in the following non-normative example deployment scenarios:

Application Integration: The OAuth connections made with different toolkits are linked to an application's user account or session (e.g., represented by an application's user identifier or an anonymous session identifier). This is common where a user authorizes an application (e.g., a cloud platform or an agentic AI service) to orchestrate multiple tools, some of which together with their OAuth providers can be contributed by the public.
Multi-tenant OAuth-as-a-Service (also known as Token Vault): In cases where OAuth responsibilities of a client are managed by a multi-tenant OAuth-as-a-Service provider, a successful OAuth connection is linked to a tenant's user identifier in addition to the tenant identifier. This is a generalization of the last deployment scenario, where an application using this OAuth-as-a-Service becomes a tenant. A tenant can usually choose some off-the-shelf toolkits using (partially) completed OAuth providers, or add their own toolkits with custom OAuth providers to support the tenant's service.

When controlled by an attacker, the open configurations of OAuth providers pose a new threat to this centralized OAuth client design. If the client fails to properly identify, track, and isolate the OAuth connection context (representing a combination of OAuth provider, toolkit, and tenant) in use during an authorization flow, an attacker can exploit this to mount Cross-toolkit OAuth Account Takeover (COAT) attacks (see and ). The COAT attacker uses a malicious toolkit to steal a victim's authorization code issued by an honest OAuth provider of an honest toolkit, and applies authorization code injection (as defined in ) against a new OAuth connection with the attacker's identity. This results in a compromised OAuth connection between the attacker's application identity and the victim's toolkit access. The impact is equivalent to an account takeover: the attacker can operate the honest toolkit with the victim's account (hijacked either under the same application, or even across tenants that share a vulnerable OAuth-as-a-Service).

Attack Description Preconditions: It is assumed that

the implicit or authorization code grant is used with multiple OAuth connection contexts, of which one is considered "honest" (H-Toolkit using H-AuthProvider with H-AS) and one is operated by the attacker (A-Toolkit using A-AuthProvider with A-AS), and
the client stores the connection context chosen by the user in a session bound to the user's browser, and
the authorization servers properly check the redirection URI by enforcing exact redirection URI matching (otherwise, see Cross Social-Network Request Forgery in for details).

In the following, it is further assumed that the client is registered with H-AS (URI: https://honest.as.example, client ID: 7ZGZldHQ) and with A-AS (URI: https://attacker.example, client ID: 666RVZJTA). Assume that the client issues the redirection URI https://client.com/honest-cb for the honest toolkit and https://client.com/attack-cb for the attacker-controlled toolkit. URLs shown in the following example are shortened for presentation to include only parameters relevant to the attack. Attack on the authorization code grant:

A victim user selects to start the grant using A-AS of A-Toolkit (e.g., by initiating a tool use on an agentic AI service).
The client stores in the user's session that the user has selected this OAuth connection context and redirects the user to A-AS's authorization endpoint with a Location header containing the URL https://attacker.example/authorize?response_type=code&client_id=666RVZJTA&state=[state] &redirect_uri=https%3A%2F%2Fclient.com%2Fattack-cb.
When the user's browser navigates to the A-AS, the attacker immediately redirects the browser to the authorization endpoint of H-AS. In the authorization request, the attacker uses an authorization request URL of H-AS and replaces the state with the one freshly received. Therefore, the browser receives a redirection with a Location header pointing to https://honest.as.example/authorize?response_type=code&client_id=7ZGZldHQ&state=[state] &redirect_uri=https%3A%2F%2Fclient.com%2Fhonest-cb.
Due to implicit or prior approvals, the user might not be prompted for re-consent. H-AS issues a code and sends it (via the browser) back with the state to the client.
Since the client still assumes that the code was issued by A-AS of A-Toolkit, as stored in the user's session (with state verified), it will try to redeem the code at A-AS's token endpoint.
The attacker therefore obtains the code and can either exchange the code for an access token (for public clients) or perform an authorization code injection attack as described in .

Note that merely issuing distinct redirection URIs per OAuth connection context, as used in this example, does not prevent the attack: in Step 5, the client fails to verify that the connection context incorporated in the redirection URI on which the authorization response was received (H-Toolkit) matches the connection context recorded in the user's session for this flow (A-Toolkit), as required in . The same vulnerability would also arise if the client had issued a single shared redirection URI (e.g., https://client.com/cb) for both toolkits. This Cross-toolkit OAuth Account Takeover (COAT) attack is a generalization of the Cross-app OAuth Account Takeover as defined in and the mix-up attack as defined in . This COAT attack exploits confusion between the OAuth connection contexts (i.e., combinations of OAuth provider, toolkit, and tenant) of a centralized client rather than being limited to confusion between two distinct authorization servers. Variants:

COAT under the OAuth-as-a-Service context: the attack above can be launched with a malicious tenant by adding a custom toolkit with an OAuth provider that targets an honest AS used by another tenant's toolkit.
Implicit Grant: In the implicit grant, the attacker receives an access token instead of the code in Step 4. The attacker's authorization server receives the access token when the client makes either a request to the A-AS userinfo endpoint (defined in ) or a request to the attacker's resource server (since the client believes it has completed the flow with A-AS).
Cross-toolkit OAuth Request Forgery (CORF): If clients do not store the selected OAuth connection context in the user's session, but in the redirection URI instead, attackers can mount an attack called Cross-toolkit OAuth Request Forgery (CORF). This results in a compromised OAuth connection between the victim's application identity and the attacker's toolkit access. The goal of this specific attack variant is not to obtain an authorization code or access token, but to force the client to use an attacker's authorization code or access token for H-AS. This Cross-toolkit OAuth Request Forgery attack is a generalization of the Cross-app OAuth Request Forgery as defined in and, when the OAuth connection context is limited to the AS, the Naïve RP Session Integrity Attack as detailed in Section 3.4 of .
OpenID Connect: Some variants can be used to attack OpenID Connect. In these attacks, the attacker misuses features of the OpenID Connect Discovery mechanism or replays access tokens or ID Tokens to conduct a mix-up attack. The attacks are described in detail in Appendix A of and Section 6 of ("Malicious Endpoints Attacks").

Countermeasures The client MUST use all variables in its supported OAuth connection context to form a connection context identifier that uniquely identifies each AS instance configured at the client. This identifier always includes the unique toolkit identifier. Additionally,

a client allowing each toolkit to use multiple OAuth providers, where one provider's AS may be compromised as assumed in , MUST also include the OAuth provider identifier;
a multi-tenant client MUST also include the tenant identifier, if the toolkit identifier is not globally unique.

Unless otherwise specified as follows, the following requirements apply: the client MUST issue a per-context distinct redirection URI that incorporates this unique connection context identifier. When initiating an authorization request, the client MUST store this identifier in the user's session. When an authorization response is received on the redirection endpoint, the client MUST also check that the context identifier from the distinct redirection URI matches the one in the user's session. If there is a mismatch, the client MUST abort the flow. Existing countermeasures for mix-up attacks () can serve as a replacement under the following conditions:

the client has entirely dropped support for the implicit grant, and
the OAuth provider specifies an AS not by individually configured AS endpoints but instead by an abstract issuer identifier (as defined in ) that represents the endpoints, and
the issuer identifier is used either in place of the connection context identifier in the redirection URI or is separately returned according to , and
an additional runtime resolution is used to resolve the issuer to retrieve the associated AS endpoints (e.g., with the authorization server metadata or OpenID Discovery ). Clients using such resolution solely to pre-populate individual AS endpoint fields without any coupling with the issuer identifier will remain vulnerable.

Compared with existing countermeasures for mix-up attacks, which rely on an issuer identifier that each AS uses to uniquely identify itself and that the client stores, the connection context identifier approach lets the client uniquely identify each AS instance it has configured. This approach is particularly useful for clients that provision AS endpoints with manually configured metadata, where an issuer identifier may not be available.

Cross-user OAuth Session Fixation Upon completion of an OAuth flow, a client often associates the resulting tokens with the user's identity at the client (e.g., the application's user account or an anonymous guest user identity). This identity information is supposedly maintained in an established session that is already bound to the user agent and accessible to the client during the OAuth flow. In real-world deployments, however, this assumption can be broken for various reasons. For instance, when a native app's backend acts as a confidential OAuth client (commonly referred to as the backend-for-frontend pattern), the OAuth flow crosses user agents, but the established session does not: the native app obtains from the client a URI to request authorization, then opens it in an external user agent (typically the browser, as defined in ) that has no established session with the client. As a workaround, the client may introduce a session fixation vulnerability: it encodes a session identifier into the URI, which fixates a dedicated authorization session to complete the OAuth flow for the user at the client. The Cross-user OAuth Session Fixation exploits this session fixation attack vector. The attacker tricks a victim user into completing an OAuth flow that the attacker has initiated at the client. Because the authorization session fixated by the attacker designates the attacker's identity at the client, the tokens issued for the victim's protected resources become associated with the attacker. In general, this session fixation vulnerability may be viewed as violating the requirement of "binding the contents of state to the browser [more precisely, the initiating user agent] session" to defend against Cross-Site Request Forgery (CSRF, see ). However, while PKCE can mitigate CSRF, PKCE alone cannot mitigate this new attack: Since the entire OAuth flow, including the authorization request and the request to the redirection endpoint, is completed by the same victim user, the cryptographic binding between the authorization request and access token request enforced by PKCE is preserved. The impact of the new attack is also more severe than that of typical CSRF attacks. Note that this section focuses on the authorization code grant in same-device scenarios. For similar attacks in cross-device flows, see .

Attack Description Preconditions: It is assumed that the client has maintained a user's session but, for usability reasons, does not want to or cannot identify the user via the session at the redirection endpoint. Example Attack:

From a vulnerable client, the attacker initiates OAuth and obtains an authorization request URI, in which the state parameter encodes a newly created authorization session of the attacker.
The attacker sends this authorization request URI to a victim user.
The victim user visits the URI and authorizes the client to access their resources. The consent prompt can be skipped due to prior or implicit approvals.
Upon receiving the state at the redirection endpoint, the client fixates the attacker's authorization session and completes the OAuth flow.
The attacker's identity at the client now gains access to the victim's resources.

The following diagram illustrates the attack: | | | | authz req URI with | | | | state=attacker_auth_session | | | |<--------------------------------| | | | | | | | | Send URI to victim out-of-band | | | | (e.g., via phishing link) | | | |--------------->| | | | | | | | | | | Visit authz req URI | | | |------------------------------>| | | | User consent (may be skipped) | | | |<----------------------------->| | | | | authz res with code + | | | | state=attacker_auth_session | | | |<-------------| | | | | Exchange code for token | | | |<------------>| | | | token for victim's resources | | | | associated with attacker's | | | | identity at the client | | | | | | | | Access resources | | | |-------------------------------->| | | | | | Resource request with token | | | | for victim's resources | | | |<--------------------------->| | Unauthorized access to | | | | victim's resources | | | |<--------------------------------| | | ]]> Variant: After the OAuth flow is initiated, the client may first generate an implementation-specific "pre-authorization" URI for the purpose of fixating an authorization session, before redirecting the user agent to the authorization endpoint. Non-normative example request: Non-normative example response: In this variant, the attacker obtains and sends the pre-authorization URI to the victim user instead of the authorization request URI. When the victim visits this URI, the attacker's authorization session is fixated before the authorization request, rather than at the redirection endpoint as in Step 4.

Countermeasures To defend against the Cross-user OAuth Session Fixation attack, the client MUST ensure that an OAuth flow initiated by one user is completed by the same user. The most straightforward countermeasure is to identify the initiating user via their existing session at the client, rather than introducing a fixated session, if usability conditions permit. However, eliminating the session fixation vector may not always be feasible due to deployment constraints. For instance, the application's session management and OAuth responsibilities may be handled by separate entities (e.g., separate services isolated under different origins, or with the OAuth responsibilities outsourced to an OAuth-as-a-Service provider as described in ), or the corresponding endpoints may be accessed from different user agents (e.g., a native app versus a browser). Such deployments have been observed in practice (see ). Hence, the client MUST validate the binding of any newly fixated authorization session (conveyed via state or the pre-authorization URI) to the existing user session (maintained at the user agent) that initiates the OAuth flow, before proceeding with the access token request. Depending on the specific current settings:

If the user session is accessible at the redirection endpoint, the client can validate this binding directly.
If the user session is not accessible at the redirection endpoint, for example, because the redirection endpoint is hosted on a different origin or accessed from a different user agent than where the user session is maintained, the countermeasure requires one of the following to make the session accessible prior to validation:
- an implementation change to co-locate the redirection endpoint under the same origin as the endpoint maintaining the user session, and/or to re-authenticate the user at the redirection endpoint from the external user agent (e.g., the browser), or
- from the current redirection endpoint, performing a further redirection back to the starting origin and/or user agent where the user session is available. For native apps, the redirect options specified in MUST be used. The location of this further redirection MUST NOT be controllable by an attacker, or it will result in Open Redirection ().

Shared Consent in Brokered OAuth In a brokered OAuth deployment, an intermediate entity (called the broker in the following) mediates between downstream clients and one or more upstream authorization servers (referred to as AS in the following). The broker acts as an OAuth client towards each AS. Towards its downstream clients, the broker either acts as an authorization server itself, exposing a standards-compliant OAuth interface, or it exposes a custom, non-OAuth interface. The attack and countermeasures described in this section apply regardless of which of these two interfaces the broker exposes to its downstream clients. Throughout this section, the terms upstream and downstream are used relative to the broker and the direction in which authorization flows. The AS is upstream as the source of authorization and tokens, while the clients the broker serves are downstream as the recipients of the access the broker obtains on their behalf. The term downstream client furthermore denotes a unit of trust rather than necessarily a single application or an OAuth client in the sense of . Multiple applications under common administrative control, such as the applications of a single tenant of a multi-tenant broker or the applications of an organization operating its own broker, may legitimately share a single registration and consent decision and thus be treated as a single downstream client. Applications under separate administrative control constitute distinct downstream clients. When the broker registers itself once at an AS and reuses this single registration for every downstream client it serves, the AS cannot distinguish between those downstream clients. As a consequence, the consent the user grants for one downstream client is silently reused for any other downstream client that integrates the same broker. A malicious downstream client integrated with the same broker can therefore obtain access to the user's protected resources without the user ever consenting to that client.

Attack Description The descriptions here follow , where additional details of the attack are laid out. Shared consent attacks require at least two downstream clients (one honest, one malicious) to be integrated with the same (honest) broker, and that broker to register itself as a client at the (honest) upstream AS. In the following, let H-Client and M-Client be downstream clients (honest and attacker-controlled, respectively) integrated with broker B. As a non-normative example, the description assumes that B exposes a standards-compliant OAuth interface to its downstream clients. The broker B registers itself once at the AS and obtains a client identifier cid_B@AS together with a redirection URI bound to the broker. The broker uses this registration whenever it issues an authorization request triggered from any of its downstream clients to the AS. At B, H-Client is registered as cid_HC@B and M-Client is registered as cid_MC@B, each with its own redirection URI bound to the respective downstream client. From the point of view of the AS, every flow that B initiates appears to come from the same client cid_B@AS, regardless of which downstream client actually triggered the flow. The broker acts invisibly to the user during the flow. It renders no consent screen of its own and no other user-visible UI that would indicate that an additional entity is involved between the downstream client and the AS. The exact form of the authorization response (authz res) returned to a downstream client is out of scope for this attack. The flaw is illustrated in the following figure: | | | | | | authz req: cid_B@AS | | | | |---------------------->| | | consent: cid_B@AS| | |<--------------------------------------------------------------->| | | | | authz res | | | | |<----------------------| | | authz res | | | |<--------------------------| | |============================ Phase 2 ============================| | | authz req: cid_MC@B | | | |------------>| | | | | | authz req: cid_B@AS | | | | |---------------------->| | | | | authz res | | | | |<----------------------| | | | authz res | | | | |<------------| | ]]> In the second phase of the figure, no consent prompt is rendered for M-Client. The AS only sees the broker's client identifier cid_B@AS, for which the user has already granted consent in the first phase, so the AS silently issues a token to B (cf. prompt=none, as defined in Section 3.1.2.1 of ) and B returns an authz res to M-Client.

Countermeasures The root cause of shared consent attacks is that the AS cannot tell on whose behalf the broker is acting and therefore cannot ask the user for consent on a per-downstream-client basis. Brokers MUST employ at least one of the following two countermeasures.

Per-Client Registration at the Upstream Authorization Server Each downstream client MUST be registered as a separate client at the AS. When initiating an authorization flow to the AS on behalf of a downstream client, the broker MUST use the registration of exactly that downstream client. The specific mechanism by which these per-client registrations are established is out of scope of this document. In practice, they are commonly established as follows: The broker provides each downstream client with a redirection URI that is hosted by the broker, and instructs the downstream client to register at every AS it expects to use, using the broker-provided redirection URI. The downstream client performs this registration at each AS, obtaining a distinct client identifier (e.g., cid_HC@AS for H-Client and cid_MC@AS for M-Client) and any associated credentials (such as a client secret). The downstream client then hands these credentials over to the broker, which gives the broker full control to act on behalf of the downstream client at the AS. This countermeasure ensures that the AS recognizes each downstream client as a distinct client, and that any consent prompt rendered by the AS is bound to a single downstream client. Consent granted for one downstream client is therefore not reusable for another. | | | | | | authz req: cid_HC@AS | | | | |---------------------->| | | consent: cid_HC@AS| | |<--------------------------------------------------------------->| | | | | authz res | | | | |<----------------------| | | authz res | | | |<--------------------------| | | | | | | | | authz req: cid_MC@B | | | |------------>| | | | | | authz req: cid_MC@AS | | | | |---------------------->| | | consent: cid_MC@AS| | |<--------------------------------------------------------------->| | | | | authz res | | | | |<----------------------| | | | authz res | | | | |<------------| | ]]>

Broker-Side Consent Screen Unless consent for the downstream client has already been granted, the broker MUST present an explicit consent screen to the user that identifies the downstream client, before initiating its own authorization request to the AS on behalf of the downstream client. The broker MUST NOT skip this consent screen based on a previously granted consent for a different downstream client. The broker MAY remember the user's consent decision per downstream client (e.g., per client_id of the downstream client), but MUST NOT remember it across different downstream clients. This countermeasure prevents the broker from silently reusing a consent granted for one downstream client when initiating a request to the AS on behalf of another downstream client, even when the AS cannot distinguish between the broker's downstream clients. | | | consent: cid_HC@B | | |<--------------------------------------->| | | | | | authz req: cid_B@AS | | | | |---------------------->| | | consent: cid_B@AS| | |<--------------------------------------------------------------->| | | | | authz res | | | | |<----------------------| | | authz res | | | |<--------------------------| | | | | | | | | authz req: cid_MC@B | | | |------------>| | | consent: cid_MC@B | | |<--------------------------------------->| | | | | | authz req: cid_B@AS | | | | |---------------------->| | | | | authz res | | | | |<----------------------| | | | authz res | | | | |<------------| | ]]>

Discussion of the Countermeasures The two countermeasures have different practical trade-offs. The Per-Client Registration countermeasure () confronts the user with at most one consent screen per authorization flow, which improves the user experience. However, it requires each downstream client to be registered at every AS the broker integrates with, which can be a substantial effort given that a single broker is typically integrated with many ASes. The Broker-Side Consent Screen countermeasure () spares downstream clients this registration effort, since the broker's single registration at each AS is reused for all downstream clients. However, the user may be confronted with up to two consent screens in a single authorization flow: one rendered by the broker identifying the downstream client, and one rendered by the AS identifying the broker. Both countermeasures are implemented entirely on the client side (the downstream client and the broker) and require no software or protocol changes to any AS.