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. 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. 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. As described in , the attacker can then utilize the obtained client authentication assertion to impersonate the client and, for example, obtain access tokens.

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. Note that the following countermeasures mandate the use of single audience value (as opposed to multiple audiences in 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. 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. Note that the countermeasures described in and do not imply any normative changes to the authorization server: requires the authorization server to only accept a JWT if the authorization server can identify itself with (at least one of the elements in) the JWT's audience value. Authentication JWTs produced by a client implementing one of these countermeasures meet this condition. Of course, an authorization server MAY still decide to only accept its issuer identifier () or the endpoint that received the JWT () as an audience value, for example, to force its clients to adopt the respective countermeasure.

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 .

Cross-toolkit OAuth Account Takeover It is increasingly common and observed that a single OAuth client supports multiple tools. Each set of tools, known as a toolkit, is mapped to an OAuth provider configuration, which includes at least the authorization server (AS) endpoints and 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 tool can then use the access token to access the user's protected resource at a resource server (RS). Multiple OAuth connections can be linked to some form of user identity based on the following common 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 a short-lived anonymous session). 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 is becoming a tenant. A tenant can usually choose some off-the-shelf toolkits using (partially-) completed OAuth providers, if not adding their own toolkits with custom OAuth providers to support the tenant's service.

When controlled by an attacker, the open configurations of OAuth providers have posed a new threat to this centralized OAuth client design. If the client fails to properly identify, track, and isolate which proper OAuth connection context (representing a combination of OAuth provider, toolkit, and tenant) is 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 the 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 cross-tenant that shares 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 the honest authorization URL 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 a re-authorization (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-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 code and can either exchange the code for an access token (for public clients) or perform an authorization code injection attack as described in .

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 exploits confusion between the OAuth connection context (i.e., a combination of OAuth provider, toolkit, 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. A variation of this attack involves the malicious tenant simply using a shared off-the-shelf toolkit that comes with a pre-built OAuth provider (with client registration included), if so allowed.
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 the Naïve RP Session Integrity Attack when the OAuth connection context is limited to AS, and is 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 NOT share OAuth providers with completed client registrations across toolkits and tenants belonging to different owners. 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, of which one 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 client MUST issue 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 was received on the redirection endpoint, the client MUST also check that the context identifier from the distinct redirection URI matches with 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 be a replacement under the following conditions:

the client has entirely dropped the support to 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.

Cross-user OAuth Session Fixation Based on similar deployment needs as outlined in , multiple OAuth connections can be linked to some form of user's identity (e.g., an application's user identifier). This identity information is supposedly maintained in a session established and already bound to the user agent. In real-world deployments, however, this prerequisite can be broken for various reasons. For instance, in OAuth deployments that cross user agents, where an authenticated native app has its backend acting as a confidential OAuth client, the client opens a tool linking URL in an external user agent (typically a browser, as defined in ) that has no authenticated sessions with the client. As a workaround, the client introduces a session fixation vulnerability: it encodes a session identifier into the URL, which fixates a dedicated authorization session to complete the OAuth flow with the user at the client. The Cross-user OAuth Session Fixation exploits this session fixation attack vector. The attacker attempts to trick a victim into completing an OAuth flow that the attacker has initiated at the client. As a result, the attacker's session will be used to establish an OAuth connection with the victim's tool resources or identity, hence resulting in the same impact as COAT (). However, this attack exploits confusion over the intended user bound to that connection context during the OAuth flow, contrasting with COAT, which exploits confusion within the OAuth connection context (OAuth provider, toolkit, tenant). 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 redirection endpoint is completed by the same victim user, the cryptographic binding of 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. For similar attacks in cross-device OAuth flows, see .

Attack Description Preconditions: It is assumed that the client has maintained a user's session. But it does not want to or cannot authenticate the user at the redirection endpoint for usability reasons, before completing the OAuth flow. Example Attack:

From a vulnerable client, the attacker initiates OAuth against a tool and obtains an authorization request URL, in which the state parameter has encoded a newly created authorization session of the attacker.
The attacker sends this authorization request URL to a victim.
The victim visits the URL and (automatically, due to prior or implicit approvals,) authorizes the client to access their resources.
Upon receiving the state at the redirection endpoint, the client fixates the attacker's authorization session and completes the OAuth flow.
The attacker's account at the client now gains access to the victim's resources.

Variant: The client may first generate a pre-authorization URL for the purpose of fixating a session before redirecting to the authorization endpoint. Non-normative example request: Non-normative example response: This variant differs from the above only by obtaining and sending the pre-authorization URL instead, which will first fixate the attacker's authorization session (rather than 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 application needs. For instance, when the OAuth client responsibilities of establishing OAuth connections and the application's session management are handled by separate entities (e.g., separate services isolated under different origins, accessed from different user agents, or when the OAuth client is outsourced to an OAuth-as-a-Service provider), as observed in practice by and . Hence, the client MUST validate the binding of any newly fixated authorization session (conveyed via state or the pre-authorization URL) 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 in 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., a browser), or
- from the current redirection endpoint, performing a further redirection back to the starting origin and/or user agent where the existing 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 ().