4. Security Considerations

Clients MUST validate the iss parameter precisely as described in Section 2.4 and MUST NOT allow multiple authorization servers to use the same issuer identifier. In particular, when authorization server details can be manually configured in the client, the client MUST ensure that the accepted iss values are unique for each authorization server.

The iss parameter enables a client to decide if an authorization server "expects" to be used in an OAuth flow together with a certain token endpoint and potentially other endpoints, like the userinfo endpoint [OIDC.Core]. When OAuth metadata is used, the iss parameter identifies the issuer and therefore the respective OAuth metadata document that points to the other endpoints. When OAuth metadata is not used, the client can use, for example, a statically configured expected iss value for each configured authorization server.

The issuer identifier contained in the authorization response is not cryptographically protected against tampering. In general, mechanisms such as JWTs (as specified in [JARM]) could be used to protect the integrity of the authorization response. However, in mix-up attacks, the client generally receives the authorization response from an uncompromised authorization server. If an attacker can tamper with this authorization response before it is received by the client, the attacker would also have direct access to the authorization code. The attacker does not need to execute a mix-up attack to steal the authorization code. Therefore, integrity protection for the authorization response is not necessary to defend against mix-up attacks.

There are also alternative countermeasures to mix-up attacks. When an authorization response already includes an authorization server's issuer identifier by other means and this identifier is checked as laid out in Section 2.4, the use and verification of the iss parameter is not necessary and MAY be omitted. For example, this is the case when OpenID Connect response types that return an ID Token from the authorization endpoint (e.g., response_type=code id_token) or [JARM] are used. However, if a client receives an authorization response that contains multiple issuer identifiers, the client MUST reject the response if these issuer identifiers do not match. The details of alternative countermeasures are outside of the scope of this specification.

Mix-up attacks are only relevant to clients that interact with multiple authorization servers. However, clients interacting with only one authorization server might add support for a second authorization server in the future. By supporting multiple authorization servers, they become vulnerable to mix-up attacks and need to apply countermeasures.