Noticed this follow up question after I sent this: > 10.6. Authorization Code Leakage: Comment on "The authorization server > SHOULD require the client to register their redirection URI": "Why is this a > should?"
Because comparing the redirect_uri value used between the two calls (authorization and token) along with client authentication is sufficient to avoid the attack described for confidential clients. For public clients it is a MUST. How about this: 10.6. Authorization Code Redirection URI Manipulation When requesting authorization using the authorization code grant type, the client can specify a redirection URI via the "redirect_uri" parameter. If an attacker can manipulate the value of the redirection URI, it can cause the authorization server to redirect the resource owner user-agent to a URI under the control of the attacker with the authorization code. An attacker can create an account at a legitimate client and initiate the authorization flow. When the attacker is sent to the authorization server to grant access, the attacker grabs the authorization URI provided by the legitimate client, and replaces the client's redirection URI with a URI under the control of the attacker. The attacker then tricks the victim into following the manipulated link to authorize access to the legitimate client. Once at the authorization server, the victim is prompted with a normal, valid request on behalf of a legitimate and familiar client, and authorizes the request. The victim is then redirected to an endpoint under the control of the attacker with the authorization code. The attacker completes the authorization flow by sending the authorization code to the client using the original redirection URI provided by the client. The client exchanges the authorization code with an access token and links it to the attacker's client account which can now gain access to the protected resources authorized by the victim (via the client). In order to prevent such an attack, the authorization server MUST ensure that the redirection URI used to obtain the authorization code, is the same as the redirection URI provided when exchanging the authorization code for an access token. The authorization server MUST require public clients and SHOULD require confidential clients to register their redirection URI and if provided in the request, MUST validate the redirection URI received in the authorization request against the registered value. EHL _______________________________________________ OAuth mailing list OAuth@ietf.org https://www.ietf.org/mailman/listinfo/oauth
