[OAUTH-WG] Using OAuth error_code to glean information from the server

[Mike Jones <[e-mail]>]

I recently received an inquiry regarding invalid_client vs. invalid_grant.  It seems that there is a potential information disclosure in the specification with respect to how these error codes are used:

invalid_client
               Client authentication failed (e.g. unknown client, no
               client authentication included, or unsupported
               authentication method).  The authorization server MAY
               return an HTTP 401 (Unauthorized) status code to indicate
               which HTTP authentication schemes are supported.  If the
               client attempted to authenticate via the "Authorization"
               request header field, the authorization server MUST
               respond with an HTTP 401 (Unauthorized) status code, and
               include the "WWW-Authenticate" response header field
               matching the authentication scheme used by the client.

invalid_grant

               The provided authorization grant (e.g. authorization

               code, resource owner credentials, client credentials) is

               invalid, expired, revoked, does not match the redirection

               URI used in the authorization request, or was issued to

               another client.

If one uses invalid_client when the client is unknown and invalid_grant when the client credentials are invalid, then an attacker could deduce whether or not a particular client exists.

First, do people agree that this is a potential information leak and that the leak is meaningful?  If so, what mitigation might be suggested?  For instance, might a server choose to use a single error code for both (and potentially other) cases?

                                                            Thanks,
                                                            -- Mike