[OAUTH-WG] Authorization Code Leakage feedback (Yaron Goland)

[Eran Hammer-Lahav <[e-mail]>]

> 10.6. Authorization Code Leakage: Comment "I fancy myself as being

> reasonably intelligent and I'm unclear what attack is actually being described

> here."



Yeah... I had to go back to -16 to be reminded of the section original title 'session fixation attack' to figure out what this was about.



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

   SHOULD require the client to register their redirection URI and if

   provided, MUST validate the redirection URI received in the

   authorization request against the registered value.



EHL