I have previously discussed about common OAuth 2 Implementation Vulnerabilities but now it is time (maybe) to list those and order them based on their criticality.
#5 The Postman Always Rings TwiceI have introduced this 'attack' in last year post . This is for provider implementer, it is not extremely severe but, hey, is better to follow the spec. Specifically
The client MUST NOT use the authorization code more than once. If an authorization code is used more than once, the authorization server MUST deny the request and SHOULD revoke (when possible) all tokens previously issued based on that authorization code.
It turned out that even Facebook and Google did it wrong... :)
#4 Match PointTo all OAuth Providers be sure to follow section 4.1.3 of the spec in particular
...if the "redirect_uri" parameter was included in the initial authorization request as described in Section 4.1.1, and if included ensure that their values are identical.
Should you fail to do it, this in combination with Lassie Come Home below is game over (even for implementer that support only the Authorization Code Grant flow).
#3 Crossing The LineAs per any other website part is important to not forget Cross Site Request Forgery aka CSRF protection in your OAuth provider impelemtation. Some examples are:
- the Doorkeeper's OAuth endpoints lack of CSRF protection
- the CSRF issue I found on the Github OAuth implementation
#2 The Devil Wears PradaIf you are an OAuth client that use OAuth for authentication (do NOT). If you absolutely have to, you'd better read User Authentication with OAuth 2.0 . Specially if you are using the OAuth Implicit Grant flow (aka Client side).
More about the topic in here and here
#1 Lassie Come Home....and the winner is 'Lassie Come Home'. Well this is hell of a danger.
There are way too many example of provider vulnerable to this attack. Just listing few here: