Top 5 OAuth 2 Implementation Vulnerabilities
Top 5 OAuth 2 Implementation Vulnerabilities


Top 5 OAuth 2 Implementation Vulnerabilities


Heya, back to my favourite topics namely OAuth .
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 Twice 
I 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 Point
To 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 Line
As 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 Prada
 If 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:
Egor Homakov hacking Github
me/myself hacking Facebook bypassing the regex redirect_uri validation
me/myself breaking the Google/Github integration

At least the mitigation for this issue is damn simple:  use exact matching against registered redirect uri to validate the redirect_uri parameter
