Software:
--------
Doorkeeper (https://github.com/doorkeeper-gem/doorkeeper)

Description:
----------
Doorkeeper is an OAuth 2 provider for Rails written in Ruby.

Affected Versions:
---------------
1.2.0 - 4.1.0 (all versions but latest patch supporting token revocation)

Fixed Versions:
-------------
4.2.0 or apply this commit[0]

Problem:
--------
Doorkeeper failed to implement OAuth 2.0 Token Revocation[1] (RFC
7009[2]) in the following ways:

1. Public clients making valid, unauthenticated calls to revoke a token
would not have their token revoked
2. Requests were not properly authenticating the *client credentials*
but were, instead, looking at the access token in a second location
3. Because of 2, the requests were also not authorizing confidential
clients' ability to revoke a given token. It should only revoke tokens
that belong to it.

(see [3][4][5][6] for above statements)

The security implication is: OAuth 2.0 clients who "log out" a user
expect to have the corresponding access & refresh tokens revoked,
preventing an attacker who may have already hijacked the session from
continuing to impersonate the victim. Because of the bug described
above, this is not the case. As far as OWASP is concerned, this counts
as broken authentication design[7].

MITRE has assigned CVE-2016-6582 due to the security issues raised. An
attacker, thanks to 1, can replay a hijacked session after a victim logs
out/revokes their token. Additionally, thanks to 2 & 3, an attacker via
a compromised confidential client could "grief" other clients by
revoking their tokens (albeit this is an exceptionally narrow attack
with little value).

Unless I'm mistaken, all clients (public or confidential) that send
well-formed, RFC 7009 compliant requests are affected by this bug.

Solution:
-------

Modify the controller so if the request comes from a public client
revoke the token without auth/auth. If the client is confidential,
authenticate the client per RFC 6749 Sec. 2.3[8] and authorize its
ownership of the provided token. As per [0].

Timeline:
--------
2016-08-03: Bug discovered
2016-08-03: CVE requested, assigned, privately disclosed to maintainer,
bugfix/patch authored
2016-08-08: Maintainer tweaked patch
2016-08-12: Jonathan Clem ( jclem) also discovered bug and publicly
disclosed[6]
2016-08-18: Patched version 4.2.0 is released

Acknowledgements:
-----------------
Special thanks to the maintainer, Tute Costa (https://github.com/tute),
for quickly collaborating with me to prepare & apply a patch.

References:
----------
[0]:
https://github.com/doorkeeper-gem/doorkeeper/commit/fb938051777a3c9cb071e96fc66458f8f615bd53
[1]: https://github.com/doorkeeper-gem/doorkeeper/pull/374
[2]: https://tools.ietf.org/html/rfc7009#section-2.1
[3]:
https://github.com/doorkeeper-gem/doorkeeper/blob/v4.1.0/app/controllers/doorkeeper/tokens_controller.rb#L13-L35
[4]:
https://github.com/doorkeeper-gem/doorkeeper/blob/master/lib/doorkeeper/helpers/controller.rb#L28-L30
[5]:
https://github.com/doorkeeper-gem/doorkeeper/blob/master/lib/doorkeeper/oauth/token.rb#L5-L23
[6]: https://github.com/doorkeeper-gem/doorkeeper/issues/875
[7]:
https://www.owasp.org/index.php/Top_10_2013-A2-Broken_Authentication_and_Session_Management
[8]: https://tools.ietf.org/html/rfc6749#section-2.3