bugtraq - Facebook Url Redirection Vuln.

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Date: Thu, 11 Jul 2013 14:32:15 +0300
From: CANSIN YILDIRIM <canyildirim@...edu.tr>
To: bugtraq@...urityfocus.com
Subject: Facebook Url Redirection Vuln.

-----------------------------------------------------
#Title: Facebook Url Redirection Vulnerability
#Discovery Date: 10/July/2013
#Author: Cansın Yıldırım
#Twitter: @YildirimCansin
#Website: www.cansinyildirim.com
-----------------------------------------------------

Introduction:

The vulnerability reported on 10th July however Facebook Security Team
replied that it is an expected behaviour. That is why I don't see any
reason not to publish the vulnerability. More detailed background
information can be found on their website:
http://www.facebook.com/notes/facebook-security/link-shim-protecting-the-people-who-use-facebook-from-malicious-urls/10150492832835766

Decription:

By obtaining user-specific hash value, an attacker redirect the user
to a malicious website without asking for verification. The hash value
can be found from the link that the user send to his/her wall. After
clicking on user's link,by setting BurpSuite Proxy, the attacker
intercept the parameters in the methods.

Example url: http://www.facebook.com/l.php?u=http%3A%2F%2Fwww.youtube.com%2Fwatch%3Fv%3DSvWjWEyTLkg&h=3AQGbk0Cf&s=1

However we don't need s parameter, for the attacker most important
part of the url is the h paramater.

Example h paramater = h=3AQGbk0Cf

Now it is time to change the u paramater to a malicious site and send
it to user with the same h paramater

Example malicious url:
http://www.facebook.com/l.php?u=cansinyildirim.com&h=3AQGbk0Cf

Example malicious encoded url:
http://www.facebook.com/l.php?u=%63%61%6e%73%69%6e%79%69%6c%64%69%72%69%6d%2e%63%6f%6d&h=3AQGbk0Cf

After the victim clicks on the link, Facebook redirects the victim to
a page specified by the attacker.

Important note: I tried while the attacker and the victim were friends.

Proof of Concept:

http://vimeo.com/70087250

http://www.cansinyildirim.com/facebook-url-redirection-vulnerability-2013/