source: http://www.securityfocus.com/bid/5997/info

vBulletin does not filter HTML tags from URI parameters, making it prone to cross-site scripting attacks.

As a result, it is possible for a remote attacker to create a malicious link containing script code which will be executed in the browser of a legitimate user, in the context of the website running vBulletin.

This issue may be exploited to steal cookie-based authentication credentials from legitimate users of the website running the vulnerable software. 

http://<victim>/usercp.php?s=[Session ID]"><Script>alert(document.cookie);</Script> 