source: http://www.securityfocus.com/bid/9940/info

It has been reported that VBulletin is prone to a cross-site scripting vulnerability in the 'ptivate.php' script. This issue is reportedly due to a failure to sanitize user input and so allow for injection of HTML and script code that may facilitate cross-site scripting attacks. 

Successful exploitation of this issue may allow for theft of cookie-based authentication credentials or other attacks.

http://www.example.com/private.php?&action=newmessage&userid=[UID]&forward=[XSS]