source: https://www.securityfocus.com/bid/15342/info

XMB is prone to a cross-site scripting vulnerability because the application fails to properly sanitize user-supplied input.

An attacker may leverage this issue to execute arbitrary script code in the browser of an unsuspecting user in the context of the affected site. A successful exploit could allow an attacker to steal cookie-based authentication credentials and launch other attacks. 

http://www.example.com/u2u.php?action=send&username=[code]