source: http://www.securityfocus.com/bid/9882/info

It has been reported that Phorum is prone to a cross-site scripting vulnerability across multiple modules. The issue presents itself across multiple modules including 'login.php', 'register.php', and 'profile.php'. These modules employ two hidden variables named 'f' and 'target', which are passed user-supplied input values from HTTP_REFERER without proper sanitization.

Phorum versions 5.0.3 Beta and prior are reported to be vulnerable to this issue.

register.php?&HTTP_REFERER=[XSS]