Multiple cross-site scripting (XSS) vulnerabilities in Phorum 5.0.17a and earlier allow remote attackers to inject arbitrary web script or HTML via (1) the username parameter to register.php or (2) a signature of a logged-in user in "My Control Center," which is not properly handled by control.php.
http://secunia.com/advisories/16667
http://archives.neohapsis.com/archives/fulldisclosure/2005-09/0048.html