FUDForum 3.0.9 is vulnerable to Stored XSS via the User-Agent HTTP header. This may result in remote code execution. An attacker can use a user account to fully compromise the system via a GET request. When the admin visits user information under "User Manager" in the control panel, the payload will execute. This will allow for PHP files to be written to the web root, and for code to execute on the remote server. The problem is in admsession.php and admuser.php.
Cross-site scripting (XSS) vulnerability in install/forum_data/src/custom_fields.inc.t in FUDforum 3.0.4.1 and earlier, when registering a new user, allows remote attackers to inject arbitrary web script or HTML via a custom profile field to index.php. NOTE: some of these details are obtained from third party information.