viewthread.php in php-fusion 4.x does not check the (1) forum_id or (2) forum_cat parameters, which allows remote attackers to view protected forums via the thread_id parameter.
The ReadMe First.txt file in PHP-Fusion 4.0 instructs users to set the permissions on the fusion_admin/db_backups directory to world read/write/execute (777), which allows remote attackers to download or view database backups, which have easily guessable filenames and contain the administrator username and password.