Stud.IP 5.x through 5.3.3 allows XSS with resultant upload of executable files, because upload_action and edit_action in Admin_SmileysController do not check the file extension. This leads to remote code execution with the privileges of the www-data user. The fixed versions are 5.3.4, 5.2.6, 5.1.7, and 5.0.9.
