MatrikzGB Guestbook is prone to a vulnerability that may permit guestbook users to gain administrative rights. It is possible to exploit this issue by manipulating URI parameters. An attacker can send a crafted request to the vulnerable application, such as http://www.example.com/php/gaestebuch/admin/index.php?do=options&action=optionsok&new_username=regularuser&new_password=regularpass&new_rights=admin&user=regularuser&pass=regularpass, where regularuser and regularpass equal the user credentials of the attacker.