MTP Guestbook contains a flaw that allows a persistent cross-site scripting (XSS) attack. This flaw exists because the application does not validate the 'ins_city' and 'ins_fname' parameters upon submission to the /mtp_guestbook/insert_user.php script. This may allow an attacker to create a specially crafted request that would execute arbitrary script code in a user's browser within the trust relationship between their browser and the server.