Simple one-file guestbook 1.0 and earlier allows remote attackers to bypass authentication and delete guestbook entries via a modified id parameter to guestbook.php.
https://exchange.xforce.ibmcloud.com/vulnerabilities/28289
http://www.vupen.com/english/advisories/2006/3229
http://www.securityfocus.com/archive/1/442884/100/0/threaded