delcookie.php in Professional Home Page Tools Guestbook changes the expiration date of a cookie instead of deleting the cookie's value, which makes it easier for attackers to steal the cookie and obtain the administrator's password hash after logout.
https://exchange.xforce.ibmcloud.com/vulnerabilities/27775
http://www.securityfocus.com/archive/1/440421/100/0/threaded
http://securityreason.com/securityalert/1275