source: https://www.securityfocus.com/bid/19437/info

Simple one-file guestbook is prone to a security-bypass vulnerability. An attacker can bypass authentication measures by using a specific URL to delete all guestbook entries.

Version 1.0 of Simple one-file guestbook is vulnerable. Other versions may be affected as well.

#Simple One-File Guestbook Adminstrator Credential Bypass
#Proof of Concept URL

http://www.example.com/[path]/guestbook.php?id=4