source: http://www.securityfocus.com/bid/61735/info

Advanced Guestbook is prone to a vulnerability that lets attackers upload arbitrary files. The issue occurs because the application fails to adequately sanitize user-supplied input.

An attacker may leverage this issue to upload arbitrary files to the affected computer; this can result in arbitrary code execution within the context of the vulnerable application.

Advanced Guestbook 2.4.3 is vulnerable; other versions may also be affected. 

http://www.example.com.tw/guestbook/addentry.php