source: http://www.securityfocus.com/bid/14391/info

Advanced Guestbook is prone to an HTML injection vulnerability. This issue is due to a failure in the application to properly sanitize user-supplied input before using it in dynamically generated content.

Attacker-supplied HTML and script code would be executed in the context of the affected Web site, potentially allowing for theft of cookie-based authentication credentials. An attacker could also exploit this issue to control how the site is rendered to the user; other attacks are also possible. 

Set the following registry setting to exploit this vulnerability:
[HKEY_LOCAL_MACHINE/SOFTWARE/Microsoft/Windows/CurrentVersion/Internet Settings/5.0/User Agent/] (Default)= "Code to inject"