source: https://www.securityfocus.com/bid/16112/info

Chipmunk 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. 

Add an entry form:
http://www.example.com/guestbook/addentry.php

Homepage value: '></a><script>alert(123);</script>