source: https://www.securityfocus.com/bid/18942/info

Fantastic GuestBook is prone to multiple HTML-injection vulnerabilities because it fails to properly sanitize user-supplied input before using it in dynamically generated content. 

Attacker-supplied HTML and script code would execute in the context of the affected website, potentially allowing an attacker to steal cookie-based authentication credentials or to control how the site is rendered to the user; other attacks are also possible.

If a malicious people go into the sign in form and put in the fields First Name, Last Name and Nick Name a code like this: <script>alert("XSS")</script> when you load the page http://www.example.com/[path]/guestbook.php or you read the messages of the guestbook you will se a pop-up that "say" 'XSS'. ]