in bugtraq
Guestserver guestbook system vulnerabilities
Mon Dec 12 16:16:41 2005
Date: 11 Dec 2005 17:42:30 -0000
The poor security and input validation at Guestserver versions below 5 ( there are still lots of them out there ) allow all HTML tags inside a message and it can be exploited as much as you want to.
A Google Search for "Guestbook by Guestserver - v4.12" for example comes up with tons of vulnerable sites.