A vulnerability has been discovered in Gerry GuestBook, which can be exploited by malicious people to conduct cross-site scripting attacks.
Input passed via the "gbText" parameter to guestbook.php (when "sign" is set to "2") is not properly sanitised before being returned to the user. This can be exploited to execute arbitrary HTML and script code in a user's browser session in context of an affected site.
The vulnerability is confirmed in version 1.21. Other versions may also be affected.
Vulnerable Products
Vulnerable Software: Gerry GuestBook 1.x
Solution
Edit the source code to ensure that input is properly sanitised.