source: https://www.securityfocus.com/bid/12578/info

A remote cross-site scripting vulnerability affects the 'forum.php' script of MercuryBoard. This issue is due to a failure of the application to properly sanitize user-supplied input prior to including it in dynamically generated Web content.

An attacker may leverage this issue to have arbitrary script code executed in the browser of an unsuspecting user. This may facilitate the theft of cookie-based authentication credentials as well as other attacks.

http://www.example.com/index.php?a=forum&f='%3E%3Cscript%3Ealert(document.cookie)%3C/script%3E
http://www.example.com/index.php?a=forum&f='><script>alert(document.cookie)</script>