source: http://www.securityfocus.com/bid/13149/info

phpBB2 Plus is affected by a cross-site scripting vulnerability. This issue is due to a failure in the application to properly sanitize user-supplied input.

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/groupcp.php?g=881&amp%3bsid='%22%3E%3Cscript%3Ealert(document.cookie)%3C/script%3E