phpBB contains a flaw that allows a remote cross site scripting attack. This flaw exists because the application does not validate "topic_id" variables upon submission to the "viewtopic.php" script. This could allow a user to create a specially crafted URL that would execute arbitrary code in a user's browser within the trust relationship between the browser and the server, leading to a loss of integrity.
Upgrade to version 2.0.6 or higher, as it has been reported to fix this vulnerability. An upgrade is required as there are no known workarounds.