phpBB contains a flaw that allows a remote cross-site scripting (XSS) attack. This flaw exists because the application does not validate the user-submitted contents of HTML tags when 'Always Allow HTML' is set to 'On'. This may allow a user to create a specially crafted URL that would execute arbitrary script code in a user's browser within the trust relationship between their browser and the server.
Upgrade to version 2.0.19 or higher, as it has been reported to fix this vulnerability. An upgrade is required as there are no known workarounds.