FlatNuke contains a flaw that allows a remote cross site scripting attack. This flaw exists because the application does not validate 'border' and 'back' variables upon submission to the help.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 2.5.4 or higher, as it has been reported to fix this vulnerability. An upgrade is required as there are no known workarounds.