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I don´t see how this could be exploited unless th hacker is a webmaster or a module admin. The first thing the script does is to check is it is a XOOPS user and if it has admin privilages. Anyway, I would prefer to sanitize fct just to make sure I don´t have a malicious admin among my crew, lol. But hey, if there is a malicious admin fixing this would be the less important of all things. In 90%(maybe 90,1% not sure, ahah) of the cases, the module developers don´t care much in protecting their modules administration so, a bad module admin can compromise your site exploting admin area of the module he has admin access.
I might be wrong eheh, but I consider this a very lazy exploit.