mainfile.php in XOOPS 2.0.13.2 and earlier, when register_globals is enabled, allows remote attackers to overwrite variables such as $xoopsOption['nocommon'] and conduct directory traversal attacks or include PHP files via (1) xoopsConfig[language] to misc.php or (2) xoopsConfig[theme_set] to index.php, as demonstrated by injecting PHP sequences into a log file.
https://www.exploit-db.com/exploits/1811
http://www.vupen.com/english/advisories/2006/1895
http://www.securityfocus.com/archive/1/434698/100/0/threaded