flatCore before 1.5.7 allows XSS by an admin via the acp/acp.php?tn=pages&sub=edit&editpage=1 page_linkname, page_title, page_content, or page_extracontent parameter, or the acp/acp.php?tn=system&sub=sys_pref prefs_pagename, prefs_pagetitle, or prefs_pagesubtitle parameter.