A remote unauthenticated attacker can insert malicious content in a CMS Made Simple installation by poisoning the web server cache when Smarty Cache is activated by modifying the Host HTTP Header in his request. The vulnerability can be triggered only if the Host header is not part of the web server routing process (e.g. if several domains are served by the same web server). This can lead to phishing attacks because of the modification of the site's links, defacement or Cross-Site-Scripting attacks by a lack of filtering of HTML entities in $_SERVER variable.
Update to CMS Made Simple 2.1.3 or later. Alternatively, ensure that Smarty caching has been disabled in the admin panel.