concrete5 8.1.0 has CSRF in Thumbnail Editor in the File Manager, which allows remote attackers to disable the entire installation by merely tricking an admin into viewing a malicious page involving the /tools/required/files/importers/imageeditor?fID=1&imgData= URI. This results in a site-wide denial of service making the site not accessible to any users or any administrators.

concrete5 8.1.0 places incorrect trust in the HTTP Host header during caching, if the administrator did not define a "canonical" URL on installation of concrete5 using the "Advanced Options" settings. Remote attackers can make a GET request with any domain name in the Host header; this is stored and allows for arbitrary domains to be set for certain links displayed to subsequent visitors, potentially an XSS vector.