Concrete CMS is prone to following vulnerabilities because it fails to properly handle user-supplied input.
1. Multiple cross-site scripting vulnerabilities
2. An arbitrary-file-upload vulnerability
3. A denial-of-service vulnerability
An attacker may leverage these issues to cause denial-of-service conditions or to execute arbitrary script code in the browser of an unsuspecting user in the context of the affected site. This may allow the attacker to steal cookie-based authentication credentials and to launch other attacks.
Concrete CMS versions 5.5 and 5.5.21 are vulnerable.