source: https://www.securityfocus.com/bid/53740/info
WHMCS is prone to a cross-site scripting vulnerability and multiple HTML-parameter-pollution vulnerabilities because it fails to properly sanitize user-supplied input.
Attacker-supplied HTML and script code would run in the context of the affected browser, potentially allowing the attacker to steal cookie-based authentication credentials, control how the page is rendered to the user, and override existing hard-coded HTTP parameters which compromises the application.
WHMCS 5.0 is vulnerable; other versions may also be affected.
http://www.example.com/knowledgebase.php?action = [XSS]