source: https://www.securityfocus.com/bid/57145/info

WHMCS is prone to an authentication-bypass vulnerability because it fails to adequately verify user-supplied input used for cookie-based authentication.

Attackers can exploit this vulnerability to gain administrative access to the affected application, which may aid in further attacks.

WHMCS 5.0 and 5.1 are vulnerable; other versions may also be affected. 

http://www.example.com/whmcs/admin/login.php?correct&cache=1?login=getpost{}