Currently there is a nasty exploit floating around for WHMCS. It's recommended to block access to your WHMCS installation completely or disable the site by moving it away.
More information:
Link 1: http://forum.whmcs.com/showthr…WHMCS-5-2-7-Vulnerability
Link 2: https://twitter.com/whmcs/status/385811317179052032
Link 3: http://www.webhostingtalk.com/showthread.php?t=1309491