This is a small guide on an issue we found with WHMCS v 7.1.
We noticed malicious users registering subdomains on domains we own. like test.zenithmedia.ca while signing up for new accounts. We reported this issue to cPanel and they have issues a core fix that will be released in their next patch.
As a quick work around we implimented a fix offered by a feature of cPanel that allowes the blocking of domain registrations based on the list.