The real fault lies with whmcs, for not utilizing hosting that is secure. Since whmcs is a billing system, a highly attractive target, they should be running their system from a banking compliant hosting provider, which means restricted access locked room for the servers, as well as additional security limiting online access. 97-99% of stolen company data is done from the inside. That includes direct employees and any contractor employee's, which in this case means hostgator.
It is clear that whmcs needs to start handling it's business with security that is required by credit card transaction regulations. Until that level of security is implemented by whmcs for it's own servers, every user of whmcs is at risk. As not only could the whmcs servers harbor a data post grabber or database dump capability to a remote location, but code could be injected into the whmcs download it self to get credit card data from every company that uses whmcs.
With the potential of hundreds of millions of dollars to be had, strict security needs to be a**ered to by whmcs, as human nature is generally the weakest link, whether it be a disgruntled employee or some one willing to make some money. The lowest paid and least educated employees are generally support personal, yet they are the ones with the ability to access any system at a hosting company.
The incentive of earning thousands or hundreds of thousands or millions of dollars is simply to great to allow average support personal access to servers that hold or process credit card transactions.
This is nothing new, old news and old knowledge, but apparently whmcs is not security minded, hell they cannot even verify their own customers from within their own system, the internal ticketing system is not secure, that is by their own admission.
Whmcs needs to go back to the grindstone and implement better security procedures for its own servers and clear up the security flaw in the ticket system. They may be able to ask for login details to verify their customers, but many users of whmcs do not sell products that allow such a method to be used to verify the customer. So how does a user of whmcs verify a client if the tickets in the ticket system cannot be trusted?