As you may be aware, a security issue has been published within the last hour which allows for information disclosure.
We are aware of the issue and are investigating it, and will be issuing a fix for this issue along with any others we discover during our targeted investigation shortly. In the meantime disabling the Mass Payment feature voids the immediate threat.
You can do this by de-selecting the "Enable Mass Payment" checkbox in Setup > General Settings > Invoices and saving.
Read more @ http://blog.whmcs.com/?t=80587