A security update has been released for all versions of WHMCS V6.x.
We evaluate the severity of this issue to be of a Moderate security rating (see Security Level Definitions).
We are providing a single patch file that you may apply to any version of WHMCS 6.x.
It will be provided along with any future incremental patch sets or fully released versions of WHMCS 6.x.
Once a sufficient amount of time has passed to allow users to apply this patch to their installation, we will release further details...
Read more @ http://blog.whmcs.com/?t=116515