Whmcs has released a patch for the 4.x and 5.x versions of their software today that fix some security issues. They are not releasing the details of the security issue as of yet, but it is highly recommended that you apply the patches as soon as possible. You can find more about it at their blog.
http://blog.whmcs.com/?t=69402
Keep secure!
Chris March 12th, 2013
Posted In: Security