EDIT: This only affects you if you use Multicraft or WHMCS.

During the past few hours a small hacker group has found vulnerabilities in Multicraft and WHMCS that gives them access to entire host networks. They've already abused this vulnerability to hack multiple major hosts and cause chaos on their servers. I can confirm that the majority of hosts that have not yet been hacked have been warned of this issue and are taking the necessary precautions.

Example of what they did to a server prior to deleting everything on it: http://imgur.com/a/R3ljb

Next to all servers that use Multicraft or WHMCS are vulnerable.

If you want to make sure you do not lose any data, make an external backup right now. I advice you to do it
Download your worlds now!

EDIT: I feel like it's time to address a few things:

I am aware I did not post a whole lot of proof. I'd love to post more, but I am not allowed to for legal purposes.
This is a PSA, not a vulnerability report. If you don't believe me, so be it. Your loss.
If you want to be safe Do it

From the reddit post: http://www.reddit.com/r/Minecraft/comme ... w_serious/