Some thieving crooks have started targeting OS X users with something called ransomware. Basically, you will get a notice claiming to be from the FBI. It says that you must pay $300 so that you can use Safari again.
OS X Malicous Code in Safari
The notice is something like this:
you have been viewing or distributing prohibited Pornographic content.. To unlock your computer and to avoid other legal consequences, you are obligated to pay a release fee of $300.
We haven’t encountered this ourselves. We hope we don’t; it looks annoying. The best thing you can do when this happens is to reset Safari. Do this in the Safari menu. All items in the “Reset Safari” menu should be checked off. This will clear history, cache … everything. When you relaunch Safari, it won’t start where you left off.
Also, MacRumors suggests an even easier way. Hold down the Shift key while you relaunch Safari. This will ensure that the software doesn’t launch with your last window open. Let’s hope these folks a shut down quickly.
The other solution is one of these guys over the Enter key :)
For more details on this OS X Safari exploit, check out the MalwareBytes blog.