A hacker yesterday exploited a glitch allowing him to create level 1 characters, fly around, and target and /kill players on a whim. WoW Insider has an article covering it and claims it’s probably the largest mass killing since the Corrupted Blood exploit. A GM has stated in the forums that it has since been hotfixed and is no longer a threat to players. The source of the hack most likely came from Ownedcore, a website famous for it’s WoW exploits, private servers, etc.
Check out this screenshot to see the extent of the damage on one of many servers effected: