In a blog post, Blizzard revealed that it was aware of the exploit and that they have hotfixed it. Players would now be able play the game as usual as the exploit can't be repeated. "Earlier today, certain realms were affected by an in-game exploit, resulting in the deaths of player characters and non-player characters in some of the major cities. This exploit has already been hotfixed, so it should not be repeatable. It's safe to continue playing and adventuring in major cities and elsewhere in Azeroth." notes Nethaera, community manager, on Blizzard blog.
There was already chatter on a couple of hacker and gaming forums that the massive deaths was because of some kind of exploit. It is still not clear as to who was the person(s) responsible behind the hack.
Blizzard has said that it takes in-game exploits seriously and is currently conducting a thorough investigating. "As with any exploit, we are taking this disruptive action very seriously and conducting a thorough investigation", notes Blizzard.