Eight months ago, the Grum botnet was estimated to be the largest in the world, pumping out a third of the global volume of spam email. But things changed over the following six months as Atif Mushtaq, senior staff scientist at security firm FireEye, noticed an abrupt drop in the number of active command-and-control servers — the network's nerve centers — and sensed the perfect time to mount an offensive. Thanks to a coordinated, global attack, the infestation shriveled to 20,000 zombies; infected computers awaiting instructions that will never come. TechCrunch takes an informative look at how Grum operated, and how a weakness in its code allowed some determined spam fighters to take it permanently offline.