To keep things secret, the criminals hadn't registered these domains, but they had coded several hundred of them into their botnet software. But the researchers learned these domain names by looking at the botnet code to find out what the hacked computers would do when McColo went down. Shortly before the McColo network was knocked offline by Global Crossing and Hurricane Electric, researchers registered the hundreds of backup domains themselves.

When the botnets couldn't go to McColo's IP (Internet Protocol) space for instructions, they started looking for their backup domains, but these were controlled by security researchers. Now, disconnected from their control servers, and unable to connect to a backup, two of the Internet's worst botnets, and , have been decapitated.

"There have got to be hundreds of thousands of bots out there that aren't phoning home right now" said Joe Stewart, a botnet expert with SecureWorks who has tracked the McColo situation.

These bots might well be disabled for good, provided McColo's computers do not get brought back online. But that's exactly what happened a week ago, when a reseller of Swedish ISP TeliaSonera reconnected McColo temporarily.

The mistake was quickly noted, and TeliaSonera quickly disconnected McColo. But security vendor FireEye reckons that the bad guys were able to regain control of thousands of botnet computers during this brief window of opportunity. When McColo went back on the Internet, its IP address space worked again and cybercriminals were able to send instructions to their botnet computers. They would not have been able to do this had the FBI been able to shut down McColo's San Jose, California, data center, as it did with Creative Internet.