As of Thursday, more than 60,000 PCs worldwide that have been infected with malicious code are now under the control of researchers, marking the effort one of the most highly successful coordinated against organized cybercrime.
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Microsoft revealed on Wednesday that it gained a court order that compelled VeriSign, the .com registry, to remove 277 ".com" names from its rolls, effectively cutting off communication between the Waledac's controllers and their infected machines.
The legal action is unprecedented at the domain name level, said Andre' M. DiMino, co-founder of The Shadowserver Foundation, a group that tracks botnets and helped take down Waledac. In June 2009, a federal court ordered the shutdown of 3FN, a rogue ISP supplying connectivity to botnets such as Pushdo and Mega-D, but this appears to be the first major action at the domain-name level.
"It's definitely pretty groundbreaking," DiMino said. "To disable and disrupt a botnet at this level is really pulling the weed out by the root."
But behind the scenes, Microsoft's legal action was just one component of a synchronized campaign to bring down Waledac.
Last year, researchers with the University of Mannheim in Germany and Technical University Vienna in Austria published a research paper showing how it was possible to infiltrate and control the Waledec botnet. They had studied Waledac's complicated peer-to-peer communication mechanism.
Microsoft -- which was annoyed by Waledec due to its spamming of Hotmail accounts -- contacted those researchers about two weeks ago to see if they could perform their attack for real, according one of the University of Mannheim researchers, who did not want to be identified.
"They asked me if there was also a way besides taking down those domains of redirecting the command-and-control traffic," said the Mannheim researcher.
Waledac distributes instructions through command-and-control servers that work with a peer-to-peer system. Led by a researcher who did his bachelor thesis on Waledac, the action began early this week.
"This was more or less an aggressive form of what we did before," the Mannheim researcher said. "We disrupted the peer-to-peer layer to redirect traffic not to botmaster servers but to our servers."
At the same time, Microsoft's legal efforts brought down domain names that were used to send new instructions to drones.
The result has been dramatic: Up to 90 percent of the infected machines, which amount to at least 60,000 computers, are now controlled by researchers, half of which are in the U.S. and Europe and the rest scattered around the globe.
Another researcher at Technical University Vienna who studied the Storm worm as part of his doctorate's degree said "still we were surprised that the attack ... was so successful."
Waledac is suspected to have been created by the same people as the Storm worm, which infected millions of computers with spam-spewing software starting in 2007.
DiMino said the next step is to notify the network providers associated with those infected computers. Then, those ISPs can alert their customers than their computer is infected and need an antivirus scan.
"The work is not finished yet," DiMino said. "We are going to continue to work together to clean up those drones."
As of Thursday, Shadowserver had assisted in taking down one of two remaining Waledac servers, with the last one is expected to fall soon.
Also involved in the operation were researchers from the University of Bonn and Symantec.