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Sophisticated peer-to-peer (P2P) botnets like Storm that have no centralized command and control architecture have frustrated researchers because they're tough to dismantle. But a group of European researchers has come up with a way to disrupt these stealthy botnets -- by “polluting” them.

The researchers, from the University of Mannheim and the Institut Eurecom, recently infiltrated Storm to test out a method they came up with of analyzing and disrupting P2P botnets. Their technique is a spinoff of traditional botnet tracking, but with a twist: it not only entails capturing bot binaries and infiltrating the P2P network, but it also exploits weaknesses in the botnet’s P2P protocol to inject “polluted” content into the botnet to disrupt communication among the bots, as well as to study them more closely. The researchers tested their pollution method out on Storm -- and it worked. They presented their research this month at Usenix.

“Our measurements show that our strategy can be used as a way to disable the communication within the Storm botnet to a large extent,” the researchers wrote in their paper. “As a side effect, we are able to estimate the size of the Storm botnet, in general a hard task... Our measurements are much more precise than previous measurements.”

Their Storm stats: the researchers crawled Storm every 30 minutes from December of last year through February of this year, and saw between 5,000 and 40,000 machines online at a time. Not surprisingly, the Christmas and New Year’s holidays accounted for a big jump in numbers. And the U.S. has the most Storm bots, with 23 percent, according to the researchers, who said they spotted Storm bots in 200 countries.

The researchers also tested another P2P mitigation method called an “eclipse attack,” which basically aims to separate a segment of the P2P network from the main body by luring it to their phony bots, but it didn’t work.

The pollution attack, meanwhile, “overwrites” the P2P botnet’s key, an identifier that’s used to get command information to the bots. Storm generates keys to find other bots, the researchers noted. “Since the Storm bots continue to publish their content as well, this is a race between the group performing mitigation attempts and the infected machines,” the researchers wrote. “Our experiments show that by polluting all those hashes that we identified to be storm hashes, we can disrupt the communication.”

Jose Nazario, a security researcher with Arbor Networks who has studied Storm, says the pollution technique isn't a new concept, but the researchers may be among the first "to expose such a methodology publicly," he says. "This has been a taboo subject of exploration, as people do not want to mess with other peoples' PCs by injecting commands," he says.

Bottom line: the so-called “publish/subscribe” type of communication used by Storm and other P2P botnets is vulnerable to this type of exploitation.

Not surprisingly, getting to the operators behind Storm wasn’t so simple. Storm’s two-tier architecture -- tier one being the P2P networks Overnet and Storm itself, and tier two, the better-hidden computers that send the actual commands -- made that difficult. “In future work, we plan to analyze in detail the second-tier computers and try to find ways to identify the operators of the Storm Worm,” the researchers said in their paper.

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