The researchers who successfully shut down much of the Pushdo botnet's infrastructure last week didn't go in planning to take down a large chunk of the botnet -- that was a secondary but major byproduct of some related botnet research they were conducting.

Thorsten Holz, senior threat analyst at LastLine and assistant professor of computer science at Ruhr-University Bochum, Germany, says he and his colleagues were working on a research project involving various botnets, including Pushdo, MegaD, and Rustock, matching infected IP addresses with their respective botnets. They decided to they needed C&C servers to evaluate an algorithm they were developing for the project, which ultimately led them to decide to take down some Pushdo C&C servers to assist their research, he says. "Pushdo's command and control infrastructure turns out to be pretty vulnerable to takedown efforts, so we identified the C&C servers in eight different hosting providers," Holz says. "It was the ideal target to get the servers down and analyze the data."

"It was not our goal to completely take down the entire botnet. We were looking for insights into it to learn more about command and control servers," he says.

The Pushdo C&C servers provided just the data the researchers needed to test their new tool. "It was unclear to what extent we [could] disturb the Pushdo operation, and we were positively surprised that it worked that well," Holz says.

The effort ended up successfully killing 80 percent of the botnet's spam production, according to estimates by Joe Stewart, director of malware research for the counter threat unit at Secureworks. But like with most botnets, the near-takedown is likely only temporary, he says. "I'm pretty sure this is short-term. These [Pushdo] guys have been doing it for awhile," Stewart says. "It's easy for them to stand up new servers, and [get] different hosting locations where they can put their servers."

Holz and his fellow researchers contacted the domain providers that they found were hosting Pushdo's botnet's 30 command and control servers, and got all but a few of them to pull the plug on the servers after providing them with evidence of the botnet operation. While nearly 20 of the servers were taken offline, as of today the botnet still appears to be spamming with a skeletal C&C infrastructure, according to security experts.

The Pushdo, aka Cutwail, botnet is infamous for being a large-scale spammer as well as a purveyor of malicious code such as Zeus and other Trojans for stealing online banking credentials. Last Line's Holz says he and his team found that the botnet had several hundred thousand bots, and two-thirds of its C&C servers were housed in Europe, with a few in the U.S. and in Russia as well. The researchers declined to name the hosting providers that helped -- as well as those that did not -- in the takedown so as not to tip off cybercriminals.

"We focused on Cutwail, the spam component for now," Holz says. "We are still in contact with the other hosting providers to get additional machines taken offline, and we then need to analyze the data to understand what we found and what additional information we can expect from there and how to leverage the data."

The researchers ultimately plan to analyze and report on all of these findings, he says. "We also plan to hand over data to ISPs to use to remediate infected machines," Holz says.

It was relatively simple to disable Pushdo's C&C servers because many of them were located in a small number of networks, he says.

Pushdo uses a proprietary communications protocol for its servers and bots to converse with one another and the botnet is now using some encryption as well, Holz says. "The infected machine contacts Pushdo on Port 80" to get its instructions, he says.

Pushdo is the latest in a series of botnet takedown efforts this year -- Holz and colleagues teamed up in February with Microsoft, Shadowserver, the University of Washington, and Symantec in the the takedown of the Waledac botnet, and soon thereafter PandaLabs, along with the FBI, Defence Intelligence, and Georgia Tech, revealed how they had derailed the Mariposa botnet in December of last year.

But takedowns rarely last. Botnets often get reinvented and spam levels eventually rebound as the bad guys pull up stakes and move somewhere else. Holz says he expects the same will occur with Pushdo.

What's the missing link to keeping crippled botnets offline for good? "Put everybody [involved] in jail," Secureworks' Stewart says. "If you don't do that, you're just trying a technical fix to the problem ... They will be back, because there's lots of money in this for them."

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