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Half a million former bot machines are at risk of reinfection or are still under cybercriminal control

It has been a week since a half-million bot-infected machines were suddenly freed from their "master" botnet servers after ISPs pulled the plug on the illicit McColo hosting service. So now what happens to those orphaned bot machines?

Researchers have spotted these errant bots over the past week attempting to phone home to their former command and control (C&C) servers. While the industry continues to celebrate a nearly 70 percent nosedive (albeit temporary) in spam volume without McColo to host the world's biggest spamming botnets anymore, these orphaned bots are still at risk -- and possibly still spewing spam, security experts say.

"They are probably already infected with multiple things. You hardly ever find just one bot on these computers," says Joe Stewart, director of malware research for SecureWorks. "You may find three or four different spam bots on the same machine. And who knows what else -- password stealers and other rogue ware."

Many of these bots -- which were members of the world's most prolific spam botnets, Srizbi, Mega-D, and Rustock "--are likely still spamming away for other botnets, or even possibly other servers on the big three that weren't hosted on McColo, security experts say.

Stewart notes that disconnected bots could even get reinfected by their former botnet. They are often infected with a Trojan that constantly checks in for updates via a malicious "pay-for-install" site. "It just downloads whatever binaries are put there by whoever's paying to get them installed," he says. So Rustock, for example, could pay for this service to reinfect its lost bots all over again, and update it with a link to a new C&C server.

The Shadowserver Foundation, a volunteer organization that gathers intelligence on the Internet's dark side, is building a so-called "sinkhole" server that poses as a defunct malicious domain server in order to find out what these machines leave behind. This could possibly help enterprises identify infected machines in their organizations, for example.

In general, most owners, especially consumers, of these infected machines are unaware that their desktop computer or laptop is under the control of cybercriminals; if they do know, aren't sure what to do about it.

Antibotnet appliance vendor FireEye this week posted instructions for cleaning up Srizbi bot-infected machines left behind in the McColo takedown. But some security researchers are skeptical that the McColo takedown is really an opportunity to clean up infected bots. First, the victims must be aware that they are infected. Second, consumer victims aren't likely to know how to clean their machines from the bot infection.

"It's a question of who is going to inform them that they need to clean it," SecureWorks' Stewart says. And even if researchers or vendors were to spot infected machines, they could report their IP addresses to the corresponding ISPs. "It takes a lot of time to clean up these machines, with multiple things infecting them...and it's difficult to do remotely," he says.

Michael Whitehurst, global vice president of customer support for Marshal8e6, says the botnets and spammers will recover, and likely soon. "If these bots don't have any instructions, they sit idle and wait. We're really curious about...when the bot-runners put in place new command and control, how easily the bots will be able to connect to them," Whitehurst says. "And how quickly spam volumes are going to ramp up."

The good news is that users with heavily infected machines eventually get them cleaned somehow if their performance suffers, or they just buy new ones. "Fortunately, these machines eventually go away," SecureWorks' Stewart says. "We're always seeing a stream of machines leave the botnet. But then the cycle continues with new bot victims."

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About the Author(s)

Kelly Jackson Higgins, Editor-in-Chief, Dark Reading

Kelly Jackson Higgins is the Editor-in-Chief of Dark Reading. She is an award-winning veteran technology and business journalist with more than two decades of experience in reporting and editing for various publications, including Network Computing, Secure Enterprise Magazine, Virginia Business magazine, and other major media properties. Jackson Higgins was recently selected as one of the Top 10 Cybersecurity Journalists in the US, and named as one of Folio's 2019 Top Women in Media. She began her career as a sports writer in the Washington, DC metropolitan area, and earned her BA at William & Mary. Follow her on Twitter @kjhiggins.

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