Dismantling a Botnet

Just because you arrest a spam kingpin doesn't mean his botnet is history. (See Spamless in Seattle.)

The arrest of Robert Soloway, the so-called "Spam King" who commanded a botnet of zombies that reportedly sent out billions of spam email messages every day, is a case study in just how difficult killing a botnet can be. His capture does not equate to the release of thousands of newly healthy client machines that may once have been his infected hostages.

"You are basically cutting a tapeworm in two. The infrastructure is still there, and it can be picked up by anyone who can find it or knows where it is," says Ira Winkler, author of Zen and the Art of Information Security. "Authorities might be able to see what servers he connects to that command the rest of the bots, but it is unlikely that they will kill all of the bots."

Researchers say Soloway had his own botnet for spamming -- not for launching denial-of-service attacks like some botnets do, nor was it part of one of the infamous botnet "gangs" out there. And everyone was watching him and his movements. "The botnet wasn't terribly sophisticated, but it was custom enough that it sort of stood out," says Jose Nazario, software and security engineer for Arbor Networks. "It seemed to be primarily his own botnet, and he [probably] had a couple of guys in contract helping him out. People had their eye on him for quite a while."

Even if authorities try to shut down his botnet, there are plenty more wanna-be spam kings and botherders waiting in the wings who probably already are snapping up the infected bots Soloway used, researchers say. "There's always someone there ready to fill the void," says Joe Stewart, senior security researcher for SecureWorks. "I don't expect to see a decrease of spam in my inbox."

Even if Soloway's bots are freed, the machines are likely still infected, so another botherder can re-hijack them for his own botnet. "It's easy to steal someone else's bots," Arbor's Nazario says. Even scarier is if your machine just so happened to be one of a Soloway bot, it may already have been recruited as a member of another botnet and you wouldn't even know it, according to Nazario.

So how do you dismantle a botnet? It's no easy task, and it requires infiltration of the botnet. SecureWorks's Stewart says the most effective way to take down a botnet is to go after the actual hands-on operation, and that's not the spam king. "There's always a central server or some sort of central control mechanism, even if it's a peer-to-peer network. Someone has the keys to control it all."

That means taking down the command and control, or master, server. "Now all the zombies are dead and defunct because they can't send mail if they don't have contact with their master server," he says. For peer-to-peer botnets -- a distributed setup where each bot can send its commands on its own so it's difficult to pinpoint the source of the command and control -- you have to "convince" the controller to shut down the botnet operation, he says.

"There's usually a way to build in an update to their software to convince them it's time to shut down," Stewart explains. "If you take away that person and their controlling of the botnet and get a list of all these infected machines before they are resold," then you have a shot at taking it down.

The key is chipping away at the guys in the trenches. "It's good to get rid of a spam kingpin. But you need to know where he's getting his services from," Stewart says.

Trouble is, many of the worker-bee spammers are in countries like China and Russia, out of the reach of U.S. law enforcement, Stewart says. "It's a global economy here in the spam underworld."

And investigators and ISPs are often in a quandary over whether to shut down a botnet altogether for fear of inadvertently sabotaging an investigation of a researcher or another law-enforcement official, for instance. (See Battling Bots, Doing No Harm.)

Winkler says to truly dismantle a botnet, you need to not only find the servers that the root commands, but also find the bot-infected end systems themselves. "If you observe the behavior of the bot servers to see what systems they connect to, you can then try to get the ISPs to tell the owners to clean up all the systems, which is the most work."

Nazario says he doesn't have any firsthand knowledge that law enforcement officials are completely killing the Spam King's botnet. That would take studying the tools and techniques he used, and then the monumental task of getting antivirus and OS vendors to provide signatures to remove his malware. "That would be a major event," he says. "They [vendors] have so many things on their plate that they have to triage and prioritize them."

Shutting down a botnet usually requires not only taking down command and control servers, but with a quick, coordinated effort of officials around the globe, says Kris Kendall, principal consultant for Mandiant. "That's a hard problem," he says. "Battling botherders on their own turf is challenging."

Investigators into the Soloway botnet may instead want to monitor it and see who's using it and how. "They could sit where he would and watch who's using it and follow them," Nazario says. "That way they could find out more about the spamming underworld and his underworld."

So the bots from Soloway's botnet aren't in the clear yet. His arrest was really more symbolic: "They were really looking to send a message to other spammers that they are being watched and can be caught. Until we start seeing a large number of these arrests on fairly regular basis, it's not going to have a significant impact," Stewart says.

— Kelly Jackson Higgins, Senior Editor, Dark Reading