SecureWorks Unveils Research on Spamming Botnets

SAN FRANCISCO -- RSA 2008 Conference -- The world’s largest spamming botnets can send over 100 billion spam messages a day, according to a newly released report by SecureWorks.

In an interview here, Joe Stewart, director of malware research for SecureWorks, also disputed that the massive "Kraken" botnet revealed by Damballa earlier this week is new. Kraken is actually the existing Bobax botnet, which SecureWorks ranks number two among spamming botnets, he said. (See New Massive Botnet Twice the Size of Storm.)

Damballa on Monday released details on Kraken, which it says is an all-new botnet that's twice the size of Storm, with 400,000 bots, including machines in 50 of the Fortune 500 companies.

But Stewart, who headed up SecureWorks's study, says Kraken and Bobax share common strings and functionality at the core, so they are the same botnet. "A lot of the code has changed over time; features have been added and removed. But at the core, it is the same code project, maintained by the same author [or authors] this entire time," he says.

Damballa, meanwhile, maintains that Kraken is indeed a distinct botnet and not Bobax. Bill Guerry, vice president of product management for Damballa, said today that his company has previously performed analysis on Bobax, and that the two botnets have distinct command-and-control characteristics. Damballa plans to release more technical details on Kraken later today to help confirm this assertion, he says.

SecureWorks's Stewart said yesterday that part of the discrepancy in identifying the botnets may have to do with the fact that virus writers don't label their code with one static name. "So we, as researchers, are left with naming things based on what we can see. There's no central source of information on what malware contains what strings, and we can only rely on what we've seen ourselves, or the scraps the AV companies give us," he says.

Another issue may be that SecureWorks and Damballa each took a slightly different look at botnet size in their research: SecureWorks's study focused specifically on botnets that send spam and their malware, while Damballa looked at the overall number of bots and the command-and-control makeup of Kraken, regardless of the type of payload it was sending.

"From what I can tell, Damballa is counting bots that are resolving the command-and-control network name and trying to connect on TCP port 447. We are counting only the part of the botnet that can send spam," Stewart notes. "Neither count can ever be a true count of the botnet size, because just like many firewalls/ISPs will block direct-to-MX outbound connections from their users, many corporate firewalls will block TCP port 447 outbound, as no known-good protocols traverse that port."

Damballa's Guerry says Kraken uses a different command-and-control than Bobax, running over an encrypted and custom TCP and UDP connection over Port 447. And there are often similarities among malware that botnets use. "When you look at malware, there are always similarities," he says (see video). .

SecureWorks's list of the top spamming botnets, in order of size is: Srizbi, with 315,000 bots; Bobax, with 185,000 bots; Rustock, with 150,000 bots; Cutwail, with 125,000 bots; Storm, with 85,000 bots (only 35,000 of which send email); Grum, with 50,000 bots; OneWordSub, with 40,000 bots; Ozdok, with 35,000 bots; Nucrypt, with 20,000 bots; Wopla, with 20,000 bots; and Spamthru, with 12,000 bots.

Srizbi, which uses encrypted command and control to send a variety of spam, can send up to 60 billion spam messages a day, according to SecureWorks, and is also known as Cbeplay and Exchanger. It's infamous for its advertising links to porn-related video files of celebrities, which are really malware that aims to infect PCs and recruit them as bots.

Bobax -- aka Bobic, Oderoor, Cotmonger, and Hacktool.Spammer -- can send about 9 billion spams per day, and is also encrypted. It used to send mostly mortgage spam, according to SecureWorks, but is doing more low-interest loan spam.

Interestingly, Storm -- aka Nuwar, Peacomm, and Zhelatin -- came in fifth place in size, spamming at a rate of 3 billion a day, according to the SecureWorks report. Its capacity is limited by the fact that only machines behind NAT firewalls send spam, while the other bots are used as fast-flux HTTP and DNS hosts for the spamming system.

Stewart noted that the spamming botnets are all starting to routinely use rootkits to keep bots infected as long as possible. Srizbi, for instance, uses rootkits. "Srizbi is big and nasty and hard to [detect]," he says.

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