Joe Stewart, director of malware research for Dell SecureWorks Counter Threat Unit, says Rustock hit the No. 1 spot thanks to its developers constant evolution of the code base. The rootkit-based malware can hide from antivirus software and employs several advanced techniques to evade detection.
Unlike in the past, bigger isn't necessarily a better strategy for a spamming botnet. Most botnets operators are keeping their botnets smaller to remain under the radar and to avoid takedown operations, such as those suffered by Mega-D, Waledac, and others. "The model seems to be fly-under-the-radar just to the point that no one cares about you," Stewart says. "These botnets have had good success at it," he says of those on the list of top spamming botnets he released today.
And there weren't any newcomers on the list -- most of the botnets have been around for several years. Cutwail is the second-largest, with 100,000 bots, followed by Lethic, 75,000 bots; Grum, 65,000 bots; Festi, 60,000 bots; and Maazben, 30,000 bots. The rest of these spamming botnets have anywhere from 5,000 to 30,000 bots at a time, and they include Asprox, Fuflo, Waledac, Fivetoon/DMSSpammer, Xarvester, Bobax, Gheg, and Bagle.
Noticeably missing from this year's list is the infamous Mega-D botnet, also known as Ozdok, which researchers at FireEye were able to successfully disrupt in 2009. The alleged botnet's operator was arrested last year, and Mega-D has not been heard from since, according to Stewart's research. "Mega-D is officially gone," Stewart says. Waledac, which was the focus of a large takedown effort last year, is still showing some signs of life.
Bobax/Kraken is an example of a big botnet that's lost some weight but remains strong: It's about a tenth of the size it once was, but is still operational, Stewart says. "They are still able to spam and make money," he says.
The spamming botnet space is basically a mature market, he says. "We've got established players in this game, and they've figured out the model," Stewart says. "Newcomers don't seem to be attracted to SMTP-based spam. That ecosystem is not growing, but spam is not going away."
To be a big botnet like Rustock and not attract a takedown effort takes a lot of effort. Rustock's developer has added new techniques to the codebase to keep the botnet alive and well, such as having samples wait up to five days before spamming; having its command-and-control (C&C) servers run a TOR exit mode; using HTTP for communicating between the bots and C&C servers and disguising the HTTP requests to look like online forum posts, with encrypted content; and masking IP addresses.
SecureWorks' Stewart has experienced first-hand the frustration of trying to track Rustock. "Some of the [evasion techniques] have been a pain for me," he says. "They've tripped me up on occasion, and I've had to write custom code to get around it. They know people like me are out there watching."
The most popular seeding mechanism for the biggest spamming botnets is pay-per-install tactics and viruses. The Festi botnet also wages distributed denial-of-service (DDoS) attacks, including one against the personal websites of the operator of Russian online payment processor ChronoPay, Stewart says. "It's not afraid to get and out cause some havoc," Stewart notes.
"If you want to make a living at it, you can run with 20,000 to 30,000 bots, and people generally aren't going to pay attention" to you, Stewart says. "If you want to be the biggest spammer in the world, it's going to take a lot more effort" to remain alive without attracting too much attention, he says.
SecureWorks' previous list of the top spamming botnets was two years ago, and that list looked like this: 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.
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