Botnets' Landscape Changes as Spammers Get Back in the Swing of Things

Spammers have been hard at work at regaining their past momentum. Over the past year, the botnet landscape has changed, especially since the McColo shutdown.

It's been roughly two months since the much-heralded shutdown of McColo, yet spam levels have remained below where they were previously.

While the amount of spam hitting enterprise networks is building as botnet operators regain their momentum, the botnet landscape has changed significantly. Some of the former kings of the hill, botnets such as Srizbi, were badly hurt by the shutdown.

In its place, botnets such as Cutwail have gained steam. According to SecureWorks, Cutwail now has 175,000 compromised computers under its control, and is the top botnet to watch in 2009. Behind it on SecureWorks' list is Rustock, which still claims 130,000 bots. The lesser-publicized Donbot is third with 125,000 bots.

"Rustock seems to be using more domain names instead of hard-coded IP addresses," Joe Stewart, director of malware research at SecureWorks, told eWEEK. "Srizbi hasn't done anything; it's still down. The others weren't really impacted for very long."

The list is further rounded out by Ozdok, Xarvester, Grum, Gheg, Cimbot and Waledac. Together, those botnets are responsible for at least 90 percent of spam, Stewart said.

Depending on whom you ask, the amount of spam declined 50 to 70 percent in the wake of the McColo shutdown in November. Two months later, researchers at McAfee and Symantec say spam levels have still not fully recovered, though their figures differ somewhat. Symantec says the amount of spam is now at 80 percent of its pre-McColo level, while McAfee puts the number at 60 percent.

"Spam levels haven't risen back up likely because spammers have not been able to relocate their services to other spam-friendly ISPs [internet service providers]," said Dave Marcus, director of research and communications at McAfee Avert Labs. "They weren't prepared to be taken offline and now may be scratching their heads as to how to build more resilient operations. Our hope is that it will now be more difficult for spammers to find other ISPs."

Some security researchers expect botnet operators to change tactics. Botnet operators, they said, will likely look to have multiple command and control servers to avoid being affected severely by takedowns similar to what happened to McColo and Intercage/Atrivo in 2008.

"The McColo shutdown was a huge blow for spammers," said Dermot Harnett, principal analyst of anti-spam engineering at Symantec. "Spam volumes will continue to creep back up because the profit motive still exists for spammers. As long as spammers continue to see a return on their investments they will become even more determined to ensure that spam reaches its pre-McColo shutdown levels before too long."