Botnet Postmortem: Rustock Now Less Than Half The Size It Was At Takedown

It has been four months since Microsoft and federal authorities knocked the prolific spamming botnet Rustock offline, and some 700,000 of the estimated 1.6 million bots worldwide are still infected with its malware. A new Microsoft report published today based on intelligence it gathered from cleanup and removal of Rustock from infected bots also shows that Rustock-infected machines are typically riddled with other malware, as well.

Rustock, which was able to send some 30 billion spam email messages each day, including phony prescription drugs and fake Microsoft lottery scams, was crippled after seven Internet hosting locations in the U.S. were raided in the takedown operation in March. FireEye, researchers at the University of Washington, Pfizer, the Dutch High Tech Crime Unit, and the Chinese CERT all assisted in the operation.

Microsoft's Digital Crimes Unit and MMPC ran an experiment with the Win32/Rustock malware family of rootkit-enabled backdoor Trojans and found that within five minutes, multiple malware and unwanted software was downloaded onto a Rustock-infected machine. That confirmed Microsoft's suspicion that Rustock bots were likely infected with more than just Rustock's malware, Richard Boscovich, senior attorney for Microsoft Digital Crimes Unit, said in a blog post today.

The experiment used Win32/Harnig, a Rustock dropper, used for getting bots. "Within five minutes of installation, a wide variety of additional malware and potentially unwanted software had been downloaded and installed onto the infected computer -- and many of these threats are themselves designed to eventually download even more malware," Boscovich said.

Among the 19 other malware programs that had infested the machine was rogue adware, spyware, various Trojan downloaders, and a worm.

Meanwhile, Microsoft says the number of Rustock bots has been cut by 56.12 percent, with the most bots in India, with 99,032; followed by the U.S., with 55,731; Turkey, with 50,465; Italy, with 32,041; Russia, with 27,535; Germany, with 25,318; Brazil, with 21,967; France, with 21,625; Mexico, with 19,064; and Poland, with 18,015.

Russia had the highest rate of reduction in victims since Rustock was first taken down, with 70.61 percent of its machines getting cleaned up, followed by India, with 69.3 percent, and Brazil, with a reduction of 53.24 percent.

"In short, since the time of the initial takedown we estimate the Rustock botnet is now less than half the size it was when we took it down in March. That’s great news, and the infection reduction has happened much more quickly than it did for Waledac over a similar period of time last year, but we still have a long way to go," Microsoft's Boscovich said in his post.

Meanwhile, Microsoft Digital Crimes Unit researchers saw one Rustock bot send 7,500 spam emails in 45 minutes -- a rate of 240,000 spam emails a day.

A copy of the full report, "Special Edition Security Intelligence Report: Battling the Rustock Threat," is available for download here.

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