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Microsoft: We took out Storm botnet

Its malware scanner cleaned more than 500k PCs infected with the bot in '07

April 22, 2008 12:00 PM ET

Computerworld - Microsoft Corp. today took credit for crushing the Storm botnet, saying that the malware search-and-destroy tool it distributes to Windows users disinfected so many bots that the hackers threw in the towel.

"They realized they were in our gun sights," said Jimmy Kuo, a principal architect with Microsoft's malware protection center, the group responsible for the Malicious Software Removal Tool (MSRT). Microsoft updates and automatically redistributes the software tool to Windows users each month on Patch Tuesday.

Last year, said Kuo, the criminals behind the Storm Trojan -- malware designed to compromise PCs and add them to a botnet, or collection of infected machines -- tried to keep pace with Microsoft and the MSRT. "They were anticipating our monthly release [of MSRT]," said Kuo, "with new versions that were ready to go immediately before our release."

The bunch controlling the Storm botnet knew that it took Kuo's group several days to create new definitions for the MSRT, and that Microsoft held to a once-a-month release schedule for the tool. And they used that lag time and set schedule to their advantage.

"They knew that it takes [us] a week or more to create new definitions, and they were prepared to update their botnet immediately prior to MSRT releasing," he said, adding that the hackers would get a new version of the Trojan onto already-infected members of the Storm botnet to try to hold on to the machines after Windows had downloaded the newest version of the MSRT.

The idea was to preempt detection by swapping out the Storm bot already on the PC with a version less likely to be identified by the MSRT.

It didn't work, said Kuo. "They found out that even that was a losing battle," he said. "Even though they were able to maintain parts of their botnet, they knew they were in our gun sights. And ultimately they gave up."

According to Kuo, it was the hammering Microsoft gave the Storm botnet that sent the hackers packing.

In the last four months of 2007, the MSRT disinfected more than 526,000 PCs plagued by the Storm bot, he claimed. The bulk of those -- more than 291,000 -- were cleaned in September, when Microsoft first added Storm detection to the MSRT. In October, the number dipped to around 90,000, then bounced back to about 100,000 each month in November and December. The front-loaded numbers, said Kuo, were typical, since the first month that the MSRT has a new malware definition, the tool cleanses all machines that have ever been infected. In the following months, it can only disinfect PCs that have been infected since the last release of the tool.

Storm, which first appeared in early 2007 -- and got the moniker because it was first disseminated in spam messages that claimed to have news of a massive series of winter storms that swept Europe -- has been linked to the Russian Business Network (RBN), a shadowy network of malware and hacker hosting services once based in St. Petersburg.

Others have confirmed Storm's decline, and credited Microsoft.

Earlier this month, Joe Stewart, the director of malware research at SecureWorks Inc., unveiled research on the world's top 11 botnets, and using SMTP "fingerprinting" and traffic extrapolations, estimated the size of each of those spam-sending botnets. Storm, said Stewart, was No. 5 on that list of 11 and likely controlled about 85,000 PCs -- a far cry from its height in 2007 and about one-fourth as many as the leading botnet, Srizbi.

"Storm is pretty insignificant at this point," Stewart said in an interview two weeks ago. "It got all this attention, so Microsoft added it to its malicious software detection tool [in September 2007], and that's removed hundreds of thousands of compromised PCs from the botnet."

But while Kuo was happy to take the credit on behalf of Microsoft for shrinking Storm, he was realistic about the overall impact.

"What we did was to drive them [the Storm bot herders] elsewhere," he said. "They're probably out there still making money with some other botnet."

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