Microsoft Tuesday said the coordinated take-down of the Rustock botnet and follow-up efforts had purged the malware from over half of the PCs once controlled by Russian hackers.
"This shows that disruptive action [against botnets] is viable and possible," said Richard Boscovich, a senior attorney with Microsoft's Digital Crime Unit.
"Once you start taking apart the infrastructure of botnets, you drive up the cost of [botnet gangs] doing business," Boscovich added in an interview Monday. "Disruptive action is just as good as trying to arrest someone."
Since March, when Microsoft lawyers and U.S. Marshals seized Rustock command-and-control (C&C) servers at five Web hosting providers in seven U.S. cities, the number of Windows PCs infected with the malware has dropped worldwide from 1.6 million to just over 700,000 as of June 18, Boscovich reported in a blog post today.
Microsoft also released a detailed report on Rustock, the take-down effort it led, and the impact of its anti-botnet campaign ( download PDF ).
In the U.S., an estimated 86,000 Rustock-infected PCs in March had been reduced to some 53,000 by June, a drop of 38%. Other countries saw even bigger reductions: In India, the March tally of 322,000 infected machines plummeted by 69% to approximately 99,000 in June.
The take-down itself didn't remove the Windows PCs from Rustock control. Instead, the seizure of the U.S.-based C&C servers and Microsoft's work to snatch control of the domains that Rustock was coded to use for fallback communications, prevented the botnet from updating itself.
That in turn provided the breathing room antivirus vendors needed to issue signatures for the existing Rustock malware and users the opportunity to scrub their systems with security software.
Microsoft, for instance, has provided Rustock signatures for its Malicious Software Removal Tool (MSRT), a free utility that detects and deletes malware, since 2008.
The take-down of Rustock's communications channels effectively silenced the botnet.
Since March, the botnet -- which was once one of the largest purveyors of spam, particularly pitches for fake drugs -- has been quiet. "Botnet activity dropped abruptly to almost zero in mid-March following the take-down," Microsoft said in its report.
Prior to the take-down, Rustock was capable of sending as many as 30 million spam messages daily.
"Cleaning the users' PCs is an important part, but really this shows that a technical countermeasure along with a legal countermeasure works," said Boscovich, talking about the two-pronged approach of seizing servers and shutting down Rustock's backup communications.
And the impact goes beyond Rustock.
"The minute you take down Rustock, what does that do to those who want to send spam?" Boscovich asked. "They have to find other botnets. But if you're a botnet herder, and you just saw Rustock go down -- with years of work coding and planting malware and maintaining the botnet -- you're going to charge more. And that's an impact on spammers' cost analysis, as it becomes more and more expensive to send out spam."
Statistics from Symantec seemed to prove Microsoft was on to something.
In its June report on spam and malware trends, Symantec said that spam levels had not recovered from the Rustock take-down, and in June accounted for 72.9% of all email, down from 83.1% in March.
Alex Lanstein, a senior engineer with FireEye who worked with Microsoft on the Rustock take-down, said the numbers spoke for themselves. "The spam drop is a direct result of the take-down," Lanstein said Monday.
But Symantec also said there was evidence that another botnet, dubbed "Grum," had stepped in to partially replace Rustock. The security firm cited such factors as similar subject lines, sending domains, a change in character sets by Grum just hours after the Rustock take-down and similarities in the two botnets' distribution patterns ( download PDF ).
So are botnet take-downs just a game of "Whack-a-Mole," where bashing one botnet only sees it replaced by another?
"I think that's foolish to say," Boscovich said. "If you don't take action, what do you do, sit and watch it happen? This weeds out the smaller players, who decide that they can't afford the higher costs of sending spam. If everyone started doing more proactive work like [take-downs], we really would be able to take down a lot of players, and disrupt the entire spam ecosystem."
"It's not nitpicking, but there are always a lot of naysayers who play up the negative angle," added Lanstein. "The lasting impact comes from how much you follow through."
And in his eyes, Microsoft is committed to the battle long term. "[Microsoft] is bringing the fight to the bad guys," said Lanstein. "This is definitely not the last botnet we're going to go after."
Microsoft has not yet identified the presumed head of the Rustock botnet gang, but last week a federal judge granted its request to extend its effort to notify unnamed defendants in the lawsuit, a legal formality designed to give potential defendants an opportunity to respond to charges.
Microsoft believes the Rustock operator resides in either St. Petersburg or Moscow in the Russian Federation; last month it published legal notifications in those cities' newspapers.
Although Boscovich said it was unlikely the defendants would step forward, he sounded confident that, with the information on the seized servers and other investigations, someone would be held accountable for Rustock.
"We think we can do some more to identify the individuals to zone in on the actual defendants in this case," Boscovich said. "I believe there's a strong likelihood [that we'll identify someone], but it's not a guarantee."