Microsoft and Symantec Wednesday announced that they took down the Bamital botnet, which was used by its controllers to amass an estimated $1 million or more per year via click fraud. As part of the takedown effort, dubbed "Operation b58," employees from both technology companies -- escorted by U.S. marshals -- seized equipment from hosting facilities in New Jersey and Virginia.

"Based on the successes of prior botnet operations, Microsoft and Symantec used a combined legal and technical action to take down Bamital," said Richard Domigues Boscovich, assistant general counsel for the Microsoft Digital Crimes Unit, in a blog post.

Boscovich told Reuters, which first reported the news of the takedown, that he had "a high degree of confidence" that the botnet takedown would spell the end of Bamital. "We think we got everything, but time will tell," he said.

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The takedown kicked off with Microsoft filing a lawsuit against the Bamital botmasters and requesting that it be allowed to seize their U.S. servers. "The court granted Microsoft's request and on February 6, Microsoft -- escorted by the U.S. Marshals Service -- successfully seized valuable data and evidence from the botnet," said Boscovich. "The evidence was taken from Web-hosting facilities in Virginia and New Jersey."

Microsoft's related complaint was filed in U.S. District Court for the Eastern District of Virginia on January 31, 2012, supported by a declaration from Symantec, against "John Does 1-18, controlling a computer botnet thereby injuring Microsoft and its customers."

The 30-page civil complaint, unsealed Wednesday, accused the defendants of violating the Computer Fraud and Abuse Act by illegally accessing Microsoft customers' computers. It also accused them of violating the Electronic Communications Privacy Act by having "knowingly and intentionally accessed Microsoft customers' computers and Microsoft's computers and servers without authorization," interfering with people's search engine queries, and intercepting people's personal information.

In the accompanying 162-page complaint appendices, also unsealed Wednesday, Microsoft provided some additional details for the 18 John Does that it said ran the botnet, who were based in Russia, Romania, Australia, Great Britain, and the United States. The court documents also listed thousands of IP addresses that Microsoft said were the command-and-control (C&C) server addresses used for the Bamital botnet, which Microsoft said had been registered using fake names.

According to Symantec and Microsoft, in the past two years more than 8 million PCs have been infected by Bamital malware, which perpetrated a click-fraud scheme that rerouted legitimate requests to major search engines, including Google, Microsoft Bing and Yahoo, to attacker-controlled sites. "From analysis of a single Bamital C&C server over a six-week period in 2011 we were able to identify over 1.8 million unique IP addresses communicating with the server, and an average of three million clicks being hijacked on a daily basis," read a blog about the takedown posted by Symantec.

But people whose PCs were infected with Bamital may not have noticed signs of the malware infection. "While the cybercriminals in this case used the Bamital malware to break victims' search experience, it was done in such a sneaky way that most victims wouldn't have even noticed a problem while the botnet was still operating," said Boscovich. "However, because the takedown severed the cybercriminals' ability to manipulate and control Bamital-infected computers, victims will likely become visibly aware that their search function is broken as their search queries will time out." But he said infected PCs queries should now be redirected to a Bamital-removal website hosted by Microsoft and Symantec.

The practice of click fraud -- or click hijacking -- refers to the practice of rerouting people who click on legitimate search engine results to sites that pay attackers, or their affiliates, for the referral. To perpetrate click fraud, attackers may trick search engines into listing their fake pages.

But by using malware, attackers can instead alter the DNS settings on an infected PC to route all Internet requests through an attacker-controlled server. Such scams may redirect users to fake sites, or display attacker-promulgated banner advertisements over legitimate sites. But the gang who ran Bamital used the botnet in part to infect PCs with their malware, according to Microsoft's complaint. "Microsoft investigators found that Bamital rerouted a search for 'Nickelodeon' to a website that distributed malware, including spyware that is designed to track the activities of the computer owner," said Boscovich. "In another case, our researchers discovered that an official Norton Internet Security page that appears in a list of search results was redirected to a rogue antivirus site that distributes malware."

One of the best-known examples of click fraud to date was the DNSChanger malware, which was wielded by an Estonian gang as part of a four-year fraud campaign that amassed an estimated $14 million, before the gang was busted by Estonian police and the FBI.

The Bamital takedown marks the sixth time that Microsoft has been involved in a botnet takedown, and the second time that it's worked with Symantec to do so. But while affecting a botnet takedown by having a private company file a civil complaint may be a novel legal strategy, it's drawn criticism. Notably, Dutch security researchers last year accused Microsoft of compromising their Zeus botnet investigations after Microsoft employees, accompanied by U.S. marshals, physically removed -- from two U.S. hosting centers -- C&C servers that were hosting a Zeus botnet.

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