The cyber criminals behind ZeroAccess, one of the largest botnets in existence, have lost access to more than a quarter of the infected machines they controlled because of an operation executed by security researchers from Symantec.
According to Symantec, the ZeroAccess botnet consists of more than 1.9 million infected computers and is used primarily to perform click fraud and Bitcoin mining in order to generate revenues estimated at tens of millions of dollars per year.
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ZeroAccess has a peer-to-peer architecture where every infected computer can relay files, instructions and information to other computers -- peers -- in the botnet. This mechanism is used by its operators for command and control (C&C), making ZeroAccess more resilient to takedown attempts than botnets that depend on dedicated C&C servers.
Earlier this year, security researchers from Symantec found a practical way to liberate ZeroAccess bots from the botmasters by leveraging a known design weakness in the peer-to-peer mechanism.
However, in June the botnet's creators started distributing a new version of the malware containing modifications to address the known flaw. This led to Symantec's decision to launch a sinkholing operation in mid-July -- an operation that involves hijacking the bots in a way that would prevent attackers from regaining control of them.
"This operation quickly resulted in the detachment of over half a million bots and made a serious dent to the number of bots controlled by the botmaster," the Symantec researchers said Monday in a blog post.
The sinkholed bots hadn't been updated and still have the weakness, but they were isolated to the point where they now only communicate with servers run by Symantec, said Vikram Thakur, principal security response manager at Symantec. "We do not believe that there is any way for the botmasters to regain control of these bots."
The sinkholing operation took only a few days, but Symantec has since worked to make sure that its sinkhole is stable and shared data with ISPs (Internet service providers) and computer emergency response teams (CERTs) so they can start the process of identifying and cleaning the infected computers.
"We wanted to make sure that the foundation for remediation was solid before we announced it to the public," Thakur said.
ISPs have been provided with traffic signatures that will help identify ZeroAccess bots on their networks, so they can act to take measures even against the bots that haven't been sinkholed, Thakur said.
The Symantec researchers performed tests in the lab in order to estimate the botnet's energy costs to victims and how much money it generates for its owners.
The company estimated that the Bitcoin mining activity, which uses computational power to generate Bitcoins, a type of virtual currency, would consume an additional 1.82 kWh per day for every infected computer, if that computer would be turned on all the time.