At its peak in 2013, ZeroAccess, also known as Sirefef, consisted of more than 1.9 million infected computers that were primarily used for click fraud and Bitcoin mining.
That was until security researchers from Symantec found a flaw in the botnet's resilient peer-to-peer architecture. This architecture allowed the bots to exchange files, instructions and information with each other without the need for central command-and-control servers, which are the Achilles' heel of most botnets.
By exploiting the flaw, Symantec managed to detach over half a million computers from ZeroAccess in July 2013 and launched an effort to clean them up in cooperation with ISPs and CERTs.
In December that same year the FBI, Europol, Microsoft and several security vendors launched a second operation that further crippled the botnet and led to those behind it capitulating. The botnet operators actually sent an update to the infected machines that contained the message "WHITE FLAG."
"We believe [that action] symbolizes that the criminals have decided to surrender control of the botnet," Richard Domingues Boscovich, assistant general counsel with the Microsoft Digital Crimes Unit, said at the time in a blog post.
It didn't last long. Cybercriminals reactivated the botnet and used it between March 21 and July 2, 2014, but then -- silence. Until now.
The botnet was reactivated on January 15, when it "again began distributing click-fraud templates to compromised systems," researchers from Dell SecureWorks said in a blog post Wednesday.
To perpetrate click fraud, malware displays ads on infected computers and clicks on them, masking the clicks as legitimate user actions in order to generate advertising income for the botnet operators.
ZeroAccess is only a shadow of its former self, as the attackers did not attempt to infect new systems since December 2013. However, the new activity this year indicates that they haven't completely given up on it.
The Dell SecureWorks researchers observed 55,208 unique IP addresses participating in the botnet between January 17 and January 25 -- 38,094 corresponding to compromised 32-bit Windows systems and 17,114 to 64-bit systems. The top ten affected countries are Japan, India, Russia, Italy, the U.S., Brazil, Taiwan, Romania, Venezuela and Germany.
"Although the threat actors behind ZeroAccess have not made any measurable attempts to augment the botnet in more than a year, it remains substantial in size," the SecureWorks researchers said. "Its resiliency is a testament to the tenacity of its operators and highlights the danger of malware using P2P networks."