A peer-to-peer botnet known for ad-click fraud and estimated at making $100,000 per day for its operator now has infected some 2.2 million users worldwide.

New data released today from the third quarter 2012 malware report by Kindsight, an Alcatel-Lucent company, says ZeroAccess is the most active botnet and is growing through its new "super-node" model where an infected host is connected directly to the Internet without a home router or network address translation device.

"The ZeroAccess botnet has grown significantly to become the most active botnet we've measured this year," said Kevin McNamee, security architect and director of Kindsight Security Labs. "Cybercriminals are primarily using it to take over victim computers and conduct ad-click fraud. With ZeroAccess, they can mimic the human behavior of clicking online ads, resulting in millions of dollars of fraud."

Kindsight says 13 percent of home networks in North America are infected with malware, 6.5 percent of which are tainted with bot malware, rootkits, and banking Trojans. And mobile malware continues to rise, with an infection rate of more than 3 percent among mobile devices, 90 percent of which is mobile adware.

The ZeroAccess botnet has been on the radar screen of botnet investigators for some time. It can cost legitimate advertisers some $900,000 per day in click fraud, according to Kindsight's report. It produces some 140 million fraudulent ad clicks per day and 260 terabytes of network traffic, the report says.

There are two versions of the botnet: one that performs Bitcoin mining and uses an encrypted peer-to-peer protocol for communications, and another that uses the super-node approach for command and control.

Researchers at SophosLabs last month said they had discovered how the botnet's operators monitor it. "We have also reverse-engineered the mechanisms by which the ZeroAccess owners keep tabs on the botnet, and discovered an array of techniques used that are designed to bury the call-home network communications in legitimate-seeming traffic," Website said James Wyke, senior threat researcher with SophosLabs UK.

Meanwhile, Kindsight found that .8 percent of North America households were infected with ZeroAccess each day during the third quarter of this year.

The TDL-4 botnet was the second most active botnet in Q3. The rootkit-based bot is relatively stealthy, hiding within the master boot record of the infected machine and eluding antivirus applications. TDL-4 is known for deleted competing malware from the machines it infects, and its newest iteration has infected nearly 10 percent of the Fortune 500.

The full report from Kindsight is available here for download.

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