It has been confirmed by multiple research houses: There's a massive, 600,000-strong botnet of mainly Apple Macintoshes out there called Flashfake, and most of the infected machines are in the U.S.

A report by researchers at Russian antivirus firm Dr. Web this week of a botnet of 500,000 to 600,000 Macs sent a chill across the Mac user community, which long has been spared the bull's eye of botmasters who traditionally have converted Windows machines into their zombie machines. Security experts have warned that with the Mac's growing popularity -- especially in enterprise circles -- that it was only a matter of time before the Mac would become an attractive target, as well.

"Mac is no longer a safe refuge," says Karim Hijazi, founder and CEO at Unveillance. In the past three hours, Unveillance so far has seen more than 200,000 unique public IPs with hosts infected with the botnet's Flashback Trojan, he says, and it's very likely more than 500,000 machines.

Researchers at Kaspersky Lab also confirmed the mega-Mac botnet today: Kaspersky malware expert Igor Soumenkov says his team found more than 600,000 unique bots reaching out to its server in less than 24 hours, and more than 50 percent were based in the U.S.

This may be the largest Mac botnet ever spotted, experts say, and it's definitely a wake-up call for Mac users to start loading antivirus software and to keep up with their patching regimen.

Dr. Web identified the bots as all Macs. But Kaspersky's Soumenkov says while it's likely that most of the bots are Macs, his team can't confirm that all of the bots that connected to its server were Mac OS X machines. "The bots can be only identified by their unique User-Agent HTTP header, and it does not contain any OS-specific information," he wrote in a blog post today. There were more than 620,000 external IP addresses.

Even so, OS fingerprinting found that more than 98 percent of the network packets received by its server were likely Mac OS X machines. "So, it is very likely that most of the machines running the Flashfake bot are Macs," Soumenkov says.

This particular variant of the Flashback malware family is being deployed for click fraud, according to Kaspersky researchers. It employs an exploit that abuses a just-patched Apple OS X Java flaw to hijack a victim's search engine results, and once they are infected, their machines are used for the click-fraud scam, the company says.

The so-called Trojan BackDoor.Flashback infects users when they are redirected to a malicious website from an infected one after downloading a phony update for Adobe Flash Player. Apple this week patched the flaws that the Trojan is exploiting, but attackers have exploiting similar flaws since February.

"This latest wave of infections is a wake-up call to Mac users that their system is not immune to threats. And the need to follow best security practices, such as remaining current with patches, is ubiquitous -- it doesn't matter if you’re using Windows, Mac, or even mobile phone," says Mike Geide, senior security researcher at Zscaler ThreatLabZ.

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