September 28, 2011, 3:27 PM — Mac users can thank Microsoft for taking down a small but dangerous botnet.

On Monday, at the request of Microsoft, a Virginia federal judge ordered VeriSign to shut down nearly two dozen domains linked to servers that controlled the Kelihos botnet.

One of those domains -- cz.cc -- had hosted machines that distributed waves of fake Mac antivirus software for weeks last spring, according to Microsoft and several security companies.

The fake programs, called "scareware" or "rogueware," began hitting users in early May and continued for nearly two months. Like similar -- and much more prevalent -- software aimed at Windows PCs, the Mac scareware falsely claimed that the computer was heavily infected. Once installed, phony antivirus software nags users with pop-ups and fake alerts until they fork over a fee to purchase the worthless program.

Microsoft noted that the cz.cc domain had "previously been investigated for hosting subdomains responsible for delivering MacDefender," the most-used name of the Mac scareware.

A pair of security companies that analyzed MacDefender in May and June confirmed Microsoft's contention today.

Among the subdomains serving up MacDefender, said both French-based Intego and U.K.-headquartered Sophos, were ones under the cz.cc domain that VeriSign shuttered.

"The sites that were leading you to MacDefender were diverse and most often compromised .com sites," said Chet Wisniewski, a Sophos security researcher, talking about how users were redirected from hacked sites to those actually pushing scareware to Macs. "I can't confirm that MacDefender was exclusively distributed through cz.cc, but anecdotally I can say it was a high percentage."

But both Wisniewski and Peter James, a spokesman for Intego, said that Microsoft's take-down was, in fact, largely moot.

"MacDefender started disappearing after the arrest of Pavel Vrublevsky in June," said Wisniewski, referring to the man who was charged with hacking into a rival of ChronoPay , a Russian company that allegedly processed credit card payments for scareware spreaders.

Vrublevsky's arrest had followed an operation in the U.S. and several other countries where authorities busted cyber gangs responsible for distributing massive amounts of scareware to Windows PCs.

Operation Trident Tribunal had targeted criminals who purportedly infected nearly a million PCs with scareware, and raked in $72 million in revenues from the scam.

"[MacDefender] seems to have faded away," agreed James of Intego. "I think a combination of the amount of press it got, and some arrests of people behind fake antivirus, stopped it or made it go dormant."