[UPDATE 4/11/12: Apple last night announced on its support website that it is developing software that will remove the Flashback Trojan and that it is "working with ISPs worldwide to disable this command and control network" for the Flashback botnet.]

What may have been the largest known botnet made up of Apple Macintosh computers appears to be gradually waning in activity, and there's now a free detection and removal tool available online for Mac users to check whether they are infected by the so-called Flashback Trojan.

Kaspersky Lab -- which is offering the free tool -- counted up to 670,000 infected OS X machines in the botnet last week; today has seen just 227,493 so far, up from 208,301 yesterday. Over the weekend, Kaspersky saw a major dip in the number of active infected Macs, from a head count on Friday, April 6, of 650,748, to 248,723 on Saturday, and then 237,103 on Sunday.

Alex Gostev, Kaspersky's chief security expert, says the number of bots counted here are active ones, and that the numbers don't reflect the total number of infected machines. Kaspersky's online detection and removal tool is available here.

"The drop in unique bots is most likely caused by efforts on the DNS [Domain Name System) levels. For example, a certain DNS could ban access to Flashback domains, which stops users from connecting to the malicious C&C servers, as well as our sinkhole," Gostev says.

But the floodgates have been opened for targeting Macs, and security experts say this is only the beginning. "With more than 100 million Mac OS X users globally, we expect future threats to arise -- we’ve already seen them increase, with attacks such as DNSChanger, Fake AV/Scareware, and the most recent version of the Flashback Trojan/Flashfake botnet. The spike in attacks started in September 2011 and has reached its highest peak in March 2012," Gostev says. "Cybercriminals recognize Mac OS X is gaining market share, especially in developed countries, and we expect them to continue to create ways to infect users."

[Mac users might not have a lot of exploits to worry about, but their lack of security worries makes them an APT attacker's dream come true. See Anatomy Of A Mac APT Attack. ]

The Mac attack scare started last week when researchers at Russian antivirus firm Dr. Web announced they had spotted a botnet of 500,000 to 600,000 Macs, a finding that later was confirmed by Kaspersky Lab and Unveillance. The news was a painful wakeup call for the Mac user community, which long has been spared the bull's eye of botmasters who traditionally have gone after Windows machines. It was no surprise to security experts, however, who for some time have warned that with the Mac's growing popularity -- especially in enterprise circles -- it was only a matter of time before attackers would zero in on the Mac as well.

Kaspersky malware expert Igor Soumenkov late last week said 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. Kaspersky sinkholed bots that were communicating with one of the Flashback/Flashfake Trojan domain names last week.

Unveillance, meanwhile, has spotted new infections this week: Last night, it counted more than 800,000 unique public IPs sporting Flashback infections, and that number is rising today as well, according to Karim Hijazi, CEO and president of botnet-tracker Unveillance.

"I am sure we will see it drop, but it will take time for the tool to make the rounds through the infected populous," Hijazi says.

Noticeably silent in all of this is Apple, which rarely speaks to the press or comments publicly on security updates or issues. Apple had not responded to a press inquiry as of this posting.

Meanwhile, Forbes reports that Apple attempted to take down Dr. Web's sinkhole server for the Flashback botnet, possibly mistaking it for a malicious command and control domain. Boris Sharov, chief executive of Dr. Web, told Forbes that a Russian registrar informed him that Apple had asked it to shut down the sinkhole domain. "They told the registrar this [domain] is involved in a malicious scheme, which would be true if we weren't the ones controlling it and not doing any harm to users," Sharov told Forbes. "This seems to mean that Apple is not considering our work as a help. It's just annoying them."

But Sharov said in the interview that he thinks Apple mistook his sinkhole for a malicious C&C server.

Kaspersky's Gostev says while his company has worked independently of Apple on this case, from what Kaspersky has seen, "Apple is taking appropriate action by working with the larger Internet security community to shut down the Flashfake C2 domains."

The Flashback variant is being used for click fraud, and exploits a very recently patched Apple OS X Java flaw in order to hijack search engine results. Users are redirected to a malicious website from an infected one after downloading a phony update for Adobe Flash Player. Apple patched the flaws that the Trojan is exploiting last week, and attackers have exploiting similar flaws since February.

Kaspersky provided details of its bot-count in a blog post last night.

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