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Dr. Web disputes Flashback Mac Trojan bot army estimates

Much bigger than Symantec says

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Efforts by Apple and anti-virus vendors to kill the vast botnet assembled by notorious Flashback Mac Trojan may be much less successful than previously thought.

Symantec last week suggested the Mac botnet shrank from a peak of 670,000 to 140,000 following the release of clean-up tools.

But the Russian anti-virus firm Dr. Web says the figures are wrong because infected machines that contact a particular server freeze up and do not contact vendor-established sinkholes. These machines are not counted, even though they are still infected.

Dr. Web thinks 570,000 systems are still infected by the Flashback Trojan. Symantec concedes that Dr. Web has a point and in response has revised figures upwards, albeit to the more modest figure of 185,000.

"A recent Dr. Web blog post reveals our sinkholes are receiving limited infection counts for OSX.Flashback.K," Symantec explains in an updated advisory. "A sinkhole registered at IP address 74.207.249.7 is causing Flashback connections to hang as it never closes the TCP handshake, in effect preventing Flashback from hitting subsequent domains."

The Flashback Trojan created a zombie army of 650,000 Apple Macs, or more, by exploiting a Java security vulnerability that Apple patched is early April, some six weeks after a patch for Windows machines became available. Mac users only needed to visit an infected site to get hit - no user interaction was required.

The initial success of the worm has spawned a series of copy-cat attacks. Flashback-S. Mac security specialist Intego reports that Flashback-S cleans itself from the Java cache to avoid detection. Flashback-S spreads by taking advantage of the same Java vulnerability as the original Trojan.

Analysts from Kaspersky have traced the initial spread of Flashback from somewhere between 30,000 to 100,000 vulnerable WordPress blogs. It thinks a Russian cyber-criminal partner programme is behind the distribution of the malware.®

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