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Flashback trojan targeting OS X shuns virtual machines

Mac malware grows up

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Underscoring the growing sophistication of Mac-based malware, a trojan preying on OS X users has adopted several stealth techniques since it was discovered last month.

Updates to the Flashback trojan, which gets installed by disguising itself as an Adobe Flash update, now prevent the malware from running on Macs that use VMware Fusion. Such virtual machine software is routinely used by security researchers to test the behavior of a malware sample because it's easier to delete a virtual instance when they're finished than it is to wipe the hard drive clean and reinstall the operating system.

When users get tricked into clicking on the recently introduced Flashback.D installer, the program checks to see if the Mac is running Fusion. If it is, it doesn't execute, researchers from antivirus provider Intego blogged on Thursday. Windows malware has done the same thing for years.

Flashback developers have also rejiggered their code so that it no longer installs itself in an easy-to-spot subfolder off the OS X ~/Library location. Instead, it plants a backdoor inside a more obscure folder associated with the Safari. Deleting the files prevents the browser from working.

Such virtual-machine blocking and cloaking of malicious files have become standard fare in Windows malware. Their addition to Flashback suggests the same techniques are being adopted by criminals targeting Macs.

“These changes show that the malware authors are sophisticated, and that they're altering their code to ensure that the malware is not detected,” Intego researchers wrote.

A separate post from researchers at competing antivirus firm F-Secure said the VM-awareness dates back to the release of the earlier Flashback.B version of the malware.

“It appears that Mac malware authors are anticipating that researchers will begin to use virtualized environments during analysis, and are taking steps to hamper such efforts,” the post stated.

Developers are bringing additional innovations to Mac malware. According to security reporter Brian Krebs, Trojan-Dropper: OSX/Revir.A, another recently discovered Mac trojan, “challenges a widely-held belief among Mac users that malicious software cannot install without explicit user permission.” ®

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