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Macs are getting more visibility in the world of attacks as a Trojan preying on OS X users has adopted several stealth techniques.

Updates to the Flashback Trojan, which victims install when it disguises as an Adobe Flash update, now prevent the malware from running on Macs that use VMware Fusion. Such virtual machine software routinely sees use by security researchers to test the behavior of a malware sample because it’s easier to delete a virtual instance when they finish than it is to wipe the hard drive clean and reinstall the operating system.

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When users click 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 said. Windows malware has done the same thing for years.

Flashback developers have also changed their code so 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.

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Such virtual-machine blocking and cloaking of malicious files have become standard fare in Windows malware. Their addition to Flashback shows attackers are adopting the same techniques only 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 said.

Another 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.

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