Security firms Friday warned Mac users of a new Trojan horse that masquerades as a PDF document.

The malware, which was spotted by U.K.-based Sophos and Finnish antivirus vendor F-Secure, uses a technique long practiced by Windows attackers.

"This malware may be attempting to copy the technique implemented by Windows malware, which opens a PDF file containing a '.pdf.exe' extension and an accompanying PDF icon," said F-Secure today: http://www.f-secure.com/weblog/archives/00002241.html

That practice relies on what is called the "double extension" trick: adding the characters ".pdf" to the filename to disguise an executable file.

The Mac malware uses a two-step process, composed of a Trojan "dropper" utility that downloads a second element, a Trojan "backdoor" that then connects to a remote server controlled by the attacker, using that communications channel to send information gleaned from the infected Mac and receiving additional instructions from the hacker.

Because it doesn't exploit a vulnerability in Mac OS X -- or any other software -- the malware instead must dupe users into downloading and opening the seemingly-innocuous PDF document, which is actually an executable.

Once run, the dropper downloads the second-stage backdoor and opens a Chinese-language PDF. F-Secure said that the PDF was another sleight-of-hand trick: "[The dropper component] drops a PDF file in the /tmp folder, then opens it to distract the user from noticing any other activity occurring," the company said in a description of the attack: http://www.f-secure.com/v-descs/trojan-dropper_osx_revir_a.shtml

Both Sophos and F-Secure noted that the malware doesn't work reliably, and currently can't connect to the command-and-control (C&C) server because the latter isn't fully functional.

Mac malware is typically crude in comparison with what targets Windows PCs.

Because the C&C server is not yet operational and since it found samples of the Trojans on VirusTotal -- a free service that runs malware against a host of antivirus engines -- F-Secure speculated that the malware is still in the testing phase.

Although Apple's Mac OS X includes a bare-bones antivirus detector, it has not been updated to detect the just-noticed Trojan dropper or backdoor. Checks of several Computerworld Macs running Lion, for instance, found that Apple last updated its detector on Aug. 9.

Mac users had their biggest malware scare earlier this year, when a series of fake security programs , dubbed "scareware," were aimed at them: http://scforum.info/index.php/topic,6070.0.html

Several antivirus companies, including Sophos, F-Secure and Intego, offer security software for the Mac: http://scforum.info/index.php/topic,5808.0.html

(PCW)