Security researchers have encountered new email-based targeted attacks that exploit a vulnerability in Microsoft Office to install a remote access Trojan horse program on Mac OS systems.
The rogue emails appear to target Tibetan activist organisations and distribute booby-trapped Microsoft Word documents that exploit a known remote code execution vulnerability in Microsoft Office for Mac, according to malware experts from security firm AlienVault.
"This is one of the few times that we have seen a malicious Office file used to deliver Malware on Mac OS X," said AlienVault security researcher Jaime Blasco.
Security researchers from Mac antivirus vendor Intego believe that the attacks might become more widespread. "This malware is fairly sophisticated, and it is worth pointing out that the code in these Word documents is not encrypted, so any malware writer who gets copies of them may be able to alter the code and distribute their own versions of these documents," they said.
"The attack will be very effective on those who have not updated their copies of Microsoft Office, or aren't running antivirus software," the Intego researchers said.
If the vulnerability is exploited successfully, the rogue Word files will install a previously unknown Mac OS X Trojan horse. The remote attackers can instruct this malware to download, upload and delete files, or to start a remote shell on the system.
AlienVault believes that this attack was carried out by the same gang that last week distributed a similar Mac Trojan by exploiting a vulnerability in outdated Java installations.
This type of targeted attack, also known as spear phishing, has become common in recent years and is usually associated with government or corporate cyberespionage operations. However, the vast majority of spear phishing emails have so far targeted Windows users.
"While, in the past, we did not see this type of attack targeting Macs, it is clear that the game has changed, and that we are entering a new period of Mac malware," the Intego researchers said.
Mac users are advised to keep the software installed on their computers up to date, especially the popular applications, and to run an antivirus program at all times. Several Mac antivirus products are available for free.