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Windows Trojan Found Targeting Mac OS X Users

Researchers at ESET have discovered a Trojan that initially focused on Windows users, but appears to be changing direction. The Trojan now has its sights on Mac OS X users, and its actions have prompted Apple to update XProtect with signatures to detect it.

The Yontoo Trojan spreads on Windows by pretending to be a video codec. Users expecting to see the viral video clip of the moment, or some other attention demanding clip, are prompted to install a codec file before the images will play. At that point the system is compromised and used to push advertising with the additional kick of a backdoor that allows for other malware to be delivered.

Mac OS MalwareThis is nothing new as far as malware on Windows goes. However, the same campaign has been targeting OS X users, but there wasn’t much thought put into it. Mac users are prompted to install a codec file that clearly says it’s for Windows 7, so it would seem the criminals behind this campaign forgot to edit their templates.

Yet, if the victim accepts the installation, Yontoo will still infect their system, and they will be added to the malicious network of ads and backend controls. So clearly the malware itself was repurposed for Mac somewhere along the line. In response to the number of reports, Apple updated XProtect to prevent Yontoo from running, naming the offending file OSX.AdPlugin.i in their signature list.

“An infection technique and criminal scheme that was previously found to be successful against Windows users is being tried against Mac users. Why? Economics. And while we cannot predict the future, we can predict the outcome of economic calculations. If there is a way to make money from attacking Macs, you can bet someone will try it,” ESET’s Stephen Cobb wrote

“With the exception of malware designed for political or hobby-activist purposes, all malware development is driven by fairly straightforward risk-benefit, return-on-investment analysis. In other words, most malware campaigns are not mounted unless somebody thinks there is money to be made, with a low probability of getting caught.”

Yontoo is still active online, but most AV protections will detect it, and OS X users should have the latest XProtect signatures already. If you are seeing unwanted ads on your Mac, check Safari’s plug-in list for Yontoo and remove it to disable the malware.

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Steve Ragan is a security reporter and contributor for SecurityWeek. Prior to joining the journalism world in 2005, he spent 15 years as a freelance IT contractor focused on endpoint security and security training.