Mac malware is a rare breed, at least compared with Windows and Android. But when Mac malware does manifest, it's often especially nasty. Trojan.Yontoo.1 is no exception.

Trojan.Yontoo.1 is not new for security Relevant Products/Services researchers, but more than one firm is reporting the rise of this malware on computers running Apple's OS X. The Trojan is sneaking in through an adware plug-in on popular browsers, including Chrome, Firefox and Safari. Microsoft's Internet Explorer was not mentioned.

Alex Horan, a senior product manager at CORE Security, told us the malware is a sign that Apple has enough market share to be a viable target for attack Relevant Products/Services.

"In one sense it is an endorsement of their growth, on the other hand it means they have to take a responsible and proactive approach to security," Horan said. "In the past, Apple has claimed to be virus-free, when in reality they simply were not an attractive target. Unfortunately they may have created a feeling of immunity for their users, who, as a result, may be slow to apply patches and remain vulnerable to attacks for a long period of time."

A Mac Malware Poster Child

Russian antivirus company Dr.Web is reporting a general rise in adware targeting Mac OS X since January. And the firm reports that Trojan.Yontoo.1 is the most prominent.

"Criminals profit from affiliate ad network Relevant Products/Services programs, and their interest in users of Apple-compatible computers grows day by day," Dr.Web reported in a blog post. "Recently discovered, Trojan.Yontoo.1 can serve as a striking example of such software Relevant Products/Services."

According to Dr.Web, there are several ways for the Trojan to get into a computer. In order to spread the Trojan, the firm said, criminals crafted movie trailer pages that prompt users to install a browser plug-in. In fact, security analysts said, the prompt only imitates a common dialogue displayed when a plug-in needs to be installed or additional configuration is necessary.

"After clicking on 'Install the plug-in', the user is redirected to another site from which Trojan.Yontoo.1 is downloaded," Dr.Web said. "Criminals have also provided for a number of alternative ways to spread this threat Relevant Products/Services. The Trojan can also be downloaded as a media player, a video quality enhancement program or a download accelerator."

An Insidious Attack

When launched, Dr.Web said, Trojan.Yontoo.1 displays a dialog window that asks the user if they want to install Free Twit Tube. However, the firm continued, after the user presses "Continue," instead of the promised program, the Trojan downloads from the Internet and installs the plug-in Yontoo for Safari, Chrome and Firefox.

"While a user surfs the Web, the plug-in transmits information about the loaded pages to a remote server," Dr.Web wrote. "In return, it gets a file that enables the Trojan to embed third-party code into pages visited by the user."