Russian security firm Doctor Web this week highlighted a new trojan affecting OS X systems and which installs an adware plug-in capable of injecting ads into users' browsing experience. From MacRumors:

As with other trojans, this new Yontoo malware relies on tricking users into installing the package, which in this case masquerades as a movie trailer video plug-in, download accelerator, or other software a user might believe they want or need on their system.When launched, Trojan.Yontoo.1 displays a dialogue window that asks the user if they want to install Free Twit Tube. However, after the user presses ?Continue?, instead of the promised program, the Trojan downloads (from the Internet) and installs the plugin Yontoo for Safari, Chrome and Firefox. These browsers are most popular among Mac OS X users. While a user surfs the web, the plugin transmits information about the loaded pages to a remote server.

 New 'Yontoo' Adware Trojan Targets Major Browsers on OS X