There's new Java-based Mac malware in the wild, but before you run screaming out the door to buy an antivirus app, read the rest of this post.
The new malware, known as GetShell.A, requires you to approve the installation of a Java applet. OS X, being the polite operating system that it is, warns you that the applet is from a root certificate that "is not trusted." If you still decide to go ahead and install this applet, your device will be infected.
What's fascinating about this malware is that it is multi-platform. Once you allow the applet to be installed, it downloads platform-specific code for OS X, Linux, and Windows to attempt opening a backdoor on your machine. Here's the interesting thing -- the OS X code won't run without Rosetta on an Intel-based platform as it is a PowerPC binary. That means that any Mac without Rosetta -- basically any machine running Lion or Mountain Lion -- is immune to the malware.
Apple Mac OS X Snow Leopard