A new family of Mac OS X malware used to breach Facebook, Apple and a couple of other high-profile companies is using an exploitation technique to bypass the basic defense mechanism in OS X.
Introduced in Mac OS X Mountain Lion, Gatekeeper acts as a control mechanism to allow or block software based on the source it comes from. Gatekeeper has three levels of security that allow a Mac OS X user to install applications that come from a) any source, whether the code is signed or not; b) applications you download from the AppStore only or c) applications that come from the AppStore or that are digitally signed by a developer.
By default, Gatekeeper is instructed to only allow applications that come from the AppStore or that are digitally signed by a developer, which was supposed to minimize the impact of malware. After all, applications downloaded from the AppStore are co-signed by Apple, and legit application developers would rarely spice their applications with malicious code.
To circumvent this first line of defense, the creators of Mac.OSX.Backdoor.Pintsized.A relied on an undocumented exploit to trick Gatekeeper into running unsigned applications regardless of the security level enforced in Gatekeeper. The Gatekeeper compromise is likely bundled with the Java exploit that also delivers the backdoor.
After bypassing Gatekeeper and achieving persistency, Pintsized.A would open a reverse shell to a command and control center and await for further instructions. To make detection harder in case its C&C traffic is inspected at the network perimeter, the backdoor encrypts it with a modified version of the OpenSSH utility.