Fans of Apple computers often boast about superior security. But as Macs have gained in popularity over the past few years, this has brought much more attention from hackers. At a presentation scheduled to take place today at the Black Hat DC computer-security conference in Washington, DC, one security expert will reveal a technique for attacking the Mac operating system--OS X--without leaving a trace.

Similar techniques have targeted both Windows and Linux machines for several years. They allow an attacker to cover her tracks, eliminating vital evidence that an investigator might normally use to prove that a machine has been compromised; they might also allow the investigator to put together details of the incident. Bringing the technique to the Mac, however, required a significantly more sophisticated approach.

The technique that will be outlined at Black Hat DC allows an attacker to remove virtually all trace of an attack against OS X, after compromising the system using another exploit.

Vincenzo Iozzo, a student at the Politecnico di Milano, in Italy, explains that the technique allows an attacker to break into a machine without leaving a trace in its permanent memory, which means that evidence of the attack will disappear as soon as the victim's computer is turned off. Such a technique could be used, for example, in combination with another software flaw to covertly replace a legitimate version of Apple's Safari Web browser with a malicious one that logs the user's keystrokes and sends them to the attacker.

Normally, when a user runs an application, the code runs in various parts of the computer's memory. In OS X, a file format called Mach-O is used to specify where in the computer's memory the application's processes should run. Iozzo studied the Mach-O file format in order to predict in advance where these processes could be found. The technique identifies an active process (such as that for Safari) and injects malicious code into the space in memory where it is running. When the system reads from the expected location, it executes the attacker's code instead of the legitimate program. Since the technique leaves no trace, Iozzo says that it can only be detected using software that watches for intrusions on a network.

Predicting where to inject the malicious code is made more difficult by a security feature in OS X that stores the variables needed to keep the attack untraceable in random locations within memory. However, Iozzo discovered a way to anticipate where the variables would be stored based on pieces of information that remain unchanged.