The little LED light on Mac laptops that switches on when the webcam is in use makes it hard for attackers to spy on Mac users without alerting them to the fact. But that doesn’t mean it is impossible for someone to use the camera to secretly record video and audio anyway.

Patrick Wardle, director of research at Synack and a former NSA analyst this week described a method that attackers could use to secretly watch Mac users by piggybacking on legitimate user-initiated Skype, FaceTime or other webcam sessions.

The method does not require hackers to exploit any security vulnerabilities in the Mac OS. Neither does it involve bypassing the LED indicator. Instead, it takes advantage of the legitimate functionality of OS X to surreptitiously record video and audio from the webcam while the camera is being used by another application.

Wardle said research conducted by Synack on other "webcam-aware" OS X malware showed that it is possible to write malware capable of detecting when a Mac’s webcam is being used by another application and then ask to use the camera at the same time.

In most cases, such code would not need any specific entitlements to access the Mac OS X webcam programmatically.

“Currently, there is no publicly known malware using this technique,” Wardle says. “Rather this is a new attack or capability that malware could employ to enhance itself.”

He pointed to several malware samples that are currently available for recording Mac users via their webcam.  Examples include OS X Mokes and OS X Eleanor. But none of the malware samples are capable of recording the user without turning on the LED light, which is a dead giveaway, he says.

“This new attack is a fairly simple capability that malware could add,” he says. Piggybacking on a legitimate user-initiated webcam session allows the attacker to secretly record the user without any visible indication, because the LED light would already be on.

“One might wonder what the benefit would be in recording videos that are taking place,” rather than recording targeted conversations, he says. But what’s important to remember is that when people use their webcams, they are usually discussing interesting or sensitive things. These are just the type of conversations that would be of interest to an attacker, Wardle says.

The piggybacking approach pretty much also offers the only way for an attacker to secretly record a Mac user without being noticed, he added.

“I don’t know of an easy way for malware to record off the webcam, arbitrarily, without the LED indicator light being activated,” he says. “The malware could be undetected up to this point - but if it arbitrarily starts recording and the LED indicator light goes on, that’s a huge give away, and likely will reveal the presence of the malware.”

Wardle says he has developed a tool that can help mitigate the threat to a certain extent. The tool simply alerts the user to the fact that some process or application is using the webcam and microphone. If a secondary process then hops in and tries to access or record data from the webcam, the tool will pick that up as well and alert the user, he says.

The tool gives users the option to allow or block the process that is accessing the webcam, he said.

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