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Researchers Demo Method For Turning A PC Into An Eavesdropping Device

The audio chipsets in many modern PCs allow audio jacks to be flipped from lineout to line-in, says team from Israel’s Ben-Gurion University.

Researchers at Israel’s Ben-Gurion University of the Negev have devised a way to turn any computer into an eavesdropping device by surreptitiously getting connected headphones or earphones to function like microphones.

In a paper titled "SPEAKE(a)R: Turn Speakers to Microphones for Fun and Profit," the researchers this week described malware they have developed for reconfiguring a headphone jack from a line-out configuration to a line-in jack, thereby enabling connected headphones to work as microphones.

The exploit works with most off-the-shelf headphones and even when the computer doesn’t have a connected microphone or has a microphone that has been disabled, according to the researchers.

The malware takes advantage of the manner in which some audio chipsets in modern motherboards and soundcards work. In a typical computer chassis, the audio jacks that are built into the front or rear panel are used either as line-in or input jacks or line-out jacks for audio output.

The chipsets in such cards support a little-used jack re-mapping or a jack re-tasking option for changing the function of the audio ports from line-in to line-out via software.

Audio chipsets from Realtek Semiconductor, for instance support this capability, though it is not documented in any technical specifications for the product, the researchers from Ben-Gurion University noted. Realtek codecs are the most widely used in PCs but other codec manufacturers allow jack repurposing as well, the researchers said.

In addition, researchers have for some time known that speakers can, with a little tweaking, be made to function like a microphone, they said. “Loudspeakers convert electric signals into a sound waveform, while microphones transform sounds into electric signals,” their paper noted.

In a speaker, electrical signals are used to create a changing magnetic field that moves a diaphragm in order to produce sounds. In a microphone, a diaphragm moves through a magnetic field to induce an electrical signal. “This bidirectional mechanism facilitates the use of simple headphones as a feasible microphone, simply by plugging them into the PC microphone jack,” the research paper said.

This capability, coupled with the fact that audio jacks can be programmatically altered to switch from output only to input jacks, creates a vulnerability that attackers can use to turn any computer into an eavesdropping device, according to the BGU researchers.

Tests to evaluate the quality of audio signals generated by off-the-shelf headphones plugged into jacks that had been modified by SPEAKE(a)R showed it is possible to acquire intelligible audio from several meters away.

Craig Young, a cybersecurity researcher for Tripwire says the fact that audio-chipsets allow this sort of reprogramming is interesting. But the chances of someone being able to pull off an attack are not easy, he says. For one thing, in order for the exploit to work, an attacker would likely need full access to the computer that is being used for eavesdropping. Anti-malware tools would also likely be able to easily spot and block the malware from working, he says.

“Based on the description of the malware, it is using the RealTek retasking feature to reconfigure an output line to be used as an input line,” he says. In a Windows environment, this is generally controlled within the registry, he says. “Anti-malware software should be able to detect either an unauthorized registry change or unexpected communication with the hardware,” he says.

The attack would also only work under a limited set of conditions.  The victim for instance would need to have done something to have the malware installed on the system. The victim’s headphone would need to be connected to the compromised system and the victim also needs not to be using the headphones for this attack to work, Young says.

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