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Researchers find microphones are susceptible to light-based signal injection

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The paper:

Pretty amazing.  Alexa can be hacked with a laser pointer.

--- Quote ---We propose a new class of signal injection attacks
on microphones based on the photoacoustic effect: converting
light to sound using a microphone. We show how an attacker
can inject arbitrary audio signals to the target microphone
by aiming an amplitude-modulated light at the microphone’s
aperture. We then proceed to show how this effect leads to
a remote voice-command injection attack on voice-controllable
systems. Examining various products that use Amazon’s Alexa,
Apple’s Siri, Facebook’s Portal, and Google Assistant, we show
how to use light to obtain full control over these devices at
distances up to 110 meters and from two separate buildings.
--- End quote ---

It is a cool idea, but photo-acoustic  tends to be rather low amplitude, especially at higher frequencies. So the Laser would need to be quite high in power. From distant memory the sound was not that load even with a 5 mW laser.

They tried 5mW and 60mW lasers.  Both worked with different devices depending on range.

They also indicate that this is a problem specifically with MEMs microphones.

I wonder how it works?  They claim it is causing mechanical movement of the microphone membrane, not some electrical effect.  I guess it is thermal heating of the air near the membrane?

The classic photo-acoustic is heating some material (solid, but also gas) with light and the heat than via thermal expansion (for solid usually by conduction to a gas layer) causes sound. This was a microphone can be used to detect the AC part of temperature, especially surface temperature.

Some microphones may have an extra effect from light entering the microphone and acting like a photodiode inside.


--- Quote from: Kleinstein on November 05, 2019, 10:05:46 pm ---Some microphones may have an extra effect from light entering the microphone and acting like a photodiode inside.

--- End quote ---
I bet this is the actual source of trouble. The MEMS microphones have a hole directly to the chip, which is of course extremely sensitive to any light.
Try with a dynamic microphone and your laser injection will be busted :)


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