Researchers at Ben-Gurion University of the Negev have demonstrated a novel way to spy on electronic conversations. A new paper 
released today outlines a novel passive form of the 
TEMPEST attack
 called Glowworm, which converts minute fluctuations in the intensity of
 power LEDs on speakers and USB hubs back into the audio signals that 
caused those fluctuations.
The Cyber@BGU
 team—consisting of Ben Nassi, Yaron Pirutin, Tomer Gator, Boris Zadov, 
and Professor Yuval Elovici—analyzed a broad array of widely used 
consumer devices including smart speakers, simple PC speakers, and USB 
hubs. The team found that the devices' power indicator LEDs were 
generally influenced perceptibly by audio signals fed through the 
attached speakers.
Although the fluctuations in LED signal strength generally aren't 
perceptible to the naked eye, they're strong enough to be read with a 
photodiode coupled to a simple optical telescope... more 
Check out the other eavesdropping hits that have come out of Ben-Gurion University... here