Inaudible Attack on Smart Speakers With Intentional Electromagnetic Interference

This article demonstrates an inaudible attack on smart speakers using electromagnetic interference (EMI). The EMI induces voltages on the order of a few millivolts on conductors, which are then converted into baseband signals by exploiting the inherent nonlinearity of microphones. The EMI signal is specially preprocessed to minimize the useless harmonics generation at the microphone output signals, which significantly improves the recognition rate as well as nullify the previous countermeasures based on the harmonics detection. The sensitive carrier frequency found by our proposed method can improve the attack distance as well. A measurement-based methodology is applied to locate the sensitive regions for noise coupling without knowing the layout of the printed circuit board (PCB), and the transfer function is also obtained to insure the main coupling location. Our experiments show that in open space, intentional EMI under 2.5 W can inject commands at distances up to 2.5 m on smart speakers.