  We propose a new class of signal injection attacks on microphones by
physically converting light to sound. We show how an attacker can inject
arbitrary audio signals to a 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 control over
these devices at distances up to 110 meters and from two separate buildings.
Next, we show that user authentication on these devices is often lacking,
allowing the attacker to use light-injected voice commands to unlock the
target's smartlock-protected front doors, open garage doors, shop on e-commerce
websites at the target's expense, or even unlock and start various vehicles
connected to the target's Google account (e.g., Tesla and Ford). Finally, we
conclude with possible software and hardware defenses against our attacks.
