  An actuator is a device that converts electricity into another form of
energy, typically physical movement. They are absolutely essential for any
system that needs to impact or modify the physical world, and are used in
millions of systems of all sizes, all over the world, from cars and spacecraft
to factory control systems and critical infrastructure. An actuator is a "dumb
device" that is entirely controlled by the surrounding electronics, e.g., a
microcontroller, and thus cannot authenticate its control signals or do any
other form of processing. The problem we look at in this paper is how the wires
that connect an actuator to its control electronics can act like antennas,
picking up electromagnetic signals from the environment. This makes it possible
for a remote attacker to wirelessly inject signals (energy) into these wires to
bypass the controller and directly control the actuator.
  To detect such attacks, we propose a novel detection method that allows the
microcontroller to monitor the control signal and detect attacks as a deviation
from the intended value. We have managed to do this without requiring the
microcontroller to sample the signal at a high rate or run any signal
processing. That makes our defense mechanism practical and easy to integrate
into existing systems. Our method is general and applies to any type of
actuator (provided a few basic assumptions are met), and can deal with
adversaries with arbitrarily high transmission power. We implement our
detection method on two different practical systems to show its generality,
effectiveness, and robustness.
