  Differential signaling is a method of data transmission that uses two
complementary electrical signals to encode information. This allows a receiver
to reject any noise by looking at the difference between the two signals,
assuming the noise affects both signals in the same way. Many protocols such as
USB, Ethernet, and HDMI use differential signaling to achieve a robust
communication channel in a noisy environment. This generally works well and has
led many to believe that it is infeasible to remotely inject attacking signals
into such a differential pair. In this paper we challenge this assumption and
show that an adversary can in fact inject malicious signals from a distance,
purely using common-mode injection, i.e., injecting into both wires at the same
time. We show how this allows an attacker to inject bits or even arbitrary
messages into a communication line. Such an attack is a significant threat to
many applications, from home security and privacy to automotive systems,
critical infrastructure, or implantable medical devices; in which incorrect
data or unauthorized control could cause significant damage, or even fatal
accidents.
  We show in detail the principles of how an electromagnetic signal can bypass
the noise rejection of differential signaling, and eventually result in
incorrect bits in the receiver. We show how an attacker can exploit this to
achieve a successful injection of an arbitrary bit, and we analyze the success
rate of injecting longer arbitrary messages. We demonstrate the attack on a
real system and show that the success rate can reach as high as $90\%$.
Finally, we present a case study where we wirelessly inject a message into a
Controller Area Network (CAN) bus, which is a differential signaling bus
protocol used in many critical applications, including the automotive and
aviation sector.
