  This chapter presents an overview on actuator attacks that exploit zero
dynamics, and countermeasures against them. First, zero-dynamics attack is
re-introduced based on a canonical representation called normal form. Then it
is shown that the target dynamic system is at elevated risk if the associated
zero dynamics is unstable. From there on, several questions are raised in
series to ensure when the target system is immune to the attack of this kind.
The first question is: Is the target system secure from zero-dynamics attack if
it does not have any unstable zeros? An answer provided for this question is:
No, the target system may still be at risk due to another attack surface
emerging in the process of implementation. This is followed by a series of next
questions, and in the course of providing answers, variants of the classic
zero-dynamics attack are presented, from which the vulnerability of the target
system is explored in depth. At the end, countermeasures are proposed to render
the attack ineffective. Because it is known that the zero-dynamics in
continuous-time systems cannot be modified by feedback, the main idea of the
countermeasure is to relocate any unstable zero to a stable region in the stage
of digital implementation through modified digital samplers and holders.
Adversaries can still attack actuators, but due to the re-located zeros, they
are of little use in damaging the target system.
