  Undetectable attacks are an important class of malicious attacks threatening
the security of cyber-physical systems, which can modify a system's state but
leave the system output measurements unaffected, and hence cannot be detected
from the output. This paper studies undetectable attacks on cone-invariant
systems and multi-agent systems. We first provide a general characterization of
zero-dynamics attacks, which characterizes fully undetectable attacks targeting
the non-minimum phase zeros of a system. This geometrical characterization
makes it possible to develop a defense strategy seeking to place a minimal
number of sensors to detect and counter the zero-dynamics attacks on the
system's actuators. The detect and defense scheme amounts to computing a set
containing potentially vulnerable actuator locations and nodes, and a defense
union for feasible placement of sensors based on the geometrical properties of
the cones under consideration.
