  The objective in this paper is to study and develop conditions for a network
of multi-agent cyber-physical systems (MAS) where a malicious adversary can
utilize vulnerabilities in order to ensure and maintain cyber attacks
undetectable. We classify these cyber attacks as undetectable in the sense that
their impact cannot be observed in the generated residuals. It is shown if an
agent that is the root of a rooted spanning tree in the MAS graph is under a
cyber attack, the attack is undetectable by the entire network. Next we
investigate if a non-root agent is compromised, then under certain conditions
cyber attacks can become detectable. Moreover, a novel cyber attack that is
designated as quasi-covert cyber attack is introduced that can be used to
eliminate detectable impacts of cyber attacks to the entire network and
maintain these attacks as undetected. Finally, an event-triggered based
detector is proposed that can be used to detect the quasi-covert cyber attacks.
Numerical simulations are provided to illustrate the effectiveness and
capabilities of our proposed methodologies.
