  This work presents a rigorous analysis of the adverse effects of
cyber-physical attacks on discrete-time distributed multi-agent systems, and
propose a mitigation approach for attacks on sensors and actuators. First, we
show how an attack on a compromised agent can propagate and affect intact
agents that are reachable from it. This is, an attack on a single node
snowballs into a network-wide attack and can even destabilize the entire
system. Moreover, we show that the attacker can bypass the robust $H_{\infty}$
control protocol and make it entirely ineffective in attenuating the effect of
the adversarial input on the system performance. Finally, to overcome
adversarial effects of attacks on sensors and actuators, a distributed adaptive
attack compensator is designed by estimating the normal expected behavior of
agents. The adaptive attack compensator is augmented with the controller and it
is shown that the proposed controller achieves secure consensus in presence of
the attacks on sensors and actuators. This controller does not require to make
any restrictive assumption on the number of agents or agent's neighbors under
direct effect of adversarial input. Moreover, it recovers compromised agents
under actuator attacks and avoids propagation of attacks on sensors without
removing compromised agents. The effectiveness of the proposed controller and
analysis is validated on a network of Sentry autonomous underwater vehicles
subject to attacks under different scenarios.
