  We address the problem of attack detection and attack correction for
multi-output discrete-time linear time-invariant systems under sensor attack.
More specifically, we focus on the situation where adversarial attack signals
are added to some of the system's output signals. A 'security index' is defined
to characterize the vulnerability of a system against such sensor attacks.
Methods to compute the security index are presented as are algorithms to detect
and correct for sensor attacks. The results are illustrated by examples
involving multiple sensors.
