  In this paper, we introduce a new vulnerability of cyber-physical systems to
malicious attack. It arises when the physical plant, that is modeled as a
continuous-time LTI system, is controlled by a digital controller. In the
sampled-data framework, most anomaly detectors monitor the plant's output only
at discrete time instants, and thus, nothing abnormal can be detected as long
as the sampled output behaves normal. This implies that if an actuator attack
drives the plant's state to pass through the kernel of the output matrix at
each sensing time, then the attack compromises the system while remaining
stealthy. We show that this type of attack always exists when the sampled-data
system has an input redundancy, i.e., the number of inputs being larger than
that of the outputs or the sampling rate of the actuators being higher than
that of the sensors. Simulation results for the X-38 vehicle and for the other
numerical examples illustrate this new attack strategy possibly brings
disastrous consequences.
