Zero-stealthy attack for sampled-data control systems: The case of faster actuation than sensing

In this paper, we introduce a new vulnerability of cyber-physical systems to malicious attack. It arises when the physical system, that is modeled as a continuous-time LTI system, is controlled by a digital controller, i.e., the output is measured only at discrete sensing times. Since the anomaly detector monitors the output signal, nothing abnormal can be detected through the output if the output looks normal at sensing times. This implies that if an (actuator) attack drives the internal states passing through the kernel of the output matrix at each sensing time, then the attack compromises the system while it remains stealthy. We show that this type of attack is feasible when the control system uses multi-rate sampling, and the sampling rate for input signal is higher than that for output measurements. Simulation results for the X-38 vehicle illustrate this new attack strategy possibly brings disastrous consequences.