A Countermeasure against Zero-dynamics Sensor Attack via Generalized Hold Feedback

The zero-dynamics attack is well-known for its lethality and stealthiness. This infamous attack has usually been studied as a type of actuator attack. In this paper, however, we focus on the zero-dynamics attack having a form of a sensor attack. In particular, when the system monitors abnormal behavior of the plant using the anomaly detector, an undetectable sensor attack can be generated, which deceives the anomaly detector. It is noticed that this sensor attack is not so effective when the plant is stable even if the attack is still undetectable. In view of this point, we propose to reexamine the so-called ‘generalized hold’ as a countermeasure against the undetectable sensor attack. Specifically, using the fact that the output feedback composed of the ‘generalized hold’ can stabilize the unstable systems by selecting an appropriate hold function, we show that the plant can be safe from the undetectable sensor attack.