  Actuator injection attacks pose real threats to all industrial plants
controlled through communication networks. In this manuscript, we study the
possibility of constraining the controller output (i.e. the input to the
actuators) by means of a dynamic filter designed to prevent reachability of
dangerous plant states - preventing thus attacks from inducing dangerous states
by tampering with the control signals. The filter synthesis is posed as the
solution of a convex program (convex cost with Linear Matrix Inequalities
constraints) where we aim at shifting the reachable set of control signals to
avoid dangerous states while changing the controller dynamics as little as
possible. We model the difference between original control signals and filtered
ones in terms of the H-infinity norm of their difference, and add this norm as
a constraint to the synthesis problem via the bounded-real lemma. Results are
illustrated through simulation experiments.
