  This paper focuses on the problem of cyber attacks for discrete event systems
under supervisory control. In more detail, the goal of the supervisor, who has
a partial observation of the system evolution, is that of preventing the system
from reaching a set of unsafe states. An attacker may act in two different
ways: he can corrupt the observation of the supervisor editing the sensor
readings, and can enable events that are disabled by the supervisor. This is
done with the aim of leading the plant to an unsafe state, and keeping the
supervisor unaware of that before the unsafe state is reached. A special
automaton, called attack structure is constructed as the parallel composition
of two special structures. Such an automaton can be used by the attacker to
select appropriate actions (if any) to reach the above goal, or equivalently by
the supervisor, to validate its robustness with respect to such attacks.
