  Industrial facilities and critical infrastructures are transforming into
"smart" environments that dynamically adapt to external events. The result is
an ecosystem of heterogeneous physical and cyber components integrated in
cyber-physical systems which are more and more exposed to cyber-physical
attacks, i.e., security breaches in cyberspace that adversely affect the
physical processes at the core of the systems.
  We provide a formal compositional metric to estimate the impact of
cyber-physical attacks targeting sensor devices of IoT systems formalised in a
simple extension of Hennessy and Regan's Timed Process Language. Our impact
metric relies on a discrete-time generalisation of Desharnais et al.'s weak
bisimulation metric for concurrent systems. We show the adequacy of our
definition on two different attacks on a simple surveillance system.
