  Due to physical isolation as well as use of proprietary hardware and
protocols, traditional real-time systems (RTS) were considered to be
invulnerable to security breaches and external attacks. However, this
assumption is being challenged by recent attacks that highlight the
vulnerabilities in such systems. In this paper, we focus on integrating
security mechanisms into RTS (especially legacy RTS) and provide a metric to
measure the effectiveness of such mechanisms. We combine opportunistic
execution with hierarchical scheduling to maintain compatibility with legacy
systems while still providing flexibility. The proposed approach is shown to
increase the security posture of RTS systems without impacting their temporal
constraints.
