  Integrated sensing and communication (ISAC) has already established itself as
a promising solution to the spectrum scarcity problem, even more so when paired
with a reconfigurable intelligent surface (RIS) as RISs can shape the
propagation environment by adjusting their phase-shift coefficients. Albeit the
potential performance gain, a RIS also poses a security threat to the system:
in this paper, we explore both sides of the RIS presence in a multi-user MIMO
(multiple-input multiple-output) ISAC network. We first develop an alternating
optimization algorithm, obtaining the active and passive beamforming vectors
maximizing the sensing signal-to-noise ratio (SNR) under minimum
signal-to-interference-plus-noise ratio (SINR) constraints for the
communication users and finite power budget. We also investigate the
destructive potential of RIS by devising a RIS phase-shift optimization
algorithm that minimizes sensing SNR while preserving the same minimum
communication SINR previously guaranteed by the system. We further investigate
the impact of the RIS's individual element failures on the system performances.
The simulation results show that the RIS performance-boosting potential is as
good as its destructive one and that both of our optimization strategies show
some resilience towards the investigated impairments.
