  Reconfigurable intelligent surfaces (RISs) have demonstrated significant
potential for enhancing communication system performance if properly
configured. However, a RIS might also pose a risk to the network security. In
this letter, we explore the impact of a malicious RIS on a multi-user
multiple-input single-output (MISO) system when the system is unaware of the
RIS's malicious intentions. The objective of the malicious RIS is to degrade
the \ac{SNR} of a specific \ac{UE}, with the option of preserving the SNR of
the other UEs, making the attack harder to detect. To achieve this goal, we
derive the optimal RIS phase-shift pattern, assuming perfect channel state
information (CSI) at the hacker. We then relax this assumption by introducing
CSI uncertainties and subsequently determine the RIS's phase-shift pattern
using a robust optimization approach. Our simulations reveal a direct
proportionality between the performance degradation caused by the malicious RIS
and the number of reflective elements, along with resilience toward CSI
uncertainties.
