  In this letter, we study an attack that leverages a reconfigurable
intelligent surface (RIS) to induce harmful interference toward multiple users
in massive multiple-input multiple-output (mMIMO) systems during the data
transmission phase. We propose an efficient and flexible weighted-sum projected
gradient-based algorithm for the attacker to optimize the RIS reflection
coefficients without knowing legitimate user channels. To counter such a
threat, we propose two reception strategies. Simulation results demonstrate
that our malicious algorithm outperforms baseline strategies while offering
adaptability for targeting specific users. At the same time, our results show
that our mitigation strategies are effective even if only an imperfect estimate
of the cascade RIS channel is available.
