  While reconfigurable intelligent surface (RIS) technology has been shown to
provide numerous benefits to wireless systems, in the hands of an adversary
such technology can also be used to disrupt communication links. This paper
describes and analyzes an RIS-based attack on multi-antenna wireless systems
that operate in time-division duplex mode under the assumption of channel
reciprocity. In particular, we show how an RIS with a non-diagonal (ND) phase
shift matrix (referred to here as an ND-RIS) can be deployed to maliciously
break the channel reciprocity and hence degrade the downlink network
performance. Such an attack is entirely passive and difficult to detect and
counteract. We provide a theoretical analysis of the degradation in the sum
ergodic rate that results when an arbitrary malicious ND-RIS is deployed and
design an approach based on the genetic algorithm for optimizing the ND
structure under partial knowledge of the available channel state information.
Our simulation results validate the analysis and demonstrate that an ND-RIS
channel reciprocity attack can dramatically reduce the downlink throughput.
