Device-to-device (D2D) communication in cellular networks refers to a technology that enables direct transmission and reception between devices in proximity without infrastructures such as base stations (BSs). It has consistently attracted attention due to its key role extending cellular coverage through the configuration of clusters, in which the D2D devices belonging to a cluster can either be a cluster head or a cluster member. In this paper, we investigate the effect of channel state information (CSI) forgery attacks of a single malicious user (i.e., attacker) belonging to a cluster. Particularly, we investigate two major threat models: 1) clustering failure attack, where an attacker reports its overestimated CSI instead of the original one to the BS during clustering, 2) quality of service (QoS) degradation attack where an attacker reports its underestimated CSI instead of the original one to the cluster head during the intracluster D2D communications (i.e., D2D multicasting). We define metrics to measure each CSI forgery attack as clustering failure probability and cluster sum-rate, respectively, and further derive a closed-form expression for the clustering failure probability. In addition, we propose threshold-based defense mechanisms as countermeasures against CSI forgery attacks and find suboptimal threshold values. Through simulations, we evaluate the performance of the defense mechanisms in terms of clustering failure probability and cluster sum-rate, comparing with optimal simulation results. INDEX TERMSWireless network security, device-to-device (D2D) communications, D2D clustering, channel state information (CSI), CSI forgery. JONGHYUN KIM (Member, IEEE) received the Ph.D. degree in computer science from The