Existing cooperative diversity protocols are designed with the inherent assumption that users exhibit cooperative behavior all the time. However, in a practical cooperative wireless system users may misbehave in malicious or selfish manner. Thus existing cooperative diversity protocols are inherently vulnerable to misbehaving users as they lack a mechanism to detect the presence of such users. In this paper we examine the physical layer consequences of a malicious user which exhibits cooperative behavior in a stochastic manner. We assume that the malicious user exploits the inherent uncertainty of the wireless channel to hide its malicious behavior. We consider a malicious user which exhibits cooperative and malicious behaviors according to firstorder Markov chain. By behaving stochastically the malicious user attempts to mimic the underlying Markov property of Rayleigh fading channels. Based on this model we examine physical layer performance of cooperative Detect-and-Forward (DF). We show that a malicious user incurs significant degradation in cooperative diversity gain. Our results indicate that misbehaving users may pose formidable challenge to practical implementation of cooperative diversity. Hence, it may be difficult to implement practical wireless cooperative networks without a mechanism to ensure cooperation.