Cooperative wireless communications offers a new dimension of diversity by emulating transmit antenna diversity to provide reliable communications. In cooperative diversity, single-antenna radios behave as relays between a source and destination, and the performance improvements are due to cooperation of the source and the relay. However, a misbehaving relay can degrade the envisaged performance improvements severely. In practice, there are no mechanisms to ensure adherence of the relay to cooperation strategy. Due to this dependency on relay's behavior cooperative diversity presents a new security challenge at the physical layer. In this paper, we investigate performance of cooperative diversity in the presence of a semi-malicious relay which does not conform to rules of cooperation at all time. The relay behavior is characterized by a probabilistic cooperation model which exploits the uncertainty in the wireless channel. Based on this model, we obtain the performance degradation in cooperative diversity both by analysis and simulation.