Dynamic Source Routing (DSR) is a well known source routing protocol for ad hoc networks. The algorithm depends on the cooperative participation of the nodes that enables route discovery from a source node to a destination node. However, if a group of nodes do not cooperate, the performance of the DSR protocol may be severely degraded. This paper presents a probabilistic attack model on the DSR protocol and analyses its effect on the routing performance. Simulations results of the model show that the effect of the attack is catastrophic only if a large number of nodes are compromised and there is no detection mechanism. As an interesting observation, the analysis also shows that the attack model can also be used to improve the performance of the DSR protocol.