In this paper we consider communication from a source to a destination over a wireless network with the help of a set of authenticated relays. We focus on a special ``diamond'' network, where there is no direct link between the source and the destination; however the relay nodes help to establish such a communication. There is a single adversarial node which injects signals to disrupt this communication. Like the source, it can only influence the destination through the relays. We develop an approximate characterization of the reliable transmission rate in the presence of such an adversary. This is done by developing an outer bound, and demonstrating an achievable strategy that is within a constant number of bits of the outer bound, regardless of the channel values. A deterministic version of the same problem is solved exactly, yielding insights which are used in the approximate characterization.