Wireless networks are being used increasingly in industrial, health care, military and public-safety environments. In these environments security is extremely important because a successful attack against the network may pose a threat to human life. To secure such wireless networks against hostile attack requires both preventative and detective measures.In this paper we propose a novel intrusion detection mechanism that identifies man-in-the-middle and wormhole attacks against wireless mesh networks by external adversaries. A simple modification to the wireless MAC protocol is proposed to expose the presence of an adversary conducting a frame-relaying attack. We evaluate the modified MAC protocol experimentally and show the detection mechanism to have a high detection rate, no false positives and a small computational and communication overhead.