The design of secure routing techniques is a crucial and challenging requirement in mobile ad hoc networking. This is due to the fact that the highly dynamic nature of the ad hoc nodes, their limited transmission range, and their reliance on an implicit trust model to route packets make the routing protocols inherently susceptible to attacks. We propose a solution to prevent two important types of replay attacks on the Ad Hoc On-Demand Distance Vector (AODV) routing protocol. Our technique is based on strengthening the neighbor authentication mechanism by a simple extension to the AODV protocol. Analysis of the technique indicates that it achieves security with little overhead.