AbstractVehicularad hocnetworks (VANETs) are becoming very popular and a promising application of the so‐called mobilead hocnetworks (MANET) technology. It has attracted recently an increasing attention from many car manufacturers as well as the wireless communication research community. Despite its tremendous potential to enhance road safety and to facilitate traffic management, VANET suffers from a variety of security and privacy issues which may dramatically limit their applications. In this paper, we address the problem of packet dropping attack launched against routing protocol's control packets, which represents one of the most aggressive attacks in MANET. The aim of this attack is to force nodes in the network to choose hostile nodes as relays to disseminate the partial topological information, thereby exploiting the functionality of the routing protocol to retain control packets. In particular, in optimized link state routing (OLSR) protocol, if a collusive packet dropping attack is launched during the propagation of the topology control (TC) packets, the topology information will fail in being disseminated to the entire network, which finally results in routing disruption. This paper focuses on the packet dropping attack, launched against OLSR, where two malicious multipoint relay (MPR) nodes collude to disrupt the topology discovery process. Based on the analysis of the attacker's behavior and the attack's consequence, we propose an acknowledgement‐based mechanism as a countermeasure to enhance the security of OLSR. This mechanism helps the OLSR protocol to be less vulnerable to such attack by detecting and then isolating malicious nodes in the network. The simulation results of the proposed scheme show high detection rate under various scenarios. Copyright © 2008 John Wiley & Sons, Ltd.