The effective tremendous deployment of ad hoc networks is incontestably braked by their unreliability in terms of security and quality of services. In this paper, we focus on security problems and show that despite of efforts made in the ad hoc security field, many security issues still jeopardize correct MANETs routing operation. For such threats, we propose an IDS (Intrusion Detection System) solution for which cryptographic-based solutions are ineffective. Actually, authenticated nodes legitimately present in the network are able to send faked routing messages to compromise the routing and then communication between nodes. To cope with such security attacks, we propose an IDS dedicated to the OLSR protocol and well fitted to its characteristics and operation. In addition, our IDS is implemented on all network's nodes which act cooperatively by continually analyzing routing messages semantics. When an intrusion is detected, alerts are flooded and intruders are banished from the network. We have finally implemented this IDS and performances evaluation shows the intrusion detection effectiveness.