A Mobile ad-hoc network (MANET) is an autonomous system of routers (and associated hosts) connected by wireless links. The Ad hoc On-Demand Distance Vector (AODV) routing protocol meant for MANETs is an improvement over Destination-Sequenced Distance Vector (DSDV) because it typically minimizes the number of required broadcasts by creating routes on demand basis, as opposed to maintaining a complete list of routes as in the DSDV algorithm. AODV is vulnerable to both external and internal security attacks. In Specification Based Intrusion Detection, the correct behavior of critical objects are manually abstracted and crafted as security specifications, which are compared with the actual behavior of the objects. We propose a technique to analyze the vulnerabilities of AODV protocol, specifically to monitor the network layer attacks such as Black Hole attack and to develop a Specification Based Intrusion Detection System (IDS) using soft computing technique. The proposed system is based on fuzzy logic which analyzes the performance of the wireless nodes in a MANET and provides relevant information about the various attacks. The Fuzzy logic control (FLC) system specifies a set of Fuzzy rules based on the essential features of the AODV routing protocol such as RREQ forwarding rate, Packet forwarding rate and so on. The performance of the MANET is analyzed based on the FLC system results. © Springer-Verlag Berlin Heidelberg 2010.