A Mobile Ad-hoc NETwork (MANET) is an autonomous network that consists of mobile nodes that communicate with each other over wireless links. In the absence of a fixed infrastructure, nodes have to cooperate in order to provide the necessary network functionality. One of the principal routing protocols used in ad-hoc networks is AODV (Ad hoc On-demand Distance Vector) protocol. The security of the AODV protocol is threaded by a particular type of attack called 'Black hole' attack. In this attack a malicious node advertises itself as having the shortest path to the destination node. If these malicious nodes work together as a group then the damage will be very serious. This type of attack is called cooperative black hole attack.