  A mobile ad hoc network (MANET) is a collection of autonomous nodes that
communicate with each other by forming a multi-hop radio network and
maintaining connections in a decentralized manner. Security remains a major
challenge for these networks due to their features of open medium, dynamically
changing topologies, reliance on cooperative algorithms,absence of centralized
monitoring points, and lack of clear lines of defense. Most of the routing
protocols for MANETs are thus vulnerable to various types of attacks. Ad hoc
on-demand distance vector routing (AODV) is a very popular routing algorithm.
However, it is vulnerable to the well-known black hole attack, where a
malicious node falsely advertises good paths to a destination node during the
route discovery process. This attack becomes more sever when a group of
malicious nodes cooperate each other. In this paper, a defense mechanism is
presented against a coordinated attack by multiple black hole nodes in a MANET.
The simulation carried out on the proposed scheme has produced results that
demonstrate the effectiveness of the mechanism in detection of the attack while
maintaining a reasonable level of throughput in the network.
