  Protecting the network layer from malicious attacks is an important and
challenging security issue in mobile ad hoc networks (MANETs). In this paper, a
security mechanism is proposed to defend against a cooperative gray hole attack
on the well known AODV routing protocol in MANETs. A gray hole is a node that
selectively drops and forwards data packets after it advertises itself as
having the shortest path to the destination node in response to a route request
message from a source node. The proposed mechanism does not apply any
cryptographic primitives on the routing messages. Instead, it protects the
network by detecting and reacting to malicious activities of any node.
Simulation results show that the scheme has a significantly high detection rate
with moderate network traffic overhead.
