Ad-hoc networks are attractive, since they can provide a high level of connectivity without the need of a fixed infrastructure. Nodes that are not within the same transmission range communicate through multi-hops, where intermediate nodes act as relays. Mutual cooperation of all the participating nodes is necessary for proper operation of MANET. However, nodes in MANET being battery-constrained, they tend to behave selfishly while forwarding packets. In this paper, we have investigated the security of MANET AODV routing protocol by identifying the impact of selfish nodes on it. It was observed that due to the presence of selfish nodes, packet loss in the network increases and the performance of MANET degrades significantly. Finally a game theoretic approach is used to mitigate the selfishness attack. All the nodes in MANET should cooperate among themselves to thwart the selfish behavior of attacker nodes.