MANET is the field of network that has been used for data communication based on without infrastructure communication. In the process of MANET intermediate nodes that act as the gateway are responsible for data forwarding to destination. Various routing protocols have been used in MANET that is responsible for path generation and data communication over the network. Security is the major concern in MANET that causes various issues in terms of confidentiality, integrity and availability. In this paper various attacks that degrades the performance of the network has been discussed. Attacks that degrade the performance of the network are grey hole, black hole, jamming and flooding attack. In this paper a review has been done on the working of attacks as well as approaches that can be used to mitigate attack effect over the network.