Vehicular Adhoc Network is a type of (MANET) Mobile Adhoc Network that enables vehicles on the road to intelligently interact and communicate with other vehicle and road side infrastructure unit. It is prone to several type of attacks and one such attack is Grayhole attack. Gray hole attack is one of the attack on routing specification in which malicious node selectively drops packets coming from the source. Due to lack of security in Adhoc on Demand Distance Vector (AODV) routing protocol, Grayhole attack disrupts the performance of network and render communication impossible. This paper reviews various attacks in VANET including Grayhole attack on AODV routing protocol and provides a survey of existing defence approaches to mitigate them
