As security is a very challenging issue in ad hoc networks, variety of research works related to security of ad hoc networks are being reported for last many years. In the present work, we propose a new sort of attack titled Twin-Node Neighbour Attack (TNNA), wherein two malicious nodes in close vicinity of each other exploits the provision of broadcast nature of Hello Messages in AODV routing protocol along with non-provision of any restriction regarding authentication of participating nodes. Mitigation measures are designed to lessen or perhaps remove security flaws and threats altogether. Detection and mitigation of TNNA attack are also proposed and discussed. The network's performance has been measured using four metrics viz. Packet Delivery Ratio, Throughput, Total Number of Received Packets and Average End-toEnd Delay. It is evident from simulations that the TNNA attack is significantly detrimental to the performance of WANETs using AODV routing protocol. After attack throughput of legitimate flow is found to be less than 5 % as compared to the Throughput without attack, when the data rate of malicious node is 100 Kibps. Due to stress of malicious flow of 100 Kibps, the number of transmitted (received) data packets of legitimate flow is reduced by a factor of more than 20.