Mobile Ad Hoc Networks (MANETs) are highly vulnerable to attacks caused by the wireless medium, dynamically changing network topology, cooperative routing algorithms, lack of infrastructure, etc. In recent decade, lots of researches had been made or still in process to resolve the security defects in MANETs and many security schemes from different aspects of MANETs have been proposed, such as secure routing protocol, key management, trust management, intrusion detection system (IDS), etc. Attack prevention mechanism, such as authentication and encryption, can be used as the first line of defense to reduce the possible attacks. Intrusion detection is the second line of defense to detect attacks. Therefore, rapid and accurate detection of attacks reduces damage to the networks. Main research on IDS in MANETs categorized i two major groups: the architecture and the detection engine. The techniques for detecting the malicious attacks in detection engines are generally In both methods, the simulation results show the advantage of updating normal profiles compared to conventional schemes. Keywords: Anomaly Detection, Intrusion Detection System, Mobile ad-Hoc Networks, AODV, Robust Principal Component Analysis, Hidden Markov Model
