Mobile Ad Hoc NETwork (MANET) is the most desired topic of research amidst researchers mainly because of its flexibility and independent nature of network infrastructures. MANET's unique characteristics, like the dynamic behaviour related to network topology, limited bandwidth and battery resources, make routing a challenging task. Currently, several efficient routing protocols based on topology and geographical information have been proposed for MANET. However, most of them assume a trusted and cooperative oriented environment. With the influence of adversaries, the MANET routing protocols are vulnerable to various kinds of attacks. Recently, several research efforts have been made to counter these adversarial activities. This survey investigates the latest and on demand security concerns in MANET. This paper comprehensively discusses the existing attacks in the network layer, various categories of defense mechanisms in the literature, and their benefits and defects when applying to MANET. This paper also classifies the attacks and defense mechanisms based on the routing nature, such as topological and geographical. It also discusses intrusion detection systems as well as anonymous routing in MANETs. Finally, it discusses the challenges and future directions in routing security over the MANET environment.
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