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Mobile Ad Hoc Network (MANET) is a collection of communication devices or nodes that wish to communicate without any fixed infrastructure and pre-determined organization of available links. The nodes in MANET themselves are responsible for dynamically discovering other nodes to communicate. Although the ongoing trend is to adopt ad hoc networks for commercial uses due to their certain unique properties, the main challenge is the vulnerability to security attacks. A number of challenges like open peer-to-peer network architecture, stringent resource constraints, shared wireless medium, dynamic network topology etc. are posed in MANET. As MANET is quickly spreading for the property of its capability in forming temporary network without the aid of any established infrastructure or centralized administration, security challenges has become a primary concern to provide secure communication. In this thesis, we identify the existent security threats an ad hoc network faces, the security services required to be achieved and the countermeasures for attacks in each layer. To accomplish our goal, we have done literature survey in gathering information related to various types of attacks and solutions, as well as we have made comparative study to address the threats in different layers. Finally, we have identified the challenges and proposed solutions to overcome them. In our study, we have found that necessity of secure routing protocol is still a burning question. There is no general algorithm that suits well against the most commonly known attacks such as wormhole, rushing attack etc. In conclusion, we focus on the findings and future works which may be interesting for the researchers like robust key management, trust based systems, data security in different layer etc. However, in short, we can say that the complete security solution requires the prevention, detection and reaction mechanisms applied in MANET.