Abstract: Mobile ad hoc networks (MANET) has risen as a major next generation wireless networking technology. This network is a network of mobile nodes with dynamic structure. Here each node acts as a router for forwarding data to other nodes. Due its dynamic nature, security has become a primary concern to provide protected communication between different nodes in ad hoc networks. There are a number of challenges in security design as ad hoc network is a decentralized network. There are five layers in MANET and each of these layer is vulnerable to various attacks. In this paper we discuss about various attacks and their protection mechanisms.