Mobile Ad hoc NETworks (MANETs) is a class of wireless networks that have been researched extensively over the recent years. The security challenges arise due to MANET's self-configuration and self-maintenance capabilities. In comparison to wired network the mobile ad hoc network is more exposed to being attacked. Because of its fundamental properties, such as dynamic topology, limited power and limited bandwidth, it is very hard to achieve absolute security in the mobile ad hoc network. This paper presents a thorough overview of security issues in mobile ad hoc network.