In Mobile Ad-Hoc Networks (MANETs), security is one of the most important concerns because a MANETs system is much more vulnerable to attacks than a wired or infrastructure-based wireless network. Designing an effective security protocol for MANET is a very challenging task. This is mainlydue to the unique characteristics of MANETs, namely shared broadcast radio channel, insecure operatingenvironment, lack of central authority, lack of association among users, limited availability of resources, and physical vulnerability. In this paper we present simulation based study of the impact of neighbor attack on mesh-based Mobile Ad-Hoc Network (MANET). And also we study the number of attackers and position affects the performance metrics such as packet delivery ratio and throughput. The study enables us to propose a secure neighbor detection mechanism (SNDM). A generic detection mechanism against neighbor attack for On Demand Routing Protocols is simulated on GlomoSim environment.