Ad hoc networks are subject of many recent research efforts. Especially in mobile scenarios, self-organizing networks are of high interest. While the routing aspects of mobile ad hoc networks (MANETs) are already well understood, the research activities about security in MANETs are still at their beginning. MANETs pose a number of new security problems in addition to the problems of regular networks. In addition to the classical security threats we identified additional ways how nodes may attack security in an Ad hoc network. In this paper, we explore several aspects of possible (mis)behaviour of MANET participants. We introduce a systematology of attacks on a MANET. Further, we describe the results of simulations that show, how misbehaving nodes affect MANETs based on the DSR routing protocol. Finally we outline a security architecture that provides substantial security services for Ad hoc networks.