Multicast network support is becoming an increasingly important technology for both military and commercial distributed and group based applications. The security services such as confidentiality, authenticity and data integrity are necessary for both wired and wireless networks to protect basic applications. In this paper we present a simulation based study of the impact of different types of attacks in mobile ad hoc networks. We consider the most common type of attacks namely Gray hole attack and Worm hole attack. Specifically, we study how these attacks affect the performance metrics of a multicast session such as packet delivery ratio, packet latency and packet-consumed energy.