Multicast communication in Mobile Ad Hoc Networks (MANET) has been used for group communication-based applications such as battlefield, rescue-field and emergency operation. Multicasting consumes less bandwidth even though it has large participants in its communication. Security issues become more severe due to its high scalability and routing complexity. In this paper, we analyze the vulnerabilities of PUMA (Protocol for Unified Multicasting through Announcements) and MAODV (Multicast Ad-hoc On-demand Distance Vector routing) against various possibilities of internal attacks. We also identify MA (Multicast Announcement) packet fabrication type of internal attacks in PUMA. It is very difficult to detect this attack efficiently when it changes its behaviors arbitrarily. We propose multicast activity-based overhearing technique to identify this attacker node in the multicast group. NS-2 simulation results show the attacker impacts on the performance metrics such as Packet Delivery Ration (PDR) and delay of PUMA and MAODV multicast routing protocols. Also simulation results supports that our technique is well-efficient to detect attacker with respect to less control overhead and false negative alarm rate.