Security is an essential requirement in mobile ad hoc networks (MANETs). Compared to wired networks, MANETs are more vulnerable to security attacks due to the lack of a trusted centralized authority, easy eavesdropping, dynamic network topology, and limited resources. The security issue of MANETs in group communications is even more challenging because of the involvement of multiple senders and multiple receivers. In this paper, we present a simulationbased study of the impacts of different types of attacks on mesh-based multicast in MANETs. We consider the most common types of attacks, namely rushing attack, blackhole attack, neighbor attack and jellyfish attack. Specifically we study how the processing delay of legitimate nodes, the number of attackers and their positions affect the performance metrics of a multicast session such as packet delivery ratio, throughput, end-to-end delay, and delay jitter. To the best of our knowledge, this is the first paper that studies the vulnerability and the performance of multicast in MANETs under various security threats.