The rapidly growing popularity of massively multiplayer online games (MMOGs) has
given rise to an increase in the number of players worldwide. MMOGs enable many players to interact together through a shared sense of presence created by the game environment. The peer-to-peer (P2P) network topology overcomes communication bottleneck problems associated with centralised client/server systems. However, many architectures are proposed in an ad hoc manner and enhancing the security of such systems is an elusive research problem. In this paper, we address this important issue by making the following contributions. Firstly, we formalise the notion of P2P-based MMOGs and demonstrate that existing P2P-based MMOG architectures can be unified using our model. To our knowledge, this is the first time that this has been done in the literature. Secondly, we use our model to develop a real-time cheating detection mechanism to identify cheating players, which can be used to expose several MMOG cheating strategies. In addition, we propose a new reputation-based system as well as a blacklisting system for P2P-based MMOGs to enhance the cheating detection process. Finally, we discuss the concept of protecting MMOGs against cheating using automated programmes.