Computer networks are present throughout all sectors of our critical infrastructure and these networks are under a constant threat of cyber attack. One prevalent computer network threat takes advantage of unauthorized, and thus insecure, hardware on a network. This paper presents a prototype simulation system for network risk assessment that is intended for use by administrators to simulate and evaluate varying network environments and attacker/defender scenarios with respect to authorized and unauthorized hardware. The system is built on the agent-based modeling paradigm and captures emergent system dynamics that result from the interactions of multiple network agents including regular and administrator users, attackers, and defenders in a network environment. The agent-based system produces both metrics and visualizations that provide insights into network security risk and serve to guide the search for efficient policies and controls to protect a network from attacks related to unauthorized hardware. The simulation model is unique in the current literature both for its network threat model and its visualized agent-based approach. We demonstrate the model via a case study that evaluates risk for several candidate security policies on a representative computer network.