The expanded development and diverse mission operations of unmanned air vehicles (UAV) have exposed INFOrmation SECurity (INFOSEC) and COMmunication SECurity (COMSEC) concerns that are not easily addressed in traditional federated or currently deployed integrated modular avionics (IMA) systems. The need to operate military UAVs in civil airspace communicating over unclassified links to foreign air traffic control systems and keep sensitive and/or classified information separated without increasing space, weight and power (SWaP) poses challenges to UAV systems architecture. In this paper the MILS (Multiple Independent Levels of Security) software architecture will be discussed in relation to how it can fulfill these disparate UAV system design requirements.