Abstract: A security system for a computer system imposes specific limitations on who has access to the computer system and to exactly what operations and data. Viruses are securely contained and prevented from expanding into areas where they can destroy stored programs or data. Viruses are also prevented from being introduced or executed in a large number of instances. The totality of computer functions is broken up into a set of events with an associated set of capabilities and different capabilities are assigned to each user depending on the particular job which that user is to do on the computer system. Also, security labels are placed on each data file and other system resources, and on each process. Further, a range of hierarchy/category labels (MAC labels) is assigned to each process to define a sub-lattice in which special capabilities can apply.