A multi-level security apparatus and method for a network employs a secure network interface unit (SNIU) coupled between each host or user computer unit and a network, and a security management (SM) architecture, including a security manager (SM) coupled to the network, for controlling the operation and configuration of the SNIUs coupled to the network. Each SNIU is operative at a session level of interconnection which occurs when a user on the network is identified and a communication session is to commence. When an SNIU is implemented at each computer unit on the network, a global security perimeter is provided. In a preferred embodiment, the SNIU is configured to perform a defined session level protocol (SLP), including the core functions of user interface, session manager, dialog manager, association manager and data sealer, and network interface. The SM architecture is implemented to ensure user accountability, configuration management, security administration, and validation key management on the network. The SM functions are distributed over three platforms, i.e., a SNIU security manager (SSM), an area security manager (ASM), and a network security manager (NSM).