A system and method enforce a security policy in a message-based operating system by controlling access to an operating system authenticator. The control occurs in response to an invocation of a microkernel call that initiates a process manager's function in accordance with the access right retained stored in a microkernel. The system and method control access to a server serving a client in the message-based operating system according to a token. The token is issued by the operating system authenticator and establishes a client's access to the services provided by the server.