In the paper we present an overview of Saga Security System, a security architecture in open distributed systems. An agent in Saga Security System is called a Saga Agent. The authorization model in Saga Security System (Saga authorizationmodel) supportsthe novelconceptofa service path and provides uniform and flexible protection appropriate for advanced computational models such as objectoriented systems and cooperative agent systems. With respect to the security mechanism of Saga Security System, its key features are an access token and a Security Monitor. Access tokens are implemented using public key technology and ensure integrity of request messages issued by Saga Agents. In addition, we can regard the Security Monitor of a Saga Agent as a reference monitor for the Agent. Security of a Saga Agent during its traversal over distributed environments is controlled by the Security Monitor integrated with the Agent.