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 authorization model) supports the novel concept of a service path and provides uniform and flexible protection appropriate for advanced computational models such as object-oriented 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.