存取控制
Most modern, multi-user operating systems include role-based access control (RBAC) and thereby rely on authorization. Access control also uses authentication to verify the identity of consumers. When a consumer tries to access a resource, the access control process checks that the consumer has been authorized to use that resource. Authorization is the responsibility of an authority, such as a department manager, within the application domain, but is often delegated to a custodian such as a system administrator. Authorizations are expressed as access policies in some types of "policy definition application", e.g. in the form of an access control list or a capability, or a policy administration point e.g. XACML. On the basis of the "principle of least privilege": consumers should only be authorized to access whatever they need to do their jobs. Older and single user operating systems often had weak or non-existent authentication and access control systems.