A method and system for cross-system authentication or credentialing of clients. Credentials from one system (e.g., system 2) are placed on a client, such as with a cookie on a browser, and the credentials are then extracted by another system (e.g., system 1), and used by system 1 to impersonate the client to system 2. If the client's credentials with system 2 are valid, system 2 provides that information to system 1 (which is impersonating the client), and system 1 uses the validity of the credentials from system 2 to grant the client access to protected resources on system 1.