mongodb/jstests/auth/system_user_exception.js

// Test the special handling of the __system user
// works when the SCRAM-SHA-1 pw auth mechanisms are disabled.
(function() {
    "use strict";

    // Start mongod with no authentication mechanisms enabled
    var m = MongoRunner.runMongod(
        {keyFile: "jstests/libs/key1", setParameter: "authenticationMechanisms=PLAIN"});

    // Verify that it's possible to use SCRAM-SHA-1 to authenticate as the __system@local user
    assert.eq(
        1, m.getDB("local").auth({user: "__system", pwd: "foopdedoop", mechanism: "SCRAM-SHA-1"}));

    // Verify that it is not possible to authenticate other users
    m.getDB("test").runCommand(
        {createUser: "guest", pwd: "guest", roles: jsTest.readOnlyUserRoles});
    assert.eq(0, m.getDB("test").auth({user: "guest", pwd: "guest", mechanism: "SCRAM-SHA-1"}));

    MongoRunner.stopMongod(m);

})();