In one window: > use y@z switched to db y@z > db.createUser({user: "x", pwd: "pwd", roles: []}) Successfully added user: { "user" : "x", "roles" : [ ] } > db.auth("x", "pwd") 1 In another: > use z switched to db z > db.createUser({user: "x@y", pwd: "pwd", roles: []}) Successfully added user: { "user" : "x@y", "roles" : [ ] } > db.auth("x@y", "pwd") 1 Return to the first window: > db.auth("x", "pwd") Error: Authentication failed. 0 Mongod will report: 2015-09-10T18:47:18.852-0400 I NETWORK [initandlisten] connection accepted from 127.0.0.1:51466 #1 (1 connection now open) 2015-09-10T18:47:42.931-0400 I COMMAND [conn1] command y@z.$cmd command: createUser { createUser: "x", pwd: "xxx", roles: [], digestPassword: false, writeConcern: { w: "majority", wtimeout: 30000.0 } } ntoreturn:1 ntoskip:0 keyUpdates:0 writeConflicts:0 numYields:0 reslen:22 locks:{ Global: { acquireCount: { r: 4, w: 4 } }, Database: { acquireCount: { W: 4 } }, Collection: { acquireCount: { w: 1 } } } protocol:op_command 164ms 2015-09-10T18:47:49.627-0400 I ACCESS [conn1] Successfully authenticated as principal x on y@z 2015-09-10T18:47:57.537-0400 I NETWORK [initandlisten] connection accepted from 127.0.0.1:51468 #2 (2 connections now open) 2015-09-10T18:48:10.693-0400 I ACCESS [conn2] Successfully authenticated as principal x@y on z 2015-09-10T18:48:18.226-0400 I ACCESS [conn1] SCRAM-SHA-1 authentication failed for x on y@z from client 127.0.0.1 ; AuthenticationFailed SCRAM-SHA-1 authentication failed, storedKey mismatch