Orphaned user is a common problem in SQL server world.When I was working on internal security audit of our production servers, I realized that sp_change_users_login with report option will not work with users associated with the windows login.
Who is an Orphan Users?
An user in a database is called orphaned when the associated login does not exists in the server or login has different SID. The first scenario can happen when you delete a login from the server. For example you created a login for an employee Mydomain\Lastname.Firstname and given access to couple of databases.Later this employee left the company or moved to different department.As a part of process we will delete his/her login (Mydomain\Lastname.Firstname) from the server. Now the databases on which he had access will have orphaned users.
The second scenario will happen when you restore the database from one environment (instance) to another environment(instance) or when you drop the login and create it again.For example you have login Mydomain\Lastname.Firstname in two instances namely INST1 and INST2. This login has db_owner right on one of the database (MyDb) in INST1. Now you have taken backup of MyDb and restored it on the INST2 and surprisingly Mydomain\Lastname.Firstname will not be able to access the restored database. If he/she tries to access the database , SQL server throw an error
The server principal "Mydomain\Lastname.Firstname" is not able to access the database "MyDb" under the current security context.
When you try to create a user on the restored database for Mydomain\Lastname.Firstname , it will throw an error as below
User, group, or role ' Mydomain\Lastname.Firstname ' already exists in the current database.
This is happening because the SID of the login and user in the database is not matching. You can check this by examining the catalog views sys.server_principals and sys.database_principals.
Identifying the orphaned users
To keep the database environment clean and safe, it is important to delete/fix the orphan users. In general ,the orphaned users created by the first scenario(login deleted) should be deleted and orphaned users created by second scenario should be fixed. The easiest way to identify the orphaned users is , use the system procedure sp_change_users_login with report option as given below.
EXEC SP_CHANGE_USERS_LOGIN 'Report'
But interestingly, this procedure will list only the orphaned users associated with the SQL login. If the login associated with windows login, the above procedure will not return the desired result. Let us see an example.
CREATE LOGIN [MyDomain\TestWindowsLogin] FROM WINDOWS
CREATE USER Testwindows FROM LOGIN [MyDomain\TestWindowsLogin]
DROP LOGIN [MyDomain\TestWindowsLogin]
Now the user Testwindows in MyDb is an orphaned users as the login associated with it does not exists. Let us execute procedure sp_change_users_login to report the orphaned users on MyDb database.USE MyDb
EXEC SP_CHANGE_USERS_LOGIN 'Report'
This execution will not report the TestWindows user as orphaned user. I have used the script List Orphaned Users.sql to list the orphaned users from the all databases of an instance.In the script, I have used sp_change_users_login to list the orphaned users associated with SQL logins and custom scrip by joining sys.server_principals and sys.database_principals for orphaned users associated with windows logins.
Also note that in SQL server you can create users with out login for some special purpose. Be careful with that type users while looking for orphaned users using the catalog view sys.server_principals and sys.database_principals. This special users can be identified by using the below query.