SQL Server error 18456: Login failed for user

Error 18456 is deliberately vague on the client. The state number in the SQL Server error log tells you the real cause — here is how to read it and fix each one.

What the error means

The client sees only:

Login failed for user 'app_user'. (Microsoft SQL Server, Error: 18456)

SQL Server hides the reason from the client on purpose. The detail lives in the error log, as a state number.

Find the state

EXEC xp_readerrorlog 0, 1, N'18456';

Look for a line like Error: 18456, Severity: 14, State: 8.

States and fixes

State	Meaning	Fix
2, 5	Login does not exist	Create the login or check spelling
6	Windows login used with SQL auth	Use Integrated Security
7	Login disabled	`ALTER LOGIN [name] ENABLE;`
8	Wrong password	Reset the password
11, 12	Valid login, no server access	Grant `CONNECT SQL`; check UAC elevation
18	Password must be changed	Change password or turn off `CHECK_EXPIRATION`
38	Default database unavailable	Fix the default database
58	SQL auth disabled on server	Enable mixed mode and restart

Enable mixed-mode authentication (state 58)

EXEC xp_instance_regwrite
  N'HKEY_LOCAL_MACHINE',
  N'Software\Microsoft\MSSQLServer\MSSQLServer',
  N'LoginMode', REG_DWORD, 2;

Then restart the SQL Server service.

Fix the default database (state 38)

ALTER LOGIN [app_user] WITH DEFAULT_DATABASE = [master];

Verify

Reconnect with sqlcmd -S server -U app_user -P '***' -Q "SELECT SUSER_NAME();". A row back means you're done.