Error 18456

Login failed for user — SQL Server authentication

Product: SQL Server
Severity: Severity: high
Updated: Oct 18, 2026
Read: 5 min

A generic authentication failure. The State number in the error log identifies the actual cause; the client message is intentionally vague.

Error message

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

Symptoms

Applications cannot connect; the client reports error 18456.
The SQL Server error log shows "Severity: 14, State: N".
Connections from SSMS succeed with Windows auth but fail with SQL auth.

Cause

State 5: the login does not exist.
State 8: the password is incorrect.
State 18: the password has expired and must be changed.
State 58: the server is configured for Windows Authentication only.

Resolution

01
Step 1: Read the State from the error log
The State value maps directly to a cause. Query the current log instead of relying on the client message.
sql
```
EXEC sp_readerrorlog 0, 1, N'18456';
```
02
Step 2: Enable mixed mode authentication (State 58)
Switch the server to SQL Server and Windows Authentication mode, then restart the service.
sql
```
EXEC xp_instance_regwrite
  N'HKEY_LOCAL_MACHINE',
  N'Software\Microsoft\MSSQLServer\MSSQLServer',
  N'LoginMode', REG_DWORD, 2;
```
03
Step 3: Reset or unlock the login (States 8, 18)
Reset the password and clear the lockout. Disable CHECK_EXPIRATION only for service accounts with managed rotation.
sql
```
ALTER LOGIN app_svc WITH PASSWORD = N'<new-password>' UNLOCK;
ALTER LOGIN app_svc WITH CHECK_EXPIRATION = OFF;
```

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