SQL Server Error 18456 - Login failed for user

 SQL Server Error 18456 is a common error indicating that the login attempt for a user has failed. The error message will typically look like this:

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

Understanding Error 18456

This error can have various reasons depending on the state code provided in the error message. The state code helps to pinpoint the exact cause of the login failure. Here are some common state codes and their meanings:

State Codes and Their Meanings:

• State 1: Generic error.
• State 2: User ID is not valid.
• State 5: User ID is not valid.
• State 6: Attempt to use a Windows login name with SQL Server Authentication.
• State 7: Login disabled and password mismatch.
• State 8: Password mismatch.
• State 9: Invalid password.
• State 11 and 12: Valid login but server access failure.
• State 13: SQL Server service paused.
• State 18: Password must be changed.

Common Causes and Solutions

1. Invalid Username or Password

• State 8: Password mismatch.
• State 9: Invalid password.

Solution: Verify that the username and password are correct. Ensure that you are using the correct authentication method (Windows or SQL Server).

2. Account Locked or Disabled

• State 7: Login disabled and password mismatch.

Solution: Check if the account is locked or disabled in SQL Server. You can unlock or enable the account using the following query:

ALTER LOGIN [YourUserName] WITH PASSWORD = 'NewPassword' UNLOCK;

3. Invalid Login Name

• State 2 and 5: User ID is not valid.

Solution: Verify the login name. Ensure that the login exists in SQL Server.

4. Password Expiration

• State 18: Password must be changed.

Solution: Change the password for the user.

ALTER LOGIN [YourUserName] WITH PASSWORD = 'NewPassword';

5. Server Access Failure

• State 11 and 12: Valid login but server access failure.

Solution: Ensure the user has the necessary permissions to access the database. Grant the required permissions if necessary.

6. Windows Authentication Issue

• State 6: Attempt to use a Windows login name with SQL Server Authentication.

Solution: Ensure you are using the correct authentication mode. Switch to Windows authentication or create a SQL Server login.

7. SQL Server Service Paused

• State 13: SQL Server service paused.

Solution: Restart the SQL Server service.

Example of Troubleshooting Steps

Here is a structured approach to troubleshoot SQL Server Error 18456:

• Check the Error Log: Look for the error in the SQL Server error log to find the state code.

EXEC xp_readerrorlog 0, 1, N'Login failed';

• Verify User Credentials: Ensure the username and password are correct.
• Check Account Status: Ensure the account is not locked or disabled.

SELECT name, is_disabled FROM sys.sql_logins WHERE name = 'YourUserName';

• Review Permissions: Ensure the user has the necessary permissions.

USE YourDatabase;
EXEC sp_change_users_login 'Update_One', 'YourUserName', 'YourLoginName';

• Check Authentication Mode: Ensure SQL Server is configured for the correct authentication mode.

SELECT SERVERPROPERTY('IsIntegratedSecurityOnly');

By following these steps, you should be able to diagnose and resolve the specific cause of SQL Server Error 18456.