Resolving SQL Server "Login Failed" Error for Recreated Active Directory Users

SQL Server authentication can sometimes present unexpected challenges, especially when dealing with Windows Authentication and Active Directory (AD) user accounts. Recently, I encountered an unusual situation that highlighted the intricate relationship between SQL Server logins and Active Directory Security Identifiers (SIDs).

The Problem

A client reported a "Login failed" error when attempting to connect to SQL Server using Windows Authentication. The user account in question belonged to an employee who had left the company and later returned. Despite the account appearing valid in Active Directory and SQL Server, login attempts consistently failed.

Troubleshooting Steps

I was able to isolate the source of the error by following the steps in my SQL Server Connection Troubleshooting article:

1. Verified the error using the ODBC Data Source Administrator (odbcad32.exe).
2. Accessed SQL Server Management Studio (SSMS) using sysadmin credentials.
3. Deleted and recreated the server login and database users for the affected account.
4. Observed that the "Login Properties" in SSMS looked identical before and after recreation.
5. Confirmed that the login now worked successfully.

The Root Cause: Orphaned Security Identifiers (SIDs)

The key to understanding this issue lies in how Windows Authentication interacts with SQL Server:

1. SQL Server uses numeric Security IDs (SIDs) behind the scenes for Windows Authentication.
2. When adding a new SQL Server login with Windows Authentication, it uses the domain\username to look up the corresponding SID.
3. If a user is completely removed from Active Directory and then re-added, they will have the same domain\username but a different SID.
4. Removing a user from Active Directory does not automatically remove them from SQL Server.
5. The SQL Server login remains, but it's tied to the old, now-invalid SID.

In this case, deleting the SQL Server login and re-adding it forced SQL Server to associate the login with the new, valid SID from Active Directory.

Important Considerations

• This issue only affects SQL Server "Windows Authentication" logins, not "SQL Authentication" logins.
• Exercise caution when deleting server-level logins. The user may have access to multiple databases, so be sure to document and recreate all necessary permissions.
• You can avoid this issue entirely by using Active Directory security groups instead of assigning permissions (or, better yet, custom roles) to individual user accounts. However, be aware that deleting and recreating the security group itself could potentially cause similar problems.

Conclusion

While this situation may seem rare, it highlights the importance of understanding the underlying mechanisms of authentication in SQL Server.

When troubleshooting login issues, especially with returning employees, consider the possibility of orphaned SIDs as a potential cause. By recreating the SQL Server login, you can ensure proper alignment between Active Directory and SQL Server authentication, resolving these cryptic "Login failed" errors.

Acknowledgements

• Article title generated with the help of Claude-3.5-Sonnet
• Article excerpt generated with the help of Claude-3.5-Sonnet
• Initial draft generated with the help of Claude-3.5-Sonnet
• Cover image generated by FLUX-schnell