Microsoft Confirms September Windows Server Update Causing Active Directory Sync Issues

A Problematic Update for Windows Server 2025 - Microsoft has acknowledged that its September 2025 security update (KB5065426) is causing significant issues for Active Directory Domain Services (AD DS) synchronization on Windows Server 2025 systems. The problem primarily affects organizations relying on Microsoft Entra Connect Sync or other tools that use the Active Directory directory synchronization (DirSync) control.

In short, administrators who have installed the latest patch may experience incomplete synchronization of large Active Directory groups, particularly those exceeding 10,000 members.

What's Affected and Why It Matters

This issue only occurs on Windows Server 2025 after the September 2025 update or later patches. The problem disrupts synchronization processes that are vital for hybrid identity management, affecting organizations using Microsoft Entra ID (formerly Azure AD) to link on-premises directories with cloud services.

In environments managing thousands of users and complex group hierarchies, incomplete synchronization can cause broken access permissions, login issues, and inconsistencies between cloud and on-prem systems.

Microsoft's Temporary Workaround

While Microsoft engineers are still working on a permanent fix, the company has issued a temporary workaround. Administrators can add a specific registry key to restore synchronization and prevent further disruptions.

Here's the workaround provided by Microsoft:

However, the company strongly advises caution. Editing the registry incorrectly can lead to serious system problems, potentially requiring a full OS reinstallation. Microsoft emphasized that this fix should be applied only by experienced administrators and at their own risk, as the company cannot guarantee it will resolve all cases.

Future Fixes and Related Issues

Interestingly, Microsoft has yet to specify the root cause of this synchronization issue. A recent support document also indicates that Windows Server 2025 support for Microsoft Entra Cloud Sync is still "planned for a future release," suggesting this problem may stem from compatibility gaps in the early stages of support.

This isn't the only challenge facing Windows Server 2025 users lately. Microsoft is also addressing:

What IT Admins Should Do Now 

Until an official patch is released, system administrators should:

Final Thoughts

While Microsoft continues to refine Windows Server 2025, these recurring Active Directory and update-related issues highlight the complexity of maintaining hybrid identity infrastructures in enterprise environments.

For now, caution is key — IT teams are advised to test all Windows Server updates in controlled environments before rolling them out to production systems. As always, a solid backup and rollback plan remains the best safeguard when dealing with registry changes and security patches.