Microsoft October 2025 Security Update Triggers AD Sync Issues on Windows Server 2025

Microsoft’s October 2025 security update cycle has introduced a significant issue for administrators managing Windows Server 2025, causing Active Directory (AD) synchronization to fail under specific conditions.

The company confirmed the bug on October 14, 2025, noting that it originates from the September 2025 security update (KB5065426) and subsequent patches.

This flaw specifically impacts organizations with large AD environments, leading to incomplete data synchronization between on-premises servers and cloud services.

The issue poses a challenge for early adopters of the latest server operating system who rely on seamless directory services for identity and access management.

The Root of the Synchronization Failure

The problem affects applications that utilize the Active Directory directory synchronization (DirSync) control to manage on-premises AD Domain Services (AD DS). A primary example of an affected service is Microsoft Entra Connect Sync, which is crucial for hybrid identity environments.

According to Microsoft’s advisory, the synchronization process fails when attempting to update AD security groups that contain more than 10,000 members.

On affected systems running Windows Server 2025, this results in incomplete or failed sync cycles, which can disrupt user access and permissions managed through these large groups.

The issue is isolated to servers that have installed OS Build 26100.6584 (KB5065426) or a later cumulative update.

Key characteristics of the failure include:

• Incomplete group membership updates for security groups exceeding 10,000 users.
• Errors logged in the Event Viewer under “ADSync” or “DirSync” with failure codes.
• No impact on smaller groups or on systems without the September security update.

Microsoft’s Official Workaround

In response to the confirmed issue, Microsoft has provided an immediate workaround for affected customers. The solution involves manually modifying the Windows Registry to disable the feature change that introduced the bug.

Administrators are advised to create a new REG_DWORD value in the registry. The company has issued a standard warning that incorrect registry modifications can cause serious system problems, potentially requiring a full operating system reinstallation.

This interim fix is intended to restore normal synchronization for large security groups while a permanent solution is developed. It offers a critical stopgap for enterprises experiencing disruptions in their identity synchronization workflows.

Steps to implement the registry workaround:

• Navigate to ComputerHKEY_LOCAL_MACHINESYSTEMCurrentControlSetPoliciesMicrosoftFeatureManagementOverrides
• Create a new DWORD value named 2362988687 with data set to 0
• Restart the Microsoft Entra Connect Sync service or reboot the server for changes to take effect

Path to a Permanent Resolution

Microsoft has confirmed that the issue is limited to Windows Server 2025 and does not affect any client platforms. The company’s engineering teams are actively investigating the problem to develop a permanent fix, which will be delivered in a future Windows update.

Until then, administrators managing large-scale AD deployments on Windows Server 2025 are encouraged to implement the provided registry workaround if they encounter synchronization failures.

Organizations should continue to monitor the Windows release health dashboard for further announcements and the release of the official patch.

This proactive approach will ensure that directory services remain stable and that the permanent fix can be applied as soon as it becomes available.

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