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Microsoft is scrambling to address a significant disruption in enterprise environments after confirming that its September 2025 security updates (KB5065426 and later) are breaking Active Directory Domain Services (AD DS) synchronization on Windows Server 2025 systems. This flaw, detailed in the company's Windows release health dashboard, impacts directory synchronization tools like Microsoft Entra Connect Sync, causing incomplete replication of large Active Directory security groups—specifically those exceeding 10,000 members. For organizations reliant on AD for identity management, this translates to potential authentication failures, access control gaps, and operational chaos.

According to Microsoft, the issue manifests exclusively on Windows Server 2025 post-installation of the problematic updates. Affected applications using the DirSync control—a core component for on-premises AD DS—may fail to fully synchronize group memberships, jeopardizing security policies and user access across hybrid environments. While engineering teams work on a permanent fix, Microsoft has issued a registry-based workaround, though it comes with stern warnings:

Path: Computer\HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Policies\Microsoft\FeatureManagement\Overrides
Name: 2362988687
Type: REG_DWORD
Value: 0

Microsoft explicitly cautions admins in its advisory: "Serious problems might occur if you modify the registry incorrectly... These problems might require that you reinstall the operating system." The company offers no guarantees of recoverability, placing the burden of risk squarely on IT teams. This vulnerability underscores a troubling pattern for Windows Server 2025, which still lacks full support for Microsoft Entra Cloud Sync according to official prerequisites, hinting at underlying compatibility challenges.

The Active Directory debacle isn't isolated. Microsoft is also battling a separate known issue in Windows 11 24H2 and Server 2025 where the Windows Update Standalone Installer (WUSA) fails when installing updates from network shares. While home and non-managed devices receive automated mitigations via Known Issue Rollback (KIR), enterprise environments remain in limbo. This follows a summer of turbulence, including July's emergency patch for Azure VM boot failures and June's domain controller unavailability bug—painting a picture of a platform struggling with stability amid rapid iteration.

For sysadmins, this episode is a stark reminder of the double-edged sword of patch management: delaying updates invites security risks, but rushed deployments can cripple critical infrastructure. As businesses increasingly depend on seamless AD synchronization for zero-trust frameworks and cloud integrations, Microsoft's ability to swiftly resolve such high-impact regressions will be crucial. Until then, the registry workaround stands as a fragile lifeline—one demanding extreme caution in deployment.