Microsoft’s security update for September (KB5065426) has been implicated in a rising wave of identity and file‑sharing headaches for organizations that have adopted Windows Server 2025—yet the most alarming claim now circulating, that a DirSync/Entra Connect group‑sync bug in KB5065426 silently breaks directory synchronization for very large on‑prem security groups (>10,000 members), is not fully corroborated by Microsoft’s public guidance. Microsoft’s own update notes for KB5065426 do document several known issues after the patch — notably an Active Directory (AD) schema replication defect when the forest Schema Master runs on Windows Server 2025 and multiple SMB/file‑sharing problems reported by administrators — but the specific DirSync limit/failure scenario and the registry toggle (a FeatureManagement Overrides DWORD 2362988687 set to 0) published by third parties has not been verified in Microsoft KB text or in official release‑health advisories at the time of writing. Microsoft’s KB entry for the September 9, 2025 rollup explicitly lists an AD replication problem tied to schema master behavior and documents SMB/SMBv1 migration impacts; the vendor continues to investigate and to roll fixes via follow‑up updates.
This article summarizes the public facts, separates confirmed Microsoft statements from unverified claims in the reporting, and lays out practical guidance for hybrid identity and AD administrators who must manage risk while waiting for Microsoft’s permanent fixes.
Microsoft’s official KB for the September 2025 cumulative security update (KB5065426) lists several known issues and workarounds. The most operationally significant items that Microsoft publicly documents are:
HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Policies\Microsoft\FeatureManagement\Overrides
Name: 2362988687
Type: REG_DWORD
Value: 0
and it asserts that this change disables an offending “feature change” causing the sync break.
Cross‑checking the claim against public Microsoft documentation and major independent sources yields the following:
The narrower claim that KB5065426 introduced a deterministic DirSync failure specifically for large AD security groups above 10,000 members and that a particular registry DWORD (2362988687) will safely disable the faulty behavior has not been validated in Microsoft’s KB or in other authoritative vendor notices at the time of writing. Until Microsoft documents that exact symptom and the registry mitigation, administrators should treat the report as unverified intelligence: investigate and validate in test environments, correlate symptoms (replication events and Entra Connect logs), and escalate to Microsoft Support rather than deploying unvetted registry changes across production.
Operationally, the simplest and most reliable mitigation for the most dangerous confirmed risk remains: transfer the Schema Master to a non‑2025 DC before performing any schema extension or Exchange update, validate replication with repadmin, and only then proceed. That single prevention rule materially reduces the chance of a forest‑wide replication outage while Microsoft delivers a permanent patch.
Source: Windows Report Microsoft Confirms Directory Sync Failure on Windows Server 2025 for Large AD Security Groups
This article summarizes the public facts, separates confirmed Microsoft statements from unverified claims in the reporting, and lays out practical guidance for hybrid identity and AD administrators who must manage risk while waiting for Microsoft’s permanent fixes.
Background: what Microsoft has acknowledged so far
Microsoft’s official KB for the September 2025 cumulative security update (KB5065426) lists several known issues and workarounds. The most operationally significant items that Microsoft publicly documents are:- Active Directory replication issue (schema duplication): Domain controllers that host the forest Schema Master FSMO role on Windows Server 2025 can produce duplicate entries in multi‑valued schema attributes during schema extension operations. Those duplicates can trigger replication failures (Event ID 8418) when older DCs (for example, 2016/2019/2022) reject the altered schema object. Microsoft’s KB describes the symptom, the common trigger (Exchange schema extension steps), and the recommended remediation path (manual removal or vendor‑assisted cleanup), and labels the issue “under investigation.”
- SMB / file‑sharing regressions: Administrators have reported file and print sharing failures and SMBv1 connectivity problems after KB5065426. Microsoft’s Q&A and Tech Community threads confirm user reports where shares or RPC/NetBIOS connections fail in mixed environments; the KB also contains SMB‑related known‑issue notes and mitigation suggestions. Real‑world forum reports show many admins temporarily uninstalling KB5065426 to restore file‑sharing behavior until a patch arrives.
- Other update side effects: Community reports and vendor advisories have linked KB5065426 to other operational disturbances (print sharing problems, DRM/HDCP playback regressions, and hotpatch/PSDirect edge cases). Microsoft has been publishing further servicing updates and hotpatches to address some of these side effects.
The WindowsReport claim and verification status
A recent WindowsReport item (summarized by many downstream outlets and social posts) states that the September update KB5065426 introduced a separate DirSync (directory synchronization) failure that specifically impacts large AD security groups exceeding 10,000 members when syncing to cloud directories (Microsoft Entra / Entra Connect). The WindowsReport piece further suggests a registry‑based workaround — adding a DWORD under:HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Policies\Microsoft\FeatureManagement\Overrides
Name: 2362988687
Type: REG_DWORD
Value: 0
and it asserts that this change disables an offending “feature change” causing the sync break.
Cross‑checking the claim against public Microsoft documentation and major independent sources yields the following:
- Microsoft’s KB entry for KB5065426 does not mention a DirSync / Entra Connect group‑member synchronization bug limited to groups of >10,000 members, nor does Microsoft’s published guidance include the exact registry DWORD 2362988687 as an official workaround for directory sync. Microsoft’s KB explicitly focuses on the AD schema duplication issue (Event ID 8418) and SMB issues.
- Microsoft Entra / Entra Connect documentation describes group sync limits (histor maximums and connector behavior): legacy DirSync (older toolsets) and Microsoft Entra Connect have different group size handling characteristics, and Entra Connect has higher default limits (and configurable limits) for group member synchronization. Published Entra Connect docs and Office 365 feature notes show typical limits (DirSync historically had lower practical sync thresholds whereas Entra Connect’s newer V2 endpoints and rules allow syncing far larger groups, with documented limits up to tens of thousands depending on connector rules). That documentation suggests a 10,000‑member ceiling is not universally true and would not by itself explain a deterministic bug introduced by a Windows server update.
- Independent community and vendor reports (forums, blogs) have widely reported AD replication and SMB problems after KB5065426, which Microsoft has acknowledged; however, independent confirmation of a specific DirSync/Entra Connect bug that is reproducibly triggered by KB5065426 only for very large AD security groups is inconsistent or absent from Microsoft’s public KB and from other major outlets at this time. Where admins have reported Entra Connect or cloud sync problems recently, the causes have ranged across TLS, service‑side outages, agent upgrades, and large‑attribute errors — but not a single, widely reproduced KB5065426‑specific DirSync bug limited to 10,000+ groups.
Technical analysis: why large group syncs and Server 2025 updates interact badly (what we do know)
Even though the specific WindowsReport registry workaround lacks confirmation, the technical context around why a Windows Server update could affect directory synchronization is important to understand.1. AD and schema are brittle and version‑sensitive
- Active Directory schema objects and replication are byte‑sensitive: schema updates must be identical across all DCs. Microsoft’s KB shows that Windows Server 2025 DCs acting as Schema Master can produce duplicate values during schema extension operations, which older DCs interpret as a schema mismatch. When schema replication fails, directory replication and identity services can degrade rapidly (Event ID 8418). This is a high‑impact, well‑documented failure class.
2. Directory sync agents depend on consistent AD data and connectors
- Microsoft Entra Connect and other directory sync tools (including Cloud Sync/Provisioning Agents) rely on LDAP reads, change‑tracking, and staged exports. Large groups are heavier to enumerate and process — they increase memory, time, and API‑rate pressures on both the on‑prem AD side and the cloud ingestion side. While large groups are supported, changes in server behavior (authentication changes, LDAP signing/encryption requirements, or object serialization differences) introduced by an OS update can expose latent bugs or performance limits in sync agents. Vendor docs show Entra Connect historically adjusted group‑member limits and connector behavior to raise sync thresholds, but large group operations retain operational risk in mixed upgrades.
3. Updates can change default feature flags and serialization logic
- The Windows FeatureManagement/Overrides registry area is frequently used by Microsoft to gate new feature variants and to ship roll‑out toggles. Administrators and researchers have previously toggled similar DWORDs to circumvent buggy behavior introduced by update variants. That makes the concept of a registry override plausible in principle, but each override value is specific to a feature rollout; a particular numeric identifier like 2362988687 must be documented or traced to an official feature flag to be safe to use. No authoritative confirmation exists for that exact value at this time.
Confirmed symptoms and diagnostics to monitor now
If you run hybrid identity or have Windows Server 2025 domain controllers, watch closely for these reproducible indicators — all of which are documented either by Microsoft or by community reports as meaningful symptoms:- Event ID 8418: “The replication operation failed because of a schema mismatch between the servers involved.” This is the primary sign Microsoft references for schema duplication issues. Add this to SIEM alerts and run repadmin to collect replication detail if it appears.
- NTDS / Event ID 1203 warnings referencing schema objects. These typically accompany 8418 and point to the offending schema attributes.
- repadmin /showrepl showing persistent inbound replication failures or asymmetric replication (2025 DCs replicating among themselves while older DCs refuse schema changes). This asymmetric pattern is a key diagnostic clue.
- Entra Connect / Cloud Sync errors for group write operations: prolonged or failed delta syncs, “Large attribute” errors, or agent quarantine messages in the Entra portal (these have varied causes in the wild, from TLS or Graph auth issues to payload size problems). Correlate Entra errors with AD and Windows Update timelines.
- SMB / file‑sharing authentication failures on Windows Server 2025 hosts after KB5065426 installs (password prompts, share access denied, or NetBIOS/SMBv1 breakage). These are frequently reported in community threads and acknowledged as a side effect by Microsoft.
Practical mitigations — immediate and short term
These are prioritized, operationally safe steps that reduce blast radius while you wait for Microsoft validation or a patch. Follow them in the sequence that matches your risk tolerance.- Identify and inventory risk surface (right now)
- Check which DCs host FSMO roles: Get‑ADForest | fl SchemaMaster. If the Schema Master is a Windows Server 2025 host, treat schema changes as high risk.
- Verify the OS version of every DC and record which servers run Server 2025.
- Block the specific trigger: do not run schema‑extending operations while a 2025 Schema Master is active
- Before running Exchange PrepareSchema, PrepareAD, or any schema‑extending updates, transfer the Schema Master to a vetted non‑2025 DC (for example, Server 2022) and confirm replication health. This is the simplest and most effective prevention recommended by Microsoft and community responders.
- If you must take immediate action for directory sync issues
- Do not apply unverified registry workarounds in production. If you see a specific Microsoft KB that documents a registry toggle, follow that vendor guidance; otherwise open a Microsoft support ticket. The specific DWORD 2362988687 mentioned in third‑party reporting is unverified in official KB text. Experimenting with undocumented FeatureManagement overrides risks making things worse.
- For Entra Connect / DirSync administrators
- Verify the group sizes and connector limits: older DirSync tools had lower practical member limits; Entra Connect’s V2 endpoints increase thresholds. If you have groups >10k members, consider:
- Staging a test full sync in a lab or staging tenant before making large changes.
- Splitting very large static membership groups into smaller, role‑based collections or using dynamic group rules to reduce single‑group enumeration costs.
- Ensuring your Entra Connect agent is fully patched and using recommended TLS and Graph endpoints.
- Monitor and escalate quickly
- Add SIEM alerts for Event ID 8418 and NTDS 1203.
- Collect repadmin output, Directory Service logs, and LDIFDE schema exports before contacting Microsoft Support — these accelerate support-assisted remediation if you are impacted. Microsoft explicitly recommends opening a support case for schema cleanup rather than doing unguided ADSIEdit operations.
- If you are seeing SMB/file‑sharing regressions after KB5065426
- Temporarily roll back KB5065426 only if you have tested rollback in a staging window and accept the security risk until a patch is installed. Many admins in the field have reported immediate improvement after uninstalling the update, but that’s an operational tradeoff. Monitor Microsoft’s release notes for the follow‑up hotfix or LCU.
Longer‑term remediation and change‑control recommendations
- Treat FSMO placement as a change‑control gate in mixed‑version forests. Avoid automated or default promotions that could move FSMO roles to brand‑new OS releases without explicit validation. Automation pipelines should be updated to respect schema windows and vendor guidance.
- Maintain a lab environment that mirrors production DC version mixes and run Exchange schema updates there before production—especially important if you plan to introduce Server 2025 DCs. Validate repadmin and healthchecker tests as part of release readiness.
- Harden your incident playbook: require system‑state backups and snapshots of Schema Master/DCs before any schema work. Keep Microsoft Support engagement contact info handy; their assisted cleanup scripts are the safest path for removing duplicate schema entries.
Risk assessment: strengths and potential harms
- Strengths of Microsoft’s current posture
- Microsoft has publicly documented and acknowledged key high‑impact issues in KB5065426, notably the AD schema replication behavior, which gives administrators a definitive diagnostic signal (Event ID 8418) and a clear prevention rule (don’t have 2025 as Schema Master during schema changes). That clarity helps teams take direct action to reduce risk.
- Potential harms and operational exposures
- The AD schema duplication problem can be instantly catastrophic: schema mismatches halt AD replication and can cascade to authentication, Group Policy, mail flow, and automation failures across the enterprise. Fixing schema object duplication often requires careful, support‑assisted AD surgery.
- Unverified workarounds (undocumented registry keys) risk making replication or sync issues worse. Editing FeatureManagement overrides without vendor confirmation may disable unrelated mitigations and create new, subtle side effects.
- Entra Connect / DirSync issues tied to very large groups are operationally plausible (large group enumerations create load and long sync cycles), but attributing such failures to KB5065426 without Microsoft confirmation risks misdirecting remediation efforts. Test and correlate evidence before wide remedial action.
What to tell executives and stakeholders (short brief)
- The September 2025 security update KB5065426 contains one or more known issues that Microsoft has acknowledged. The most actionable one for on‑premises identity infrastructure is an AD schema replication defect that is triggered when a Windows Server 2025 domain controller hosts the forest Schema Master and Exchange or other schema‑changing updates are executed. Microsoft recommends not hosting the Schema Master on Server 2025 during schema operations.
- Community reporting suggests multiple other operational regressions after KB5065426 (file/share and media playback issues). These are real and widespread enough that some orgs have chosen to roll back the patch temporarily.
- The specific WindowsReport claim about DirSync failing for groups >10,000 and a particular registry fix is not yet confirmed by Microsoft. Treat that as unverified intelligence and escalate to Microsoft Support if you observe matching symptoms. Do not roll out undocumented registry changes enterprise‑wide.
Action checklist (concise, prioritized)
- Immediately identify your Schema Master: Get‑ADForest | fl SchemaMaster. If it’s a Windows Server 2025 DC, schedule a transfer to a vetted non‑2025 DC before performing any Exchange or schema work.
- Add SIEM alerts for Event ID 8418 and NTDS 1203 and run repadmin /showrepl to baseline replication health.
- If you run Entra Connect or other sync agents, confirm agent versions, TLS settings, and review sync logs for “Large attribute” or long‑running delta syncs. Stage large group operations in a lab before applying changes in production.
- Do not apply unverified registry toggles. If a vendor‑documented registry workaround appears in Microsoft KB or release‑health guidance, apply per vendor instructions and test in staging.
- If impacted by schema duplication replication failures, gather repadmin, Directory Service logs, and LDIFDE schema exports and open a Microsoft Support ticket for assisted remediation.
Conclusion
The September 2025 update (KB5065426) has legitimate, Microsoft‑acknowledged consequences for Active Directory replication and for file‑sharing behavior on Windows Server 2025. Those confirmed issues alone justify careful, conservative change control: do not run Exchange schema changes while a Windows Server 2025 DC holds the Schema Master role; monitor Event ID 8418; and engage Microsoft Support for schema cleanup.The narrower claim that KB5065426 introduced a deterministic DirSync failure specifically for large AD security groups above 10,000 members and that a particular registry DWORD (2362988687) will safely disable the faulty behavior has not been validated in Microsoft’s KB or in other authoritative vendor notices at the time of writing. Until Microsoft documents that exact symptom and the registry mitigation, administrators should treat the report as unverified intelligence: investigate and validate in test environments, correlate symptoms (replication events and Entra Connect logs), and escalate to Microsoft Support rather than deploying unvetted registry changes across production.
Operationally, the simplest and most reliable mitigation for the most dangerous confirmed risk remains: transfer the Schema Master to a non‑2025 DC before performing any schema extension or Exchange update, validate replication with repadmin, and only then proceed. That single prevention rule materially reduces the chance of a forest‑wide replication outage while Microsoft delivers a permanent patch.
Source: Windows Report Microsoft Confirms Directory Sync Failure on Windows Server 2025 for Large AD Security Groups
