KB5124008: Windows 11 Version 25H2 for x64-based Security Update (September 2026)

Full tracking and change history for KB5124008

Released

2026-09-08

Last changed

2026-09-24

Changes tracked

3

Senserva began recording day-to-day change history on 2026-07-07, and refreshes it several times a day. Release dates are historical where Microsoft publishes them; revision history is tracked from that date forward, so it does not include changes made before then.

Check if KB5124008 is installed

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Known issues, from Microsoft

Quoted from Microsoft's support article for KB5124008 (checked 2026-09-27). Verify against the article before acting.

Domain-joined devices might lose their secure trust relationship with the domain

Symptoms

After installing the September 8, 2026, Windows security update ( KB5124008 ), or later updates, some Credential Guard protected machine accounts might lose their secure channel with an on-premises Active Directory (AD) domain. Users might then be unable to sign in interactively with valid domain credentials and might receive a message stating that the trust relationship between the device and the domain failed. Offline sign-in using previously cached credentials might continue to work. AD replication and AD services on the domain controllers are not affected.

This issue occurs because KB5124008 and later updates enable the Machine Identity Isolation feature. While the update does not directly enable Machine Identity Isolation enforcement, it does cause Windows to begin honoring any existing or policy-provisioned settings that enabled Machine Identity Isolation enforcement. However, this feature is only supported for environments connected to domain controllers running at a Windows Server 2025 Domain Functional Level (DFL) and above. The feature should be disabled elsewhere. Any devices previously configured to use Machine Identity Isolation that are not connected to Windows Server 2025 domain controllers will experience this issue and will need to disable the feature.

Workaround

Important: This section contains information about modifying the registry. Before you modify the registry, back it up and make sure that you know how to restore it if a problem occurs. For more information, see How to back up and restore the registry in Windows .

To work around this issue, disable Machine Identity Isolation using the same management method that was used to enable it. Choose the applicable option below:

If Machine Identity Isolation was enabled by Intune policy, disable Machine Identity Isolation with Intune .

If Machine Identity Isolation was enabled by Group Policy, disable Machine Identity Isolation with Group Policy .

If Machine Identity Isolation was enabled directly in the registry, use these steps to disable it:

On the Windows 11, version 24H2 or 25H2 device, locate the following registry paths:

HKLM\SYSTEM\CurrentControlSet\Control\Lsa\MachineIdentityIsolation

HKLM\SOFTWARE\Policies\Microsoft\Windows\DeviceGuard\MachineIdentityIsolation

For either of these registry keys, if the value of MachineIdentityIsolation is set to 2 , change it to:

MachineIdentityIsolation = 0

After you disable Machine Identity Isolation, restart the device.

Then reset the secure channel ...

CVEs fixed by this update

Sorted exploited-first, then by CVSS. Every CVE links to its Senserva page with the full risk facts, references, and a ready-to-paste AI prompt.

Scope and sources

Affected products

• Windows 11 Version 24H2 for ARM64-based Systems
• Windows 11 Version 25H2 for ARM64-based Systems
• Windows 11 Version 25H2 for x64-based Systems

Also released in September 2026

KB5122871

679 CVEs · Critical · exploited

KB5122876

616 CVEs · Critical · exploited

KB5122878

568 CVEs · Critical · exploited

KB5122880

602 CVEs · Critical · exploited

KB5122882

641 CVEs · Critical · exploited

KB5123065

393 CVEs · Critical · exploited

KB5123066

418 CVEs · Critical · exploited

KB5123099

555 CVEs · Critical · exploited

The full month, summarized: Microsoft Patch Tuesday.

Authoritative references

Official KB5124008 page (Microsoft Support)Microsoft Update CatalogMicrosoft Security Update Guide