Patch tracker / KB5124008
KB5124008: Windows 11 Version 25H2 Security Update (September 2026)
KB5124008 is a Critical security update for Windows 11 Version 25H2, released 2026-09-08. It closes 635 CVEs with a maximum CVSS of 9.8. At least one of them is actively exploited (CISA KEV), so it is urgent, and Microsoft lists 6 known issues: test it before broad deployment. It is the third largest of the 54 updates Microsoft shipped that month, by CVEs closed.
Before you deploy: Microsoft lists 6 known issues for this update.
Our data comes from Microsoft, CISA and other public sources, and it can be wrong or out of date. If you are unsure, check the linked source before you act, and tell us when we are wrong so we can fix it.
Update summary
Fixes 635 CVEs. At least one is actively exploited (CISA KEV). Most severe CVSS 9.8 (Critical). EPSS exploit probability up to 2.8%. Among the top half of tracked Microsoft updates by EPSS exploit probability.
A newer security update for Windows 11 Version 25H2 for ARM64-based Systems has shipped since this one: KB5127648 (2026-10-13). For cumulative update families the newer package includes these fixes, so installing that one is usually the shorter path. Verify against your own update rings before you skip this package.
Microsoft documents 6 known issues with this update.
- Devices might experience a black screen or desktop loading issues after sign-in
- Domain-joined devices might lose their secure trust relationship with the domain
- USB audio devices might fail to start or produce no sound
Senserva AI Opinion and rich prompt for KB5124008
Copy this prompt into Claude, ChatGPT, or Copilot. The facts are included, sourced from this page.
Full tracking and change history for KB5124008
- Released
- 2026-09-08
- Last changed
- 2026-10-16
- Changes tracked
- 6
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.
Microsoft revised KB5124008 on 2026-10-13: CVEs fixed 628 to 639.
- 2026-10-13 CVEs fixed 628 to 639
Recorded by Senserva on the dates shown, from the MSRC release feed. This is what moved and when, which neither the vendor advisory nor NVD publishes.
Check if KB5124008 is installed
Elevated PowerShell. No output means it is not installed.
Known issues, from Microsoft
Quoted from Microsoft's support article for KB5124008 (checked 2026-10-16). Microsoft lists 6 known issues. Verify against the article before acting.
1. Devices might experience a black screen or desktop loading issues after sign-in
Symptoms
After installing the August 2026 Windows non-security preview update KB5120998 and subsequent updates, some devices might experience desktop loading issues. This issue has been primarily observed on Azure Virtual Desktop (AVD) hosts using FSLogix. This issue appears to occur more frequently with some existing user profiles.
Affected users might see a black screen after sign-in, with the desktop session failing to load automatically. In some cases, users might be unable to access their desktop until the desktop session is started manually. Application event logs might also show Windows Explorer crashes.
Resolution
This issue is resolved in Windows updates released on and after October 13, 2026 such as KB5127648 . We recommend you install the latest Windows update for your device as it contains important improvements and issue resolutions, including this one.
2. 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.
Resolution
This issue is addressed by KB5124010 . This update prevents Machine Identity Isolation (MII) enforcement by disabling it completely while improvements are made to the feature. MII enforcement will be prevented regardless of the MII configuration. ...
Windows release health for this update
From Microsoft's Windows release health for Windows 11, version 25H2; Windows 11, version 24H2, checked 2026-10-20. Microsoft's issue text is linked from each title.
Issues Microsoft traces to this update
- USB audio devices might fail to start or produce no sound
Resolved. Microsoft last updated it 2026-10-13 11:02 PT. - Domain-joined devices might lose their secure trust relationship with the domain
Resolved by KB5124010. Microsoft last updated it 2026-10-08 17:44 PT. - Remote Desktop Services might stop responding after Sept. 2026 security update
Resolved by KB5129195. Microsoft last updated it 2026-09-17 19:22 PT.
Other open issues on these Windows versions
- Remote Desktop connections might fail for non-admin users with error 0xC000000D
Mitigated. Microsoft last updated it 2026-10-13 10:13 PT.
From the Senserva blog
- Microsoft's September 2026 Out-of-Band Update
September 14, 2026 · Mark Shavlik - September 2026 Patch Tuesday: 1,169 Fixes, Two Exploited
September 8, 2026 · Mark Shavlik
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
- Also released in September 2026
KB5122871
684 CVEs · Critical · exploitedKB5122876
620 CVEs · Critical · exploitedKB5122878
572 CVEs · Critical · exploitedKB5122880
608 CVEs · Critical · exploitedKB5122882
645 CVEs · Critical · exploitedKB5123065
398 CVEs · Critical · exploitedKB5123066
423 CVEs · Critical · exploitedKB5123099
559 CVEs · Critical · exploitedThe full month, summarized: Microsoft Patch Tuesday.
- Authoritative references
See this and every Microsoft update ranked by real-world risk on the Microsoft Patch Tracker.