Patch tracker / KB5122878
KB5122878: Windows 10 Version 21H2 for 32-bit Security Update (September 2026)
KB5122878 is a Critical security update for Windows 10 Version 21H2 for 32-bit, released 2026-09-08. It closes 568 CVEs with a maximum CVSS of 9.8. At least one of them is actively exploited (CISA KEV), so this is a deploy-first update. It is the seventh largest of the 69 updates Microsoft shipped that month, by CVEs closed.
Update summary
Fixes 568 CVEs. At least one is actively exploited (CISA KEV). Most severe CVSS 9.8 (Critical). EPSS exploit probability up to 3.6%. Among the top half of tracked Microsoft updates by EPSS exploit probability.
Microsoft documents 2 known issues with this update.
- Remote Desktop Services might stop responding after September 2026 security update
- File History might stop working after installing September 2026 Windows update
Senserva AI Opinion and rich prompt for KB5122878
Copy this prompt into Claude, ChatGPT, or Copilot. The facts are included, sourced from this page.
Full tracking and change history for KB5122878
- 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 KB5122878 is installed
Elevated PowerShell. No output means it is not installed.
Known issues, from Microsoft
Quoted from Microsoft's support article for KB5122878 (checked 2026-09-27). Verify against the article before acting.
Remote Desktop Services might stop responding after September 2026 security update
Symptoms
After installing the September 2026 Windows security update, some organizations might experience issues with Remote Desktop Services (RDS).
In some environments, RDS might become unstable, resulting in RDP connections failing after several minutes, sign-in issues, or servers hanging at "Please wait for the Remote Desktop Configuration". Related tools, including Microsoft Management Console (MMC), RDS Licensing Diagnoser, and File Explorer might also become unresponsive. Additionally, the Windows Update page might stop responding and continuously display a loading indicator.
Note
This issue does not affect Windows 365 or Azure Virtual Desktop.
Resolution
This issue is resolved in Windows updates released on and after September 14, 2026 such as KB5129236 . We recommend you install the latest Windows update for your device as it contains important improvements and issue resolutions, including this one.
File History might stop working after installing September 2026 Windows update
Symptoms
After installing the September 2026 Windows security update KB5124008 , some customers using File History, might be unable to create or update backups. File History, available through Control Panel > System and Security > File History , is used to back up files to an external drive or network location. Affected devices might incorrectly display a "Reconnect your drive" message even when a compatible backup drive is connected and functioning properly. Additionally, the "Last Backup" timestamp might not update, and previously backed up files might show "No previous version available." In some cases, Event Viewer might record application crash events referencing FileHistory.exe and KERNELBASE.dll .
Resolution
Microsoft is working on a resolution for this issue in a future Windows update and will provide more information when it is available.
Applies to: Windows 10 Enterprise LTSC 2021 and Windows 10 IoT Enterprise LTSC 2021
Important
Use EKB KB5003791 to update to Windows 10, version 21H2 on supported editions.
This security update includes fixes and quality improvements that are part of the following updates:
August 11, 2026—KB5120249 (OS Builds 19045.7663 and 19044.7663)
The following is a summary of the issues that this update addresses when you install this update. If there are new features, it lists them as well. The bold text within the brackets indicates the item or area of the change we are documenting.
[Secure Boot] This update includes additional high confidence device ...
Windows release health for this update
From Microsoft's Windows release health for Windows 10, version 22H2; Windows 10, version 21H2, checked 2026-09-29. 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
Mitigated. Microsoft last updated it 2026-09-29 10:12 PT. - Remote Desktop Services might stop responding after Sept. 2026 security update
Resolved by KB5129236. Microsoft last updated it 2026-09-17 19:22 PT. - Host folder shares might be unavailable in Hyper-V-based Linux VMs
Resolved by KB5129236. Microsoft last updated it 2026-09-14 13:30 PT.
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
679 CVEs · Critical · exploitedKB5122876
616 CVEs · Critical · exploitedKB5122880
602 CVEs · Critical · exploitedKB5122882
641 CVEs · Critical · exploitedKB5123065
393 CVEs · Critical · exploitedKB5123066
418 CVEs · Critical · exploitedKB5123099
555 CVEs · Critical · exploitedKB5124008
628 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.