Win Update KB5122871 Windows Server 2025 Cumulative Update build 26100.33438 - Sept. 8

This cumulative update for Windows Server 2025 (KB5122871), includes the latest security fixes and improvements, along with non-security updates from last month’s optional preview release. Visit the Windows release health dashboard for the latest status on this release.

Announcements and messages​

This section provides key notifications related to this release, including announcements, change logs, and end-of-support notices.

Windows Secure Boot certificate expiration

Important: Secure Boot certificates used by most Windows devices are set to expire starting in June 2026. Microsoft has been updating these certificates on consumer and non-managed business devices over the past several months. Devices that haven’t received the newer certificates will continue to start and operate normally, and standard Windows updates will continue to install. Updated certificates will continue to be delivered through Windows Update in the coming months.

Improvements​

This security update contains fixes and quality improvements from KB5120233 (released August 11, 2026). The following summary outlines key issues addressed by this update. Also, included are available new features. The bold text within the brackets indicates the item or area of the change.

• [Certificates] This update improves application compatibility during Windows certificate-authority updates, helping applications continue to work as expected as Microsoft transitions to newer signing certificates
• [Date and Time] This update adjusts Morocco Standard Time to reflect Morocco's transition to permanent UTC+00:00 effective September 20, 2026. This change ensures that the correct local time is displayed after the transition.
• [Networking] This update improves network failover on Windows Server systems by helping connections respond more quickly when a network adapter is disconnected. This helps network storage and file access continue to work as expected.
• [Remote Desktop] This update improves Remote Desktop audio redirection, helping audio from remote sessions play correctly on the local device.
• [Sysmon]
  • This update improves Sysmon's Process Tampering detection by preventing false Event ID 25 alerts for normal Windows Session Manager (smss.exe) activity.
  • This update improves Sysmon reliability during file monitoring, helping prevent unexpected system crashes when archive directories are unavailable.
  • This update improves Sysmon tampering detection by making executable file validation more reliable and consistent.
  • This update improves Sysmon file archiving reliability, helping monitored file operations complete as expected even when archive directories cannot be accessed.

If you've already installed previous updates, your device will download and install only the new updates included in this package.

For more information about security vulnerabilities, see the Security Update Guide and the September 2026 Security Updates.

Windows Server 2025 servicing stack update (KB5122870) - 26100.33434​

This update makes quality improvements to the servicing stack, which is the component that installs Windows updates. Servicing stack updates (SSU) ensure that you have a robust and reliable servicing stack so that your devices can receive and install Microsoft updates. To learn more about SSUs, see Simplifying on-premises deployment of servicing stack updates.

Known issues in this update​

How to get this update​

Before you install this update​

Microsoft combines the latest servicing stack update (SSU) for your operating system with the latest cumulative update (LCU). For general information about SSUs, see Servicing stack updates.

Deployment

If you deploy dynamic updates such as this update to an existing Windows image, ensure the boot.stl file is included as part of the installation media. Failure to include the file might prevent devices from successfully starting from the installation media and can result in error code 0xc0430001.

Note: The boot.stl file is used during Secure Boot validation and must match the Windows version and architecture of the image you are updating.
To ensure the boot.stl file is included as part of the installation media, do one of the following:

• Use the Update WinPE script to update an existing Windows image. (Recommended)
• Manually copy the boot.stl file from the device Windows\Boot\EFI folder to the corresponding folder on your installation media before deploying the update.

For information about how to apply Dynamic Update packages to existing Windows images, see Update Windows installation media with Dynamic Update.

Install this update​

To install this update, use one of the following Windows and Microsoft release channels.

File information​

For a list of the files provided in this update, download the file information for cumulative update KB5122871.

For a list of the files provided in the servicing stack update, download the file information for SSU KB5122870 - version 26100.33434.





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