Microsoft Support:
This out-of-band (OOB) update for Windows Server 2025 (KB5129235) is a non-security cumulative update.
Improvements
This out-of-band update contains quality improvements from KB5122871 (released September 8, 2026). The following summary outlines key issues addressed by this out-of-band update. The bold text within the brackets indicates the item or area of the change.- [Remote Desktop Services (known issue)] Fixed: This update addresses an issue affecting Remote Desktop Services (RDS) after installing the September 2026 Windows security update (KB5122871). In affected environments, RDS might become unstable, causing RDP connection and sign-in failures or servers to become unresponsive during Remote Desktop configuration. Related tools, including Microsoft Management Console (MMC), RDS Licensing Diagnoser, File Explorer, and the Windows Update page, might also stop responding.
- [8-channel or 3D audio modes] Fixed: Some USB Audio Class 1.0 devices functioning as expected in standard stereo configurations failed when using multichannel audio features, including 8-channel or 3D audio modes. (Note: There are other audio symptoms not resolved in this OOB update. See the “Known issues in this update” section for more information.)
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
After installing KB5070881 or later updates, Windows Server Update Services (WSUS) does not display synchronization error details within its error reporting. This functionality is temporarily removed to address the Remote Code Execution Vulnerability, CVE-2025-59287.
Symptoms
After installing the September 8, 2026, Windows security update, some USB Audio Class 1.0 devices might fail to start or produce audio. Affected devices might experience one or more of the following symptoms: The device displays an error in Device Manager: "This device cannot start (Code 10).” No audio output. Volume controls are unresponsive or remain at zero. Sound settings are unresponsive or unavailable. This issue is limited to USB Audio Class 1.0 devices.
Resolution
Microsoft is working on a resolution and will update this documentation when more information is available.
After installing the September 8, 2026, Windows security update, some USB Audio Class 1.0 devices might fail to start or produce audio. Affected devices might experience one or more of the following symptoms: The device displays an error in Device Manager: "This device cannot start (Code 10).” No audio output. Volume controls are unresponsive or remain at zero. Sound settings are unresponsive or unavailable. This issue is limited to USB Audio Class 1.0 devices.
Resolution
Microsoft is working on a resolution and will update this documentation when more information is available.
How to get this update
Before you install this updateMicrosoft now 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.
Install this update
To install this update, use one of the following Windows and Microsoft release channels.
Available | Next Step |
|---|---|
| See the other options. |
| To install this update, download the standalone package(s) for this update from the Microsoft Update Catalog. This update contains MSU files that require installation in a specific order. Follow the Arm64 installation instructions or x64 installation instructions, as appropriate. If you install additional Dynamic Update packages, use versions from the same release month as this update. If a Safe OS Dynamic Update or Setup Dynamic Update is not available for the same release month, use the most recently released version. Note This latest cumulative update includes updates for AI components. AI components are applicable only to Copilot+ PCs and will not install on other Windows client PCs or Windows Server. |
File Information
For a list of the files provided in this update, download the file information for cumulative update KB5129235.
For a list of the files provided in the servicing stack update, download the file information for SSU KB5122870 - version 26100.33434.
Source:
September 14, 2026—KB5129235 (OS Build 26100.33451) Out-of-band | Microsoft Support
September 14, 2026—KB5129235 (OS Build 26100.33451) Out-of-band
support.microsoft.com
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