September 8, 2026—KB5123099 (OS Build 14393.9512)

Applies To
Windows Server 2016, all editions Win 10 Ent LTSB 2016 Win 10 IoT Ent LTSB 2016

Important

Free support for Windows 10 Enterprise 2016 LTSB and Windows Server 2016 will soon end. For information about the availability of Extended Security Updates (ESUs), see the End of support section.

Summary

This article lists the security issues and quality improvements included in this cumulative security update.

Applies to: Windows Server 2016

This security update includes fixes and improvements that are a part of the following update:

The following is a summary of the issues that this update addresses. 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 targeting data, increasing coverage of devices eligible to automatically receive new Secure Boot certificates. Certificate deployment via Windows updates continues across supported PCs and non-managed business devices in the coming months.

  • [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.

  • [Windows Code Integrity policies] Improves application compatibility during Windows certificate-authority rotation by recognizing Microsoft Windows Production PCA 2026 RSA2048-SHA256 as equivalent to PCA 2011.

  • [Remote Desktop] This update addresses an issue that affects Remote Desktop audio redirection. Audio from the remote session might not play on the local computer in certain configurations.

  • [Microsoft Compatibility Telemetry service (known issue)] Fixed: After installing Windows update KB5120418 released on August 11, 2026, some Windows Server 2016 devices might generate recurring Application Error events (Event ID 1000, with exception code 0xc0000409) associated with CompatTelRunner.exe.

If you installed earlier updates, only the new updates contained in this package will be downloaded and installed on your device.

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

For more information about Windows Server 2016, see its update history page.

For information about Windows update terminology, see types of Windows updates and monthly quality update types.

Known issues in this update

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 KB5129239. We recommend you install the latest Windows update for your device as it contains important improvements and issue resolutions, including this one.

Available Next step
Available This update will be downloaded and installed automatically from Windows Update.

File information

A list of the files included in this update is provided in a CSV (Comma delimited) (*.csv) file. You can open the file in a text editor such as Notepad or in Microsoft Excel.

Note The English (United States) version of this software update might contain files for additional languages.

Download Download the file information for cumulative update KB5123099.

Microsoft Store application updates

Windows updates do not install Microsoft Store application updates. Enterprise users should see Microsoft Store apps - Configuration Manager. Consumer users should see Get updates for apps and games in Microsoft Store.

End of support

Microsoft will no longer provide free software updates from Windows Update, technical assistance, or security fixes on the following end dates. For information about the availability of Extended Security Updates (ESUs), see Plan for Windows Server 2016 and Windows 10 2016 LTSB end of support.

Change log
Change date Change description
September 19, 2026 Added a known issue, "File History might stop working after installing September 2026 Windows update".
September 14, 2026 Added the resolution for the known issue "Remote Desktop Services might stop responding after September 2026 security update".
September 12, 2026 Added a known issue, "Remote Desktop Services might stop responding after September 2026 security update".
September 8, 2026 Added the resolution release note for the known issue "CompatTelRunner.exe might close unexpectedly" for Windows Server 2016.