Important
The installation of this Extended Security Update (ESU) might fail when you try to install it on an Azure Arc-enabled device that is running Windows Server 2012 R2. For a successful installation, please make sure all Subset of endpoints for ESU only are met as described in Connected Machine agent network requirements.
Announcement: Secure Boot certificate updates remain in progress as Microsoft transitions devices from certificates that began expiring in 2026 to newer certificates. Devices that have not yet received the newer certificates can continue to start and receive standard Windows updates. Microsoft will continue delivering the newer Secure Boot certificates to eligible devices through Windows Update.
End of support information
End of support
Windows Server 2012 R2 reached the end of support (EOS) on October 10, 2023.
Extended Security Updates (ESUs) are available for purchase and will continue for three years, renewable on an annual basis, until the final date on October 13, 2026. For information about the procedure to continue receiving security updates, see KB5031043: Procedure to continue receiving security updates after extended support has ended on October 10, 2023.
We recommend that you upgrade to a later version of Windows Server. For more information, see Overview of Windows Server upgrades.
Summary
Learn more about this cumulative security update, including improvements, any known issues, and how to get the update.
Windows Server 2012 R2
This security update includes fixes and quality improvements that are 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.
[Distributed Key Manager (DKM)] This update automatically remediates insecure DKM container ACL configurations in AD FS by default. Administrators can opt out of automatic remediation by setting RemediateDkmAcl to 0. This change helps address an elevation of privilege vulnerability that can occur when the DKM container ACL is overly permissive. For more information about how to manage this change, see CVE-2026-56155: AD FS Distributed Key Manager container ACL hardening.
[Secure Boot] This update continues the deployment of updated Secure Boot certificates using the latest high-confidence targeting data to identify eligible devices that are ready to automatically receive the new certificates. Deployment through Windows Update remains ongoing across supported devices.
For more information about the resolved security vulnerabilities, please refer to the Deployments | Security Update Guide and the September 2026 Security Updates.
For information about the various types of Windows updates, such as critical, security, driver, service packs, and so on, see Description of the standard terminology that is used to describe Microsoft software updates.
To view other notes and messages, see the Windows 8.1 and Windows Server 2012 R2 update history home page.
Known issues in this update
USB audio devices might fail to start or produce no sound
Symptoms
After installing the September 8, 2026, Windows security update (KB5124012), 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.
Some devices might function in standard stereo configurations but fail when using multichannel audio features, including 8-channel or 3D audio modes. Some customers have reported that they're able to restore audio in these cases by switching to 2-channel mode.
This issue is limited to USB Audio Class 1.0 devices.
Resolution
This issue is partially resolved in the out-of-band (OOB) update released on September 14, 2026, (KB5129243). This OOB update resolves the symptoms experienced on devices using 8-channel or 3D audio modes. Microsoft is working to address the other symptoms and will provide more information when it is available.
IT administrators who need an immediate workaround for the symptoms not addressed yet by the OOB update should contact Microsoft Support for Business for assistance.
How to get this update
Before installing this update
To install any Windows Server 2012 R2 Monthly Rollup released on or after January 14, 2025, we recommend you first install the latest Servicing Stack Update SSU KB5127356. If your device or offline image does not have the latest SSU installed, you might not be able to install this update.
Caution
Until you install the latest SSU, this update might not be offered to your device. To reduce your security risk, install the latest SSU as soon as possible.
- If you use Windows Update, the latest SSU KB5127356 will be offered to you automatically. If the latest SSU is not installed, you might not be able to install this update.
- If you use the Update Catalog, we recommend you download and install the latest SSU KB5127356. If the latest SSU is not installed, you might not be able to install this update.
- If you are a Windows Server Update Services (WSUS) administrator, you must approve SSU KB5127356 and this update KB5126836.
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 may 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 devices Windows\Boot\EFI folder to the corresponding folder on your installation media before deploying the update.
For general information about SSUs, see Servicing stack updates and Servicing Stack Updates (SSU): Frequently Asked Questions.
Language packs
If you install a language pack after you install this update, you must reinstall this update. Therefore, we recommend that you install any language packs that you need before you install this update. For more information, see Learn about adding a language pack to Windows.
Install this update
To install this update, use one of the following release channels.
| Available | Next step |
|---|---|
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This update will be downloaded and installed automatically from Windows Update. |
File information
A list of the files that are included in this update are provided in a CSV (Comma delimited) (*.csv) file. The file can be opened in a text editor such as Notepad or in Microsoft Excel.
Download the file information for cumulative update KB5126836.