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:

• September 8, 2026—KB5123066 (Monthly Rollup)

• September 14, 2026—KB5129243 (Monthly Rollup) Out-of-band

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.

• [Authentication] This update strengthens security on domain controllers by verifying that incoming Lightweight Directory Access Protocol (LDAP) binds use a Kerberos service ticket issued for the LDAP service principal name (SPN). This change can affect Azure NetApp Files and NetApp ONTAP configurations that connect to Active Directory LDAP with LDAP client session security set to none. In these configurations, access to SMB shares and creating new SMB volumes might fail. For more information, see KB5134907.

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

Multipath I/O (MPIO)-managed disks go offline

Symptoms

Multipath I/O (MPIO)-managed disks may unexpectedly go offline or display error C0000010 in the details tab of device manager. MPIO is a storage multipathing technology used to manage paths between a server and its storage. Although the storage location becomes unavailable, this issue does not result in data loss or change. MPIO-managed storage and all underlying data is immediately visible after disabling MPIO.

Resolution

This issue is resolved in an out-of-band (OOB) update released on November 6, 2016, (KB5135582).

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. The remainder of the issue, that caused some USB Audio Class 1.0 devices to fail to start or produce audio, is resolved with an OOB update released on November 6, 2016, (KB5135582).

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.

• 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.

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.

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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.

Change log

Change date	Change description
November 10, 2026	Added information about an LDAP security change that might affect access to Azure NetApp Files and NetApp ONTAP SMB shares in certain configurations. For more information, see KB5134907.