KB5124012: Windows 11 version 26H1 Security Update (September 2026)

1. Devices might experience a black screen or desktop loading issues after sign-in

Symptoms:

After installing the August 2026 Windows non-security preview update KB5120996 and subsequent updates, some devices might experience desktop loading issues. This issue has been primarily observed on Azure Virtual Desktop (AVD) hosts using FSLogix. This issue appears to occur more frequently with some existing user profiles.

Affected users might see a black screen after sign-in, with the desktop session failing to load automatically. In some cases, users might be unable to access their desktop until the desktop session is started manually. Application event logs might also show Windows Explorer crashes.

Resolution

This issue is resolved in Windows updates released on and after October 13, 2026 such as KB5127646 . We recommend you install the latest Windows update for your device as it contains important improvements and issue resolutions, including this one.

2. Domain-joined devices might lose their secure trust relationship with the domain

Symptoms

After installing the September 8, 2026, Windows security update KB5124012 , or later updates, some Credential Guard protected machine accounts might lose their secure channel with an on-premises Active Directory (AD) domain. Users might then be unable to sign in interactively with valid domain credentials and might receive a message stating that the trust relationship between the device and the domain failed. Offline sign-in using previously cached credentials might continue to work. AD replication and AD services on the domain controllers are not affected.

This issue occurs because KB5124012 and later updates enable the Machine Identity Isolation feature. While the update does not directly enable Machine Identity Isolation enforcement, it does cause Windows to begin honoring any existing or policy-provisioned settings that enabled Machine Identity Isolation enforcement. However, this feature is only supported for environments connected to domain controllers running at a Windows Server 2025 Domain Functional Level (DFL) and above. The feature should be disabled elsewhere. Any devices previously configured to use Machine Identity Isolation that are not connected to Windows Server 2025 domain controllers will experience this issue and will need to disable the feature.

Resolution

This issue is addressed by KB5124010 . This update prevents Machine Identity Isolation (MII) enforcement by disabling it completely while improvements are made to the feature. MII enforcement will be prevented regardless of the MII configuration.

3. 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, (KB5129194) . 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.

4. Host folder shares might be unavailable in Hyper-V-based Linux VMs

​​​​​​​​​

Symptoms

After installing the September 2026 security update (KB5124012), applications that use HCS-managed virtual machines might experience issues when sharing host folder with Linux VMs using Plan9. Affected virtual machines start normally, but folders shared from the Windows host using Plan9 do not appear or cannot be accessed in the guest environment.

Applications or sandbox environments that depend on these shared folders might display an error indicating that no Plan9 drive shares were mounted. Claude Cowork and the Windows Subsystem for Linux (WSL) are two of the applications affected by this issue. Standard Hyper-V virtual machines that do not use the Plan9 feature are not affected by this issue.

Resolution

This issue is resolved in Windows updates released on and after September 14, 2026 such as KB5129194 .

5. 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 KB5129194 . We recommend you install the latest Windows update for your device as it contains important improvements and issue resolutions, including this one.

6. File History might stop working after installing September 2026 Windows update

Symptoms

After installing the September 2026 Windows security update KB5124008 , some customers using File History, might be unable to create or update backups. File History, available through Control Panel > System and Security > File History , is used to back up files to an external drive or network location. Affected devices might incorrectly display a "Reconnect your drive" message even when a compatible backup drive is connected and functioning properly. Additionally, the "Last Backup" timestamp might not update, and previously backed up files might show "No previous version available." In some cases, Event Viewer might record application crash events referencing FileHistory.exe and KERNELBASE.dll .

Resolution

This issue is resolved in Windows updates released on and after September 22, 2026 such as KB5124006 . We recommend you install the latest Windows update for your device as it contains important improvements and issue resolutions, including this one.