Summary
Microsoft has released KB5129194, an out-of-band update for Windows 11 version 26H1 that adds protections for two elevation-of-privilege vulnerabilities and fixes Remote Desktop, Hyper-V and multichannel USB audio problems. The update also refreshes selected AI components for Copilot+ PCs.
Microsoft released KB5129194 on September 14, 2026, as an out-of-band update for Windows 11 version 26H1, covering all editions. The package moves supported systems to OS Build 28000.2956 and combines security protections with fixes for Remote Desktop Services, Hyper-V folder sharing and certain USB audio configurations.
An out-of-band update is issued separately from the normal scheduled cumulative-update cycle. Microsoft says KB5129194 is available through Windows Update, Microsoft Update, Windows Update for Business, the Microsoft Update Catalog and Windows Server Update Services (WSUS).
Security protections and system fixes
The update adds protections for two vulnerabilities classified as elevation-of-privilege flaws. CVE-2026-62721 affects the Windows User-Mode Power Service (UMPS), while CVE-2026-85921 affects Windows Secure Kernel Mode. Elevation-of-privilege vulnerabilities are security flaws that can allow activity running with limited permissions to obtain greater access than intended; Microsoft identifies the affected components but does not provide further technical details on this support page.
KB5129194 also addresses a problem introduced after the September 8, 2026, Windows security update KB5124012. In affected environments, Remote Desktop Services could become unstable, leading to Remote Desktop Protocol connection or sign-in failures, or servers becoming unresponsive while Remote Desktop was being configured. Microsoft also lists related failures involving tools such as Microsoft Management Console, the Remote Desktop Services Licensing Diagnoser, File Explorer and the Windows Update page.
A separate Hyper-V fix concerns Linux virtual machines that use HCS-managed virtual machines and share a host folder through Plan 9. Folders shared from the Windows host could fail to appear or could not be accessed inside the Linux guest. Plan 9 in this context is the file-sharing mechanism connecting a Windows host folder with a virtualised guest environment.
The release fixes another issue affecting some USB Audio Class 1.0 devices when they were used with multichannel features, including 8-channel or 3D audio modes. Microsoft says these devices could work normally in standard stereo but fail when multichannel audio was enabled.
AI components, installation and deployment
The package updates four Windows AI components—Image Search, Content Extraction, Semantic Analysis and Settings Model—to version 1.2608.951.0. Microsoft specifies that these components apply only to Copilot+ PCs; they will not install on other Windows client PCs or Windows Server.
KB5129194 includes servicing stack update KB5125104, Build 28000.2950. The servicing stack is the part of Windows responsible for handling update installation, and Microsoft says this component update improves the reliability of that process.
For most users, the update downloads and installs automatically through Windows Update and Microsoft Update. Organisations using Windows Update for Business receive it according to their configured policies. Administrators can also obtain standalone MSU packages from the Microsoft Update Catalog or synchronise the update through WSUS using the Security Updates classification.
IT teams applying the update to Windows installation media have an additional requirement. Microsoft says the boot.stl file must be included when applying dynamic updates to an existing Windows image. The file is used during Secure Boot validation and must match the Windows version and architecture of the image. If it is missing, devices may fail to start from the installation media and produce error code 0xc0430001.
Microsoft continues to list a separate USB Audio Class 1.0 problem associated with the September 8 update. Some affected devices may show Device Manager error Code 10, produce no audio, leave volume controls at zero or make sound settings unresponsive. The company says it is working on a resolution; the multichannel-audio fix in KB5129194 therefore should not be interpreted as a resolution for every USB Audio Class 1.0 symptom.