Microsoft released KB5129195 on September 14, 2026, for Windows 11 versions 25H2 and 24H2. The cumulative security update moves those systems to OS Builds 26200.9457 and 26100.9457, respectively.[1][3]
Microsoft has issued an out-of-band Windows 11 update to repair several problems introduced or exposed by the September update cycle.[1][5] KB5129195 combines previous cumulative fixes with additional security and reliability improvements; it is not simply an optional troubleshooting patch.[1]
This Windows-only guide focuses on identifying, installing, and verifying KB5129195. For the wider September 14 release picture, including Apple’s macOS updates, see our Windows OOB Fix and macOS 27 overview.
Who should receive KB5129195?
KB5129195 applies to all editions of Windows 11 version 25H2 and Windows 11 version 24H2.[1]
After installation, version 25H2 advances to Build 26200.9457, while version 24H2 advances to Build 26100.9457.[1][3][4]
You can check your version by pressing Windows key + R, typing winver, and selecting OK. If your computer runs another Windows release, do not manually install a package intended for 24H2 or 25H2; Microsoft issued different out-of-band updates for other supported Windows versions.[1][5]
The biggest fix: Remote Desktop instability
Microsoft says KB5129195 fixes a Remote Desktop Services problem that could appear after installing the September 2026 Windows security update, KB5122880.[1] In affected environments, Remote Desktop could become unstable, RDP connections or sign-ins could fail, or a server could stop responding while Remote Desktop was being configured.[1]
The disruption could reach beyond the Remote Desktop window.[1] Microsoft says Microsoft Management Console, RDS Licensing Diagnoser, File Explorer, and the Windows Update page could also stop responding on affected systems.[1] Independent reporting documented the same RDS failures and the broader September 14 out-of-band response across supported Windows editions.[5]
Why it matters: This fix matters most to businesses that depend on Remote Desktop for daily access, remote administration, hosted applications, or multi-user desktops.[1][5] A failed login is inconvenient on one PC; across an office, it can become an operational outage.[1]
Hyper-V and Linux shared folders
KB5129195 also fixes a Hyper-V issue affecting applications that use Host Compute Service-managed virtual machines.[1] Windows folders shared with Linux virtual machines through Plan9 could fail to appear or become inaccessible inside the guest system.[1]
That problem can affect development tools and workflows that rely on a Windows host sharing project files with a Linux virtual environment.[1] Practical check: After installing the update, administrators should verify that expected shared folders appear and that applications can read and write them normally.[1]
A partial USB audio repair—not a complete one
Microsoft fixed one specific audio failure involving some USB Audio Class 1.0 devices.[1] Hardware that worked in ordinary stereo mode could fail when switching to multichannel configurations such as 8-channel or 3D audio.[1]
However, KB5129195 does not resolve every September USB audio problem.[1] Microsoft still lists a known issue in which some USB Audio Class 1.0 devices might fail to start, show “This device cannot start (Code 10)” in Device Manager, produce no sound, leave volume controls stuck at zero, or make Windows sound settings unavailable.[1] Microsoft says it is working on a resolution.[1]
If your USB audio device still fails after KB5129195, do not assume the update installed incorrectly. Confirm the Windows build, check Device Manager, test a standard stereo configuration if appropriate, and continue monitoring Microsoft’s release notes for the remaining fix.
The security component: CVE-2026-62721
KB5129195 includes additional protections associated with CVE-2026-62721, a Windows User-Mode Power Service elevation-of-privilege vulnerability.[1] The CVE record describes insufficiently granular access control that could allow an already authorized attacker to elevate privileges locally.[2]
The distinction matters: this is a local privilege-escalation issue, not evidence that any nearby stranger can remotely take over a patched PC.[2] An attacker would first need authorized local access at a lower privilege level.[2] Applying the update reduces the opportunity to turn that foothold into broader control.[2]
Microsoft’s wording says the out-of-band release includes protections documented under an already published CVE.[1] We therefore should not describe CVE-2026-62721 as a brand-new vulnerability discovered on September 14.[2]
How to install KB5129195
For ordinary home and small-business PCs, the safest route is:
- Open Settings.
- Select Windows Update.
- Select Check for updates.
- Allow KB5129195 to download and install.[1]
- Save your work and follow any restart prompt Windows presents.
- Run
winverafterward and confirm 26200.9457 on Windows 11 25H2 or 26100.9457 on Windows 11 24H2.[1][3]
Microsoft says KB5129195 downloads and installs automatically through Windows Update and Microsoft Update.[1] It is also available through Windows Update for Business, the Microsoft Update Catalog, and Windows Server Update Services when configured for the Windows 11 product and Security Updates classification.[1]
IT administrators using standalone Microsoft Update Catalog packages should follow Microsoft’s package-order instructions.[1] The catalog workflow may include a required checkpoint cumulative update that must be installed before the target KB5129195 package; downloading one MSU file in isolation is not always sufficient.[1]
The IT Guys’ recommendation
Practical guidance: install KB5129195 promptly on supported Windows 11 24H2 and 25H2 systems, especially where Remote Desktop, Hyper-V Linux virtual machines, or multichannel USB audio are business-critical.[1][5] For managed offices, test the update on a representative device first, confirm that key applications and remote connections work, and then continue the rollout.[1]
Do not uninstall September security updates merely to recover Remote Desktop unless you have a carefully assessed emergency plan.[5] Independent reporting notes that uninstalling the affected September update restored Remote Desktop for some administrators, but it also removed the security fixes bundled with that update.[5] Installing Microsoft’s corrective out-of-band release is the safer forward-fix when it is compatible with your environment.[1]
Need help with a Windows update problem?
If KB5129195 will not install, Remote Desktop still fails, or USB audio remains unavailable, The IT Guys can help determine whether the problem is the Windows build, a driver, the remaining known issue, or something else in the device’s configuration.
We provide computer support in Port St. Lucie, Jensen Beach, Fort Pierce, and Vero Beach. Contact The IT Guys before repeated uninstall/reinstall attempts turn one update problem into several.