KB5124010: The Windows 11 Update That Already Contains 26H2 (What It Does, the AC-3 Audio Crash, 4.9 GB Download, Uninstall)

KB5124010 is the September 22, 2026 optional "preview" update for Windows 11 versions 24H2, 25H2 and 26H2. It takes them to builds 26100.9550, 26200.9550 and 26300.9550, fixes the domain trust failures that hit Credential Guard PCs, fixes a built-in camera that stopped working and File History's false "Reconnect your drive," and slowly rolls out a set of features, from a Copilot key you can remap to single-finger touchpad scrolling. It also brought one new problem: some older apps that decode AC-3 (Dolby Digital) audio now close without warning, and there's no workaround yet. Here's the surprise most people miss: KB5124010 already contains Windows 11 version 26H2. The 26H2 features are inside it, switched off. The "Windows 11, version 26H2" upgrade you see in Settings afterward is a tiny enablement package that flips them on, which is why the Microsoft Update Catalog lists one 4.98 GB file for 24H2, 25H2 and 26H2 alike. And if you're on 24H2 Home or Pro, this is the last preview update you'll ever get: that version's updates end on October 13, 2026. This page explains which update your PC needs, what KB5124010 changes, every known issue, how to install, download and uninstall it, and what to do if it won't install.

Jake runs a phone repair shop. The little PC in his waiting area plays old movies from a folder of files a customer gave him years ago, through a media player older than some of his staff. In the last week of September it started closing the moment a film began, every time. Nothing else on the PC had changed except a Windows update that Jake had clicked "Download and install" on, because it was sitting there in Settings and he likes to keep things tidy. His friend Ethan, a developer, looked at the update history, then at Microsoft's known issues, and found the answer in about ten minutes. This page is what Ethan explained over the counter: what KB5124010 is, why it was optional, the 26H2 switch hidden inside it, what's new and what's fixed, the AC-3 crash and the other known issues, how to install or download it, how to remove it, and what it means for 24H2 and for IT departments.

Not sure which Windows 11 version you have? Our guide to finding your build number shows it in ten seconds with winver. You can follow this page without it; every step starts from zero.

Do you need KB5124010? Which September update is for your PC

Windows 11 now has several versions in service at once, and each gets its own update, or shares one. KB5124010 is a single update for three of them, because 24H2, 25H2 and 26H2 are built on the same code base. Press Windows + R, type winver and press Enter to see your version and build, then find your row:

Windows 10 doesn't get KB5124010 at all; it's a different system with its own updates. If you're on Windows 11 24H2, 25H2 or 26H2 and your build ends in .9550, you already have it.

For context, KB5124010 sits between two "B" updates, the ones released on Patch Tuesday, the second Tuesday of each month. September's was KB5124008 (builds 26100.9445 and 26200.9445, September 8). An out-of-band fix, KB5129195, followed on September 14 (build .9457). KB5124010 is the "D" release: the optional preview that comes out in the fourth week. October's B update arrives on October 13, 2026.

What "preview" means: optional now, automatic on October 13

A preview update is a test run of next month's changes for people who want them early. It shows up in Settings > Windows Update > Advanced options > Optional updates, and it doesn't install itself unless you ask. That's why one PC in a home can have it and another doesn't: someone clicked, someone didn't. For organizations using Windows Update for Business, the preview isn't offered at all; its changes reach those devices in the next security update.

Everything in KB5124010 rolls into October's Patch Tuesday update on October 13, 2026, which installs automatically. So the honest answer to "should I install KB5124010?" is:

• Install it now if it fixes something you're living with: domain sign-in failures on Credential Guard PCs, a camera that stopped working, or File History saying "Reconnect your drive" when the drive is connected.
• Wait for October 13 if nothing in the fix list affects you, especially if you rely on older media players, games or business apps that might hit the AC-3 audio issue below. October's update includes these changes too, so waiting only buys time for more reports.
• Install it if you want 26H2 now, because the 26H2 enablement package needs it, or a later update, first.

Jake: "So I clicked a button that said 'optional,' and it broke the movie PC."

Ethan: "You volunteered for next month's changes a few weeks early. Most of them are good. One of them doesn't get along with your very old player, and Microsoft hasn't fixed that yet."

The surprise: KB5124010 already contains Windows 11 26H2

Windows 11 version 26H2, also called the Windows 11 2026 Update, reached general availability on September 29, 2026. It isn't a big download like the feature updates of old. Its features are delivered inside the monthly updates and stay dormant until a small package called an enablement package turns them on. That package, KB5121794, is in effect a master switch.

KB5124010 is the update that carries those dormant features. That's why the Microsoft Update Catalog lists KB5124010 separately for 24H2, 25H2 and 26H2, with the same 4,981.5 MB size for each x64 entry and the same 4,825.6 MB for each Arm64 entry: it's one package of code, and the version number on your PC depends only on whether the switch has been flipped.

To get 26H2, a PC needs:

1. Windows 11 version 24H2 or 25H2. The 26H2 package is offered to those two versions.
2. KB5124010, or a later cumulative update, installed first.
3. To be eligible in Microsoft's phased rollout. The rollout started with people who turned on Get the latest updates as soon as they're available in Settings > Windows Update, and expands over the following months. Microsoft can hold back devices with a known compatibility problem.

When your PC is ready, Settings > Windows Update shows "Windows 11, version 26H2 is available" with a Download & install button. It's a small download and a single restart. The enablement package isn't in the Microsoft Update Catalog; it comes only through Windows Update and WSUS, where it appears under the Upgrades classification as "Windows 11, version 26H2." Installing 26H2 resets your support clock: 24 months for Home and Pro, 36 months for Enterprise and Education.

Here's the practical consequence. If you install KB5124010 on 24H2 or 25H2 and never take the 26H2 package, you have 26H2's code on disk but not its features, and your PC still reports 24H2 or 25H2 with that version's end-of-updates date. On 24H2 Home and Pro, that date is October 13, 2026, so the switch matters.

Jake: "So the new version was already in the box, it just wasn't plugged in."

Ethan: "That's it. The September update delivered the furniture; the 26H2 package is the key to the room."

What the 26H2 switch turns on

Because KB5124010 carries 26H2's code, it helps to know what the switch actually activates. Version 26H2 bundles features that rolled out gradually to 24H2 and 25H2 over the past year, and turns some of them on by default. The highlights:

Three features that sat behind a temporary commercial control on 25H2 are on by default in 26H2: Windows settings backup, app-specific taskbar actions, and the File Explorer improvements. Administrator-configured policies are still honored, so a business that had them off keeps them off. Installing 26H2 also resets the support lifecycle: 24 months for Home and Pro, 36 months for Enterprise and Education.

The driver change is the one to check before an IT rollout: an old piece of hardware whose driver is only cross-signed may stop loading once enforcement applies. Most home PCs use WHCP-signed drivers delivered through Windows Update and won't notice.

What's new in KB5124010

Microsoft delivers most of the new features in a gradual rollout: you can have KB5124010 installed and still not see a feature for a few weeks, because it's turned on for devices in waves. Here are the changes that matter most for everyday use:

One thing goes away: PC-to-PC Migration is no longer available in Windows Backup. If you were planning to move to a new PC with that feature, you'll need another route, such as backing up with Windows Backup and restoring on the new PC, or copying your files across yourself.

One planned feature was pulled back: WinRE was going to reuse eligible saved Wi-Fi profiles, including some certificate-based networks, so a PC in recovery could get online on its own. Microsoft moved it to a later rollout. That's the "KB5124010 Wi-Fi" change people search for: it isn't in this update after all.

Copilot+ PCs also get updated AI components (Image Search, Content Extraction, Semantic Analysis and Settings Model, version 1.2608.951.0). They don't install on other PCs.

What KB5124010 fixes

These fixes go to every device that installs the update, not in waves:

• Domain trust on Credential Guard PCs. Domain-joined devices with Credential Guard protected machine accounts could lose their secure channel with Active Directory, so users saw "the trust relationship between this workstation and the primary domain failed" and couldn't sign in with domain credentials. KB5124010 resolves it by disabling Machine Identity Isolation enforcement completely, whatever your configuration says, while Microsoft improves the feature. Details are in the IT admin section.
• The built-in camera. The camera could stop working in the Camera app and in other apps.
• File History. Backup or restore to external or network drives could fail, with a false "Reconnect your drive" message even though the drive was connected.
• Sysmon. Three fixes for the built-in System Monitor: executable header validation, a stop error in a file system filter driver, and wrong Event ID 25 alerts for smss.exe.

The update also includes a servicing stack update, KB5124009 (build 26100.9539), which improves the reliability of installing updates. It's bundled with the cumulative update, so you don't install it separately.

Known issues in KB5124010

There are three known issues for this update. One is new with KB5124010; two were already open and carried over.

Apps that decode AC-3 (Dolby Digital) audio close unexpectedly

This is the new one, confirmed on October 2, 2026. After installing KB5124010, or any later update, some applications that rely on Windows' own AC-3 decoding can fail to launch, close without warning, or work until you use a feature that plays audio, such as playing music or a video. Games, media players and some productivity apps are affected. It affects Windows 11 versions 24H2, 25H2 and 26H2.

Two details narrow it down. First, it's mostly older software: many modern apps decode AC-3 themselves and never touch the Windows component involved. Second, it depends on what the app is doing, so an app might open fine and then close the moment it reaches an AC-3 audio track, which is exactly what Jake's waiting-room player did with old movie files.

Microsoft hasn't published a workaround and says a fix will come in a future update. Until then, practical options:

• Use a player that decodes audio itself. Modern media players that ship their own decoders don't depend on the Windows component, so the same files play normally.
• Delay KB5124010 on machines that run the affected app, if you haven't installed it yet. It's optional until October 13. Note that October's security update carries the same code, and Microsoft hasn't said whether it will include the fix.
• Uninstall KB5124010 on 24H2 or 25H2 if a critical legacy app broke and there's no alternative, accepting that you also lose this update's fixes. Steps are below.

USB audio devices show Code 10 or produce no sound

Some USB Audio Class 1.0 devices (often older USB headsets, speakers and audio interfaces) fail to start after the September 8 update and later ones, including KB5124010. Device Manager shows "This device cannot start (Code 10)," there's no sound, or volume controls stop responding. The status is "Mitigated": an out-of-band update addressed some symptoms, Microsoft is working on the rest, and business customers can ask Microsoft Support for an immediate workaround. Our USB audio Code 10 guide walks through what you can do today.

Black screen or desktop loading problems after sign-in

Mostly on Azure Virtual Desktop hosts using FSLogix profiles, and more often with some existing profiles: after sign-in the screen stays black and the desktop doesn't load, and the event log shows File Explorer crashing. Two workarounds:

• Start the desktop by hand: press Ctrl + Shift + Esc to open Task Manager, choose Run new task, type explorer.exe and press Enter.
• Known Issue Rollback (IT admins): install the Group Policy package "KB5124010 260924_20021 Known Issue Rollback" for 24H2, 25H2 and 26H2, set it under Computer Configuration > Administrative Templates, and restart. It switches off the change behind the problem until a fix ships.

Jake: "So the update didn't break my movies. It broke the old player's way of reading the sound."

Ethan: "Right. The player borrowed a part from Windows to decode that kind of audio, and that part changed. A newer player brings its own part, so it doesn't care."

How to install KB5124010, or download it as an .msu file

From Windows Update (the normal way)

1. Open Settings > Windows Update and select Check for updates.
2. Go to Advanced options > Optional updates.
3. Tick 2026-09 Cumulative Update Preview for Windows 11 (KB5124010) and select Download & install.
4. Restart when asked, then run winver to confirm the build ends in .9550.

From the Microsoft Update Catalog (offline or for several PCs)

Search the catalog for KB5124010 and download the package for your processor. The files are big because they contain the whole cumulative update:

Microsoft requires a checkpoint update, KB5043080, to be installed before this one if it isn't already on the PC. Download both into one folder, for example C:\packages, then install them in order: the checkpoint first, then KB5124010. You can double-click each .msu (it opens the Windows Update Standalone Installer, wusa.exe), or install from an elevated prompt:

DISM /Online /Add-Package /PackagePath:c:\packages\windows11.0-kb5124010-x64_95a6272121df38221bfafc90b50baa8ec3e5294e.msu

Or in PowerShell, run as administrator:

Add-WindowsPackage -Online -PackagePath "c:\packages\windows11.0-kb5124010-x64_95a6272121df38221bfafc90b50baa8ec3e5294e.msu"

If you service installation media or images with this update as a dynamic update, include the boot.stl file; leaving it out can cause error 0xc0430001.

Through WSUS

KB5124010 isn't published to WSUS as a normal item, because it's a preview. Administrators who want it there can import it manually from the Microsoft Update Catalog.

KB5124010 not installing? Work through these

When an update fails, Windows Update usually shows an error code. Most failures come from a handful of causes, and these steps fix them in order of effort:

1. Restart and try again. A pending restart from an earlier update blocks the next one more often than anything else.
2. Free up space on drive C:. The standalone package alone is nearly 5 GB, and installing needs working room on top of that. Settings > System > Storage shows what you can clear.
3. Repair the system files. Open Terminal as administrator and run DISM /Online /Cleanup-Image /RestoreHealth, then sfc /scannow, then restart and try the update again.
4. Install the .msu files by hand, checkpoint KB5043080 first, as in the section above. A manual install often succeeds when Windows Update's download keeps failing.
5. Pause third-party antivirus for the install, if you use one, and turn it back on right after.

If none of that works, an in-place repair (reinstalling the same version of Windows over itself, keeping your files and apps) resets the update components completely. Since this update is optional, you can also simply skip it: October 13's update replaces it.

How to uninstall KB5124010

On Windows 11 24H2 and 25H2 you can remove the cumulative update from Settings:

1. Open Settings > Windows Update > Update history.
2. Scroll to Related settings and select Uninstall updates.
3. Find the entry for KB5124010 and select Uninstall, then restart.

Two cautions. The servicing stack part (KB5124009) stays behind, which is normal. And removing KB5124010 removes everything it fixed, including the domain trust fix, so on a domain-joined PC with Credential Guard, check that the device can still reach its domain before you remove it. If the update comes back by itself, pause updates in Settings until Microsoft ships a fix for whatever made you remove it. Our guide to uninstalling Windows updates covers the other methods.

On 26H2 the situation is different, because the 26H2 features depend on this update's code. If you need to go back, Settings > System > Recovery offers a way back to your previous version for a limited time after a feature update; after that, the route back is a reinstall.

24H2 Home and Pro: October 13, 2026 is the last update

For many people this matters more than anything in KB5124010. Windows 11 version 24H2 Home, Pro, Pro Education and Pro for Workstations reach end of updates on October 13, 2026. KB5124010 is the last preview update 24H2 will get, and October's Patch Tuesday update is the last security update for those editions. After that, a 24H2 Home or Pro PC keeps working but stops receiving security fixes.

If you're on 24H2 Home or Pro, the simplest path forward is the one this page has already described: install KB5124010 (or October's update), then take the 26H2 package when Settings offers it. You land on 26H2 with updates until October 10, 2028, after a small download and one restart. If 26H2 isn't offered yet because of the phased rollout, turn on "Get the latest updates as soon as they're available" in Settings > Windows Update.

The Secure Boot certificate note

KB5124010's notes also repeat a reminder: the Secure Boot certificates used by most Windows devices began expiring in June 2026. Devices without the newer certificates still start and update normally, and the updated certificates arrive through Windows Update over the coming months. There's nothing to do by hand on a home PC; keep updates on.

For IT admins: deploying KB5124010 and 26H2

A few points for managed fleets:

• Channels. KB5124010 is an optional preview: available through Windows Update, as standalone packages in the Microsoft Update Catalog, and by manual import into WSUS. It isn't offered through Windows Update for Business. Its changes reach every channel in the October 13, 2026 security update.
• Checkpoint. Catalog installs need checkpoint KB5043080 first. Put both packages in one folder and install the checkpoint, then KB5124010, with wusa.exe or DISM.
• Machine Identity Isolation. KB5124010 disables MII enforcement entirely, regardless of Intune, Group Policy or registry settings, while Microsoft improves the feature. MII is only supported with domain controllers at the Windows Server 2025 domain functional level. Devices that lost their secure channel before getting this update can be repaired with Test-ComputerSecureChannel -Repair -Credential (Get-Credential) after it's installed; our trust relationship guide has the full procedure. The older Known Issue Rollback for this problem ("KB5124008 260915_17123") is no longer needed on devices with KB5124010 or later.
• AVD and FSLogix. For the black screen issue, deploy the "KB5124010 260924_20021 Known Issue Rollback" policy for 24H2, 25H2 and 26H2, or "KB5124006 260924_20071" for 26H1, and restart.
• AC-3 audio. Inventory legacy line-of-business, training and media applications that use Windows' built-in AC-3 decoding before broad deployment of KB5124010 or October's update, and pilot them first.
• 26H2 through WSUS. The enablement package appears in WSUS with Product set to Windows 11 and Classification set to Upgrades, titled "Windows 11, version 26H2." Devices need KB5124010 or a later cumulative update first.
• 24H2 Enterprise and Education keep receiving updates until October 12, 2027; only the Home and Pro family ends on October 13, 2026. 23H2 Enterprise and Education end on November 10, 2026.
• WMIC. If scripts in your fleet still call wmic, note that it's been removed from 24H2 and later; our WMIC replacement guide maps every common command to PowerShell.

Still stuck? Troubleshooting by symptom

An older game or media player closes when audio starts

That matches the AC-3 known issue in KB5124010 and later. Try a player that decodes audio itself, delay the update on that PC, or, on 24H2 or 25H2, uninstall KB5124010 until Microsoft ships the fix.

I installed KB5124010 but don't see the new features

Most features roll out gradually, so they appear on devices in waves over several weeks. Keep the update installed, keep Windows Update on, and check again later. "Get the latest updates as soon as they're available" puts you earlier in the queue.

KB5124010 isn't in Windows Update at all

It's under Advanced options > Optional updates, not on the main page. If your organization manages updates with Windows Update for Business, it isn't offered; you'll get the same changes on October 13.

Windows 11 26H2 isn't offered after installing KB5124010

The rollout is phased, and some devices are on a compatibility hold. Turn on "Get the latest updates as soon as they're available," check again over the following days, and make sure you're on 24H2 or 25H2, the versions the package is offered to.

My USB headset stopped working

If Device Manager shows Code 10 on a USB audio device, it's the USB Audio Class 1.0 known issue, which started with the September 8 update. See our USB audio Code 10 guide for current options.

Domain users can't sign in on a Credential Guard PC

Install KB5124010 or later, which disables Machine Identity Isolation enforcement, then repair the secure channel with Test-ComputerSecureChannel -Repair, signing in with cached credentials or a local administrator account first.

KB5124010: frequently asked questions

Jake's waiting room shows films again. Ethan installed a modern media player that brings its own audio decoders, and the same old files play without a hitch, while the PC keeps KB5124010 and its fixes. The office PC, still on 24H2 Home, took the 26H2 package the same afternoon after one restart, which moved its end-of-updates date from October 13, 2026 to October 10, 2028. Ethan wrote the line below on a sticky note and put it on the movie PC's monitor.

Revision note. Written October 12, 2026, for every PC that clicked "optional" and got more than it expected. "Look before you leap," says the old proverb; with Windows previews, read the known issues before you click.