KB5124008: Windows 11 Sept 2026 Update, Bugs & Fixes (Yes, its 4+GB)

September 8, 2026 was Patch Tuesday, and the update your Windows 11 PC is asking for is KB5124008 — it takes 24H2 and 25H2 to builds 26100.9445 and 26200.9445, and the honest reveal buried under a dozen "new bug!" headlines this week is that most of what those posts are describing — the black desktop background, the broken mouse cursor colors — is not a new bug in this update at all. It is the bug from last month's optional preview, and today's update is the one that fixes it.

Which September update is actually for your PC?

Jake runs a phone repair shop, and every second Wednesday of the month he gets the same three text messages from customers: "my computer says update failed," "is this update safe," and, from his uncle, "why did my Windows 10 laptop not update at all." This month is no different, except this month's batch is unusually large — Microsoft's own count puts September 2026 at over 1,000 security fixes across all its products, one of the biggest Patch Tuesdays on record.

Ethan, who fixes the shop's own machines between customers, laid it out on a sticky note taped to the front counter: "There isn't one 'Patch Tuesday update.' There are four, and which one lands on a given PC depends entirely on which Windows version it's already running. Nobody reads the version number until something breaks — then it's the first thing I ask."

Run Win + R, type winver, press Enter — that's the fastest way to see your exact build without opening Settings at all. Match it against the table below.

Windows 11, version 24H2 Home and Pro reach end of updates on October 13, 2026 — a little over a month from this release. Enterprise and Education editions of 24H2 remain supported until October 12, 2027. If you're on 24H2 Home or Pro, this is close to your last free ride; the built-in upgrade to 25H2 is free and the sensible move before the clock runs out.

What's actually new in KB5124008

This is a bigger release than most September updates, because it's the one that finally turns on features Microsoft has been testing for months. All of them roll out gradually through what Microsoft calls Controlled Feature Rollout, or CFR — the code lands on your PC with this update, but the feature itself lights up account by account over days or weeks, not the instant you restart.

A taskbar you can actually move

Open Settings > Personalization > Taskbar and, once your rollout wave arrives, you'll find a new position option: bottom, top, left, or right. There's also a genuine smaller-taskbar toggle now — not the old one that just shrank the icons while the bar stayed the same size, but one that shrinks the whole bar, the way Windows 10 always did it.

A Start menu that fits smaller screens

The Categories-layout Start menu that's been swallowing half the screen on 13- and 14-inch laptops finally gets a size control — Large, Small, or Automatic — under Start settings. It won't give you Windows 10-style free dragging, but it stops the menu from being bigger than the taskbar it sits on.

Search without Bing

Under Settings > Privacy & security > Search, two new toggles let you turn off Web Searches and Microsoft Store results in Windows Search, once your rollout arrives — no registry edits, no region switching required. Turn both off and typing in the Start menu only searches your own apps, files, and settings, which is also noticeably faster.

The bugs KB5124008 actually fixes

Here's the part worth slowing down for, because it's the opposite of what a lot of quick-turnaround coverage is implying this week. Every one of the following problems originated in an earlier August update and is resolved by installing today's KB5124008 — not caused by it.

The black desktop background / mouse cursor issue

After installing the August 27 optional preview (KB5120998) or the August 11 security update (KB5121003), some PCs — mostly on non-English installs — found that Personalization settings, including the desktop background, would fail to load and default to a plain black screen. A related bug reset custom mouse cursor styles and colors. Both are fixed as of KB5124008. If your desktop went black after August's update and is still black, install today's update rather than trying to manually re-set the background — the underlying settings loader was the actual problem.

Teams and Outlook crashing on Arm64 PCs

This is where the "excel" and "outlook" search results usually get confused with each other, so it's worth being precise. After the August 11 update, on Arm-based PCs like Surface Pro 11 and Surface Laptop 7, the new Outlook for Windows and Microsoft Teams could fail to launch or close unexpectedly — most often on freshly imaged machines that hadn't yet pulled Microsoft Store app updates. Classic Outlook, Word, and Excel were never affected by this bug; it was specific to the Store-delivered, UWP-style versions of Teams and the new Outlook. KB5124008 resolves it.

Games that wouldn't launch

Also traced to the August 11 update: ARC Raiders, MARVEL Tōkon: Fighting Souls, and THE FINALS could fail to run as expected on affected PCs. That's fixed in this release too. If a game stopped launching sometime in August and you've been putting off troubleshooting it, install September's update before you do anything more drastic.

Regional settings failing to load

On non-English installs, certain Windows settings tied to locale code components could fail to load and silently fall back to a default, with no way to manually restore them while the underlying bug was active. This is also resolved by KB5124008.

Genuinely new fixes in this release

Separately from the August carry-over fixes above, KB5124008 itself addresses a Remote Desktop Audio Redirection issue that could prevent audio from a remote session from playing on the local PC in certain setups, improves diagnostic logging for the OMA-DM client (used by device management servers to troubleshoot connection problems), and adjusts Morocco Standard Time for that country's permanent switch to UTC+00:00 on September 20, 2026. None of these need any action from you — they apply the moment the update installs.

The Secure Boot certificate warning riding along with this update

KB5124008's release notes carry an "Important" notice that has nothing to do with the taskbar or Search changes above, and it's easy to skim past. Secure Boot is the firmware-level check that verifies your PC is starting up with trusted, unmodified software before Windows even loads. The certificates behind that check — issued back in 2011 — began expiring in June 2026, and Microsoft has been quietly rolling out their 2023 replacements through monthly updates ever since, including this one.

That last line is the part worth sitting with, because it's genuinely counterintuitive: a PC on the old, expiring certificates doesn't stop, doesn't warn you, and doesn't behave any differently day to day. What it quietly loses is the ability to receive new protections for the early boot process — updates to Windows Boot Manager, Secure Boot's revocation lists, and fixes for newly discovered vulnerabilities in the boot chain itself. Scenarios that lean on Secure Boot trust, including some BitLocker hardening features, can also be affected if they need an updated certificate to function.

Windows 11 24H2 Home and Pro: the end-of-updates date sitting right next to it

The other announcement on KB5124008's release page is more concrete and has a harder deadline attached: Windows 11, version 24H2 Home and Pro editions reach end of updates on October 13, 2026 — a little over a month from this release. After that date, those editions stop receiving fixes for known issues, time zone updates, technical support, and monthly security or preview updates, including the kind of protections the Secure Boot rollout above depends on. Enterprise and Education editions of 24H2 aren't affected by this deadline; they remain supported until October 12, 2027.

Put those two notices side by side and the practical takeaway for a 24H2 Home or Pro reader is straightforward: the free upgrade to 25H2 isn't just about getting the new taskbar and Start menu earlier — it's what keeps a PC inside the window where Secure Boot certificate updates, and everything else in this post, keep arriving automatically at all.

Known issues right now

Microsoft's own release-health page for KB5124008 states plainly: it is not currently aware of any issues with this update. That's a real statement about the update's known-issues list, not a claim that nothing will ever go wrong on your specific PC — "no known issues" and "your download will definitely succeed" are two different sentences, and only one of them is Microsoft's.

Ethan put it to Jake this way: "'No known issues' means nobody's reported a pattern serious enough to make Microsoft's public tracker. It doesn't mean your particular PC, with its particular five years of installed software and however much free disk space it has left, is guaranteed a clean run. Those are two different risks, and only one of them is Microsoft's job to manage."

How to install KB5124008 — step by step

For almost everyone reading this, the update is already downloading itself in the background. Here's the deliberate path if you'd rather not wait for it:

1. Press Win + I to open Settings directly (this shortcut works from anywhere, no Start menu needed).
2. Select Windows Update in the left sidebar.
3. Click Check for updates. If KB5124008 doesn't appear immediately, give it a minute — Microsoft staggers the rollout so not every PC sees it in the same hour.
4. When it appears, click Download and install. Plan for one restart at the end; two restarts are occasionally normal this month because the bundled servicing stack update sometimes needs its own reboot before the main package finishes.
5. After the restart, press Win + R, type winver, and confirm the build reads 26100.9445 (24H2) or 26200.9445 (25H2).

If you manage more than one PC — a small business, a household with several machines — the same update is available through Windows Update for Business, WSUS (Windows Server Update Services, the on-premises tool IT admins use to control which updates reach which PCs), and the Microsoft Update Catalog, all pulling from the identical KB5124008 package. More on those channels further down.

Downloading it manually as an offline .msu file

Sometimes Windows Update itself is the thing that's broken, and the fix is to bypass it entirely. That's what a manual download does — you get the update package straight from Microsoft's Update Catalog and install it by hand, which sidesteps whatever is tripping up the automatic path.

1. Confirm your exact version first (Settings > System > About, or winver) — you need the right file for 24H2 vs. 25H2 and for your processor type (x64 or Arm64).
2. Go to catalog.update.microsoft.com and search KB5124008.
3. Pick the matching entry for your build and processor architecture, and download the .msu file (Microsoft Standalone Update — a self-contained installer format that doesn't need Windows Update running at all).
4. Double-click the downloaded file. Windows Update Standalone Installer opens automatically and walks through the install.
5. Restart when prompted, then check winver to confirm the new build landed.

One honest catch with the Update Catalog route: it serves the full update file, not the smaller differential download Windows Update normally sends you. That's directly why the file sizes below look so much larger than what Windows Update itself typically pulls down.

Download size and how long it takes

Windows updates have gotten noticeably bigger over the past couple of years — Windows 11's full standalone packages now run well over 5 GB, largely because of the on-device AI components bundled into recent builds (this release updates several of them — Image Search, Content Extraction, Semantic Analysis, and the Settings Model — to version 1.2608.951.0). Here's exactly what the Update Catalog lists for KB5124008's full offline packages:

For Windows 11 26H2 26300.9445 the size is 4934.9 MB. You can download from https://catalog.update.microsoft.com/Search.aspx?q=KB5124008 in case your windows update was not downloading this KB5124008. 

Two things worth knowing before those numbers scare you off: first, that's the full standalone package size from the Catalog, not what Windows Update sends to a PC that's already reasonably current — Windows Update only fetches the pieces your machine is actually missing, which is usually a fraction of that. Second, install time depends far more on your storage than your download speed; on an SSD with 15+ GB free, expect somewhere in the 10–20 minute range including the restart. On an older hard drive with a nearly full system partition, it can run considerably longer, and that's the scenario most "it's stuck at 40%" complaints trace back to.

When KB5124008 keeps failing to download or install

"No known issues" is a statement about the update's design, not a promise about every PC it lands on. When it fails, it's almost always the PC's own update plumbing — not a flaw in KB5124008 itself. Work down this list, cheapest and fastest first.

1. Restart and try again

Unsatisfying, but it genuinely resolves a surprising share of stuck downloads. The bundled servicing stack update occasionally needs one restart before the main cumulative update can proceed.

2. Free up disk space

Given the sizes above, aim for at least 10–15 GB free before retrying. Settings > System > Storage > Storage Sense clears temporary and cache files in a couple of clicks.

3. Run the Windows Update troubleshooter

Settings > System > Troubleshoot > Other troubleshooters > Windows Update. It resets the common stuck states in the update service automatically, without you needing to know which service was actually the problem.

4. Pause third-party security software during the install

Antivirus tools that intercept file writes are a classic, quiet update-killer, especially behind error 0x80070020 below. Pause it, install, restart, then turn it back on.

5. Repair the update components

Open Terminal as administrator and run DISM /Online /Cleanup-Image /RestoreHealth, let it finish (it can sit at 62% for several minutes — that's normal, not a freeze), then sfc /scannow, then restart and retry the update. This addresses component-store corruption, which is behind most repeat failures.

6. Install the .msu manually

Go back to the manual download steps above. This is the single highest-percentage fix on this list, because it fully bypasses whatever part of Windows Update is failing.

7. The last resort: an in-place repair install

Download the Windows 11 Installation Assistant or a current ISO from Microsoft, run Setup from inside your current Windows, and choose "Keep personal files and apps." It rebuilds the Windows system files underneath your data and installs the current cumulative update as part of the process — a genuine cure for a PC that has failed every step above.

If you're getting an actual error code rather than a silent failure, it's naming the specific problem:

All five of these are general-purpose Windows Update error codes — they show up across many months' cumulative updates, not something specific to KB5124008. That's actually good news: the fix ladder above works on them regardless of which month's update triggered them.

The BitLocker recovery key prompt

A specific and genuinely alarming-looking prompt that has followed several recent Windows updates: a blue screen asking for a BitLocker recovery key right after a restart. BitLocker is Windows' built-in drive encryption, and this prompt appears because the update changed measurements BitLocker uses to verify the boot process hasn't been tampered with — it's the encryption being cautious, not a sign of damage or malware.

Windows 11 23H2: what's different with KB5122880

If winver shows you're on 23H2, a straight fact worth knowing before you go looking for the new taskbar options: consumer (Home/Pro) support for Windows 11 23H2 already ended in November 2025. September's KB5122880, build 22631.7582, is a security-only release at this point, primarily kept alive for Enterprise and Education editions, which lose support on November 10, 2026. It carries the same Secure Boot certificate targeting expansion, Morocco time zone change, and Remote Desktop audio fix as KB5124008 — but none of the taskbar, Start menu, or Search changes, since those are exclusive to 24H2/25H2's codebase.

Deploying KB5124008 across managed fleets

Everything above assumes one PC and Windows Update running the show. If you're patching more than a handful of machines, Microsoft lists four separate channels for this specific release, and which one you use changes what you need to watch for.

Whichever channel you use, KB5124008 combines the September servicing stack update (SSU, packaged as KB5124007) with the cumulative update (LCU) in one file. That's Microsoft's standard practice now — you no longer install the SSU separately before the LCU, they arrive together.

Beyond that, the practical admin question this month is simpler than usual: unlike some releases that introduce a fresh known issue for fleets to watch, KB5124008's job is mostly closing out last month's Arm64 Teams/Outlook crash and the game-launch failures — so if your pilot ring already sat on August's KB5121003 without incident, there's no new fleet-wide regression pattern to stage around this time. Approve it in WSUS or add it to your Windows Update for Business deferral ring on your normal cadence.

Uninstalling KB5124008

If a specific feature or behavior is causing you real trouble after installing, you can remove the cumulative update — though you generally shouldn't, because doing so reopens every security hole this release closed, and this month's release closes an unusually large number of them.

1. Settings > Windows Update > Update history.
2. Scroll to Uninstall updates.
3. Find KB5124008 in the list and select Uninstall, then restart when prompted.

Two honest caveats. First, the bundled servicing stack update (KB5124007) is not removed by this process by design — it's low-level plumbing that future updates rely on, and it stays behind regardless. Second, uninstalling doesn't just undo the new taskbar or Search options; it undoes every security fix in the package, including patches for vulnerabilities that are now public knowledge simply because this Patch Tuesday shipped. Treat uninstalling as a short diagnostic step to isolate a problem, not as a way to live with an older, unpatched build.

Windows 10 in September 2026: KB5122878 and the ESU reality

Windows 10 reached end of support on October 14, 2025 — that's now nearly a year in the past, and it is still the single most important sentence in any Windows 10 post. Windows 10 22H2 gets no free security updates anymore, full stop. It still boots, browses, and prints, which is exactly why so many people don't realize anything has changed — the risk is invisible until it isn't.

The only Windows 10 PCs that received anything this Patch Tuesday are those enrolled in Consumer Extended Security Updates (ESU). Those machines got KB5122878, taking build 19045 to 19045.7725. It carries the same Secure Boot certificate expansion and Remote Desktop audio fix as this month's Windows 11 updates, plus a Windows Code Integrity change that improves app compatibility during Microsoft's ongoing certificate-authority rotation, recognizing the new "Microsoft Windows Production PCA 2026" certificate as equivalent to the older one apps already trust.

Enrolling in ESU (if you haven't yet)

Go to Settings > Windows Update and look for an "Enroll now" option — it requires build 22H2 and a Microsoft account signed in locally (a local-only account won't qualify). Enrollment covers up to 10 devices per Microsoft account and, as of this writing, is still open. You have three ways to pay for it: free if you sync your settings through Windows Backup, 1,000 Microsoft Rewards points, or a one-time payment of roughly $30 (regional pricing varies, and the European Economic Area gets it free without the backup requirement).

How long ESU actually buys you

Consumer ESU originally ran one year, to October 2026. In June 2026, Microsoft extended it by an additional year, so enrolled consumer devices are now covered through October 2027. Commercial ESU (for businesses) is priced separately at $61 per device in year one, doubling each year for up to three years, running through October 2028.

Ethan's line to Jake's uncle, whose Windows 10 laptop hadn't seen a security patch since last October and wasn't enrolled in anything: "It doesn't look sick. That's the whole problem — the holes it's missing patches for were published the same day this month's updates shipped, and now every attacker on the internet has the list. Enroll it, upgrade it, or know exactly what you're risking. 'It still works fine' isn't one of the three options."

The other pieces that show up alongside KB5124008

Open your update history on a Windows 11 PC this week and KB5124008 usually won't be alone. Depending on what's installed, you may also see separate .NET Framework and .NET runtime security updates listed by their own KB numbers, and the monthly refresh of the Windows Malicious Software Removal Tool (MSRT) — a standalone scanning tool Microsoft ships every month independent of the cumulative update, which checks for a specific list of known malware and removes it if found. None of these require separate action; they install through the same Windows Update pass.

Questions people actually ask about KB5124008

What is KB5124008?

The September 8, 2026 Patch Tuesday cumulative update for Windows 11 versions 24H2 and 25H2, taking them to builds 26100.9445 and 26200.9445. It bundles this month's security fixes with quality-of-life features — a movable and genuinely resizable taskbar, a resizable Start menu, and Bing-free Search — plus fixes for bugs introduced by August's updates.

Is KB5124008 safe to install?

Yes. Microsoft lists no known issues with this release, and because it's a security update, the vulnerabilities it patches became public knowledge the day it shipped, so delaying doesn't protect you, it exposes you longer.

What is the difference between KB5124008 and KB5122880?

KB5124008 is for Windows 11 24H2/25H2 and includes the new taskbar, Start menu, and Search features. KB5122880 is for Windows 11 23H2 and is a security-only release for Enterprise/Education stragglers, since consumer 23H2 support already ended in November 2025.

Why is my PC downloading such a large file this month?

The full offline packages run roughly 4.9–5.4 GB, driven partly by updated on-device AI components bundled into recent Windows 11 builds. Windows Update itself typically downloads far less than that, since it only fetches the pieces your PC is missing rather than the full standalone file.

How long does KB5124008 take to install?

On a PC with an SSD and plenty of free space, expect roughly 10 to 20 minutes including the restart. Older hard drives or nearly full system partitions can take considerably longer, and a low "percent complete" number is not linear progress — give it time before assuming it's stuck.

Do I need to install August's KB5120998 first?

No. KB5124008 is cumulative and already includes everything from August 27's optional preview. Install September's update directly.

What do I do if KB5124008 keeps failing to download?

Restart and retry first, confirm at least 10-15 GB of free disk space, then run the Windows Update troubleshooter. If it still fails, download and install the .msu file manually from the Microsoft Update Catalog, which bypasses Windows Update entirely.

What does Install error - 0x800f0991 mean?

It typically means missing or corrupted payload files in Windows' component store. Run DISM's RestoreHealth command followed by sfc /scannow, restart, and retry; if that doesn't clear it, install the update manually from the Update Catalog instead.

Can I download KB5124008 offline as an .msu file?

Yes. Search KB5124008 on catalog.update.microsoft.com, choose the entry matching your Windows version (24H2 or 25H2) and processor (x64 or Arm64), and double-click the downloaded .msu file to install it without going through Windows Update.

Why am I being asked for a BitLocker recovery key after this update?

The update changed boot measurements that BitLocker checks for tampering, so it's asking to be cautious rather than reporting damage. Your files are intact; find your recovery key at aka.ms/myrecoverykey, where it's almost always auto-saved under your Microsoft account.

Will KB5124008 fix Outlook or Teams crashing on my PC?

If you're on an Arm64-based PC and Teams or the new Outlook has been crashing or failing to launch since August, yes — that issue is specifically resolved by this update.

Can I uninstall KB5124008?

Yes, through Settings > Windows Update > Update history > Uninstall updates. Note that the bundled servicing stack update stays behind by design, and uninstalling reopens every security fix in the package, so treat it as a temporary diagnostic step rather than a fix you keep long-term.

What happened to my Windows 10 PC this Patch Tuesday?

If it isn't enrolled in Extended Security Updates, nothing — and that's expected, not an error. Windows 10 support ended October 14, 2025, and only ESU-enrolled PCs receive monthly security patches now.

Is Windows 10 KB5122878 the same as KB5124008?

No. KB5124008 is for Windows 11 24H2/25H2. KB5122878 is the separate September 2026 ESU security update for Windows 10 22H2, taking it to build 19045.7725, and it only installs on devices enrolled in Consumer or Commercial ESU.

Does this update affect Excel or Word?

No. The Arm64 app-crashing bug this update fixes was specific to the new Outlook and Microsoft Teams; classic Outlook, Word, and Excel were never affected.

Revision note. Written September 2026 and this will need an update once Microsoft's known-issues page for KB5124008 changes, once the Secure Boot certificate rollout reaches its next milestone, or when October's Patch Tuesday supersedes it. If you landed here at midnight with a stuck progress bar, take a breath — the fix ladder above has almost certainly got you covered, and your files are safer for having stayed to read it.