Microsoft released Windows 11 KB5120998 on August 27, 2026 as an optional non-security preview for Windows 11 versions 24H2 and 25H2. It moved Windows 11 24H2 to OS Build 26100.9278 and Windows 11 25H2 to OS Build 26200.9278. The update added major taskbar, Start, Search and platform changes, but it also became associated with several documented problems—including mouse personalization resets and black desktop backgrounds—that Microsoft later fixed.
KB5120998 is no longer the update most users should manually seek. Microsoft released the September 8 security cumulative update KB5124008, followed by later September fixes and the September 22 optional preview KB5124010. Current Windows users should normally install the latest applicable cumulative update offered through Windows Update rather than deliberately installing the older August preview.
Table of contents
Windows 11 KB5120998: Quick Facts
- Released: August 27, 2026
- Type: Optional non-security preview cumulative update
- Windows 11 24H2: OS Build 26100.9278
- Windows 11 25H2: OS Build 26200.9278
- Security update: No
- Mandatory: No
- Major additions: movable/smaller taskbar, Start customization, Search improvements, File Explorer changes, Administrator Protection and new platform features
- Major documented issues: mouse customization reset and black desktop background; ARM Teams/new Outlook issue was also listed but originated with August security updates
- Issue resolution: KB5124008 and later updates
- Current advice: install the latest applicable cumulative update rather than seeking KB5120998 specifically
What Is Windows 11 KB5120998?
KB5120998 is a cumulative preview update containing production-quality non-security improvements. Microsoft uses optional preview releases to ship fixes and feature changes before they are folded into later security cumulative updates. Because it is cumulative, a later supported update can include applicable KB5120998 fixes without requiring users to install the August package first.
Microsoft also uses gradual rollout for some Windows experiences. Two PCs with KB5120998 installed can therefore show different new features temporarily. Missing a taskbar or Start option immediately after installation does not necessarily mean the update failed.
Is KB5120998 Mandatory?
No. KB5120998 was an optional preview, not the monthly Patch Tuesday security update. At its original release, users who did not need its new features could reasonably skip it and wait for the next security cumulative update.
That next security update arrived as KB5124008 on September 8, 2026. Microsoft then issued additional September servicing, including KB5129195 on September 14 and the newer optional preview KB5124010 on September 22. The practical result is that most users should update forward, not go backward to an August preview.
Windows 11 24H2 and 25H2 Build Numbers
| Windows version | KB5120998 build |
|---|---|
| Windows 11 24H2 | 26100.9278 |
| Windows 11 25H2 | 26200.9278 |
New Taskbar Options in KB5120998
One of the most visible changes is taskbar placement. Windows 11 can now place the taskbar at the bottom, top, left or right of the screen. Most familiar taskbar behaviors, including flyouts and “never combine” options, work in the alternate positions.
There is also a Small taskbar option that reduces both taskbar height and icon size. Microsoft currently limits the Search box and smaller-taskbar experience to top and bottom placement. Settings are under Settings > Personalization > Taskbar > Taskbar behaviors.
Microsoft also improved system-tray loading reliability on non-touch PCs.
Start Menu Customization
KB5120998 adds Small, Large and Automatic Start-menu sizing. “Recommended” is being renamed “Recent,” users can hide their name and profile picture, and the redesigned Start settings page offers more independent controls over Pinned, Recent/Recommended and All sections.
Windows Search and File Explorer Improvements
Windows Search Home is cleaner, result-source labeling is clearer, and users can control whether web and Microsoft Store suggestions appear alongside local results under Settings > Privacy & security > Search. Windows can also index frequently used folders automatically, with a separate setting controlling that behavior.
File Explorer Home should open more quickly, and touch users can scroll the Recommended/Home carousel more naturally. These are reliability and usability improvements rather than a major Explorer redesign.
Lock Screen Widgets and Progress Indicators
Microsoft revised the lock-screen rollout so Weather is the only widget shown by default, while other widgets can be added manually under Settings > Personalization > Lock screen. This is a gradual rollout and may not appear on every PC at the same time.
The update also refreshes Windows progress indicators shown during startup, sign-in, restart, shutdown and Windows Update installation.
Administrator Protection
Administrator Protection introduces just-in-time administrative privileges. Instead of allowing an administrator account to carry standing elevated rights everywhere, Windows can issue an isolated admin token only when an approved administrative action needs it.
The feature uses profile separation and Windows Hello authentication to harden Windows against elevation-of-privilege attacks. Microsoft explicitly states that Administrator Protection is not a formal security boundary. It is off by default and can be configured in managed environments with Intune/OMA-URI or Group Policy.
Other Windows Improvements
- Multiple desktops: smoother virtual-desktop switching.
- Display: improved reliability in Display settings and better persistence of brightness after Energy Saver turns off.
- Task Manager: more reliable App history information.
- Windows Setup: simpler secondary-keyboard selection and improved Get Started pages.
- Restart behavior: a PC manually put to sleep can return to sleep after Windows Update finishes.
- Network: VPN-process resiliency improvements and better Japanese WLAN-report output.
- Input: Japanese IME reliability and voice-typing setting persistence.
Windows Update Orchestration Platform
KB5120998 adds support for participating applications to coordinate their updates through the Windows Update Orchestration Platform. The goal is better update scheduling and a less fragmented update experience. It does not mean every Windows application automatically starts updating through Windows Update; software developers must integrate with the platform.
Microsoft Execution Containers and Agentic Processes
Microsoft Execution Containers gain Process Isolation, a lightweight containment mechanism intended for responsive workloads such as coding agents and model-generated code. Policies can restrict access to files, networking, the user interface and other operating-system capabilities.
Windows also gains preview platform support for tagging agentic processes. Authorized components can attach an opaque agent identifier to a process token; child processes inherit it, and Web Account Manager can include it in authentication requests. Microsoft says the identifier format remains preview technology and can change.
Post-Quantum Cryptography and WMIC Removal
ML-KEM can now be used as a standalone post-quantum algorithm for TLS key exchange in addition to previously supported hybrid groups. This is a platform-level cryptography change rather than something most home users need to configure manually.
Beginning in August 2026, Windows 11 24H2 and 25H2 no longer include the old WMIC command-line utility as a Feature on Demand. WMI itself remains supported. Administrators should use modern alternatives such as PowerShell CIM cmdlets when replacing WMIC-based scripts.
Copilot+ AI Component Updates
KB5120998 updates Image Search, Content Extraction, Semantic Analysis and Settings Model components to version 1.2608.951.0. Microsoft states that these AI component updates apply only to eligible Copilot+ PCs; they do not install on every Windows 11 PC or Windows Server.
For Copilot+ hardware context, FrediTech’s HP OmniBook 5 Copilot+ review explains the broader NPU-focused PC category.
Servicing Stack Update
KB5120998 includes servicing-stack improvements through KB5120997. Microsoft lists Build 26100.9270 in the release summary and file version 26100.9274 in the file-information section. The servicing stack is the Windows component that installs operating-system updates, so SSU improvements are intended to make the update process itself more reliable.
KB5120998 Known Issues
| Issue | Who was affected | Current status |
|---|---|---|
| Mouse customization reset | Primarily non-English Windows installations | Resolved by KB5124008 and later |
| Desktop background turns black / personalization settings fail to load | Some Windows 11 24H2/25H2 devices | Resolved by KB5124008 and later |
| Teams and new Outlook may fail on ARM | Some ARM PCs, especially fresh/new images | Resolved by KB5124008 and later; root issue began with Aug. 11 security update KB5121003 |
Mouse Customization Reset
Microsoft confirmed that Windows updates released on or after August 27 could reset pointer style, cursor customization and cursor animations on some non-English Windows installations. Manual restoration could fail because affected components did not load the settings correctly. Microsoft fixed the issue in updates released September 8 and later, including KB5124008.
Desktop Background Turning Black
Some systems lost desktop personalization settings and displayed a solid black background. Slideshow or contrast-theme settings could also be affected, and simply selecting the old wallpaper again might not work while the bug was present. Microsoft resolved this in KB5124008 and later updates.
Teams and Outlook Problems on ARM PCs
Some ARM devices, including systems such as Surface Pro 11 and Surface Laptop 7, could see Microsoft Teams or the new Outlook fail to open or close unexpectedly. Classic Outlook, Word and Excel were not reported as affected by this specific issue. Microsoft’s temporary workaround was to update the Auto Super Resolution Package through Microsoft Store to version 1.0.19.0 or later.
Important attribution: Microsoft says this ARM issue began with security updates released on or after August 11, 2026 (KB5121003). It appeared on the KB5120998 known-issues page, but it should not be described as a bug uniquely introduced by KB5120998. KB5124008 resolved it.
Problems From Later September Updates
Do not attribute every September Windows problem to KB5120998. Later updates introduced or exposed separate issues involving Active Directory trust, USB Audio Class 1.0 devices, Hyper-V/Plan9 shared folders, Remote Desktop Services and File History. Those belong to the September servicing line, including KB5124008 and successors, not the August preview alone.
Should You Install KB5120998 Now?
Current Recommendation
Install the latest supported Windows cumulative update offered through Windows Update rather than manually installing KB5120998. Newer cumulative updates include applicable KB5120998 improvements plus later bug fixes and security servicing.
KB5120998 remains useful as historical context when diagnosing a PC that installed it in late August. It is not the package ordinary users should deliberately hunt down in late September.
How to Check Whether KB5120998 Is Installed
- Open Settings.
- Select Windows Update.
- Open Update history.
- Search the installed update list for KB5120998.
You can also press Win + R, run winver, and compare the OS build with 26100.9278 for 24H2 or 26200.9278 for 25H2.
winverHow KB5120998 Was Installed
At release, the normal route was Settings > Windows Update > Advanced options > Optional updates. It may no longer appear because later cumulative updates supersede it.
The Microsoft Update Catalog still lists x64 and Arm64 packages for Windows 11 24H2 and 25H2. Manual catalog installation is mainly useful for administrators, offline servicing and controlled troubleshooting. Microsoft warns that checkpoint cumulative update packages may need to be installed in a specific order. Download Windows update packages only from Microsoft.
KB5120998 Won’t Install: What to Try
- Restart Windows and retry Windows Update.
- Confirm you are actually running Windows 11 24H2 or 25H2.
- Check that the system drive has adequate free space.
- Run the Windows Update troubleshooter.
- Check Windows Release Health for current servicing problems.
- Repair the Windows component store if system corruption is suspected.
- Run System File Checker after DISM where appropriate.
- If KB5120998 has been superseded, install the latest applicable cumulative update instead.
DISM /Online /Cleanup-Image /RestoreHealthsfc /scannowDISM repairs the Windows component store; SFC checks protected system files. Neither command guarantees that every Windows Update problem will be fixed.
For general maintenance context, see FrediTech’s PC maintenance guide and device security best practices.
How to Uninstall KB5120998
If troubleshooting specifically identifies KB5120998 and the system still allows removal, go to Settings > Windows Update > Update history > Uninstall updates. Find KB5120998 and use the available uninstall option.
However, later cumulative updates can supersede older packages, and removing a cumulative update can also remove included fixes. Because the main KB5120998 bugs are already corrected in newer updates, updating forward is usually preferable to rolling back.
Secure Boot Certificate Notice
Microsoft notes that older Windows Secure Boot certificates began expiring in June 2026. Microsoft is deploying replacement certificates through Windows Update. A PC without the new certificate does not simply stop booting because the old date has passed; Microsoft says affected devices continue to start and standard Windows updates continue to install while the certificate transition proceeds.
Windows 11 24H2 End-of-Support Warning
Windows 11 24H2 Home and Pro reach end of updates on October 13, 2026. Enterprise and Education follow a different lifecycle and remain supported longer. Home/Pro users still on 24H2 should move to a supported Windows version rather than focusing on an old August preview.
Windows 11 25H2 remains supported and continues to receive cumulative servicing. You can check your Windows version under Settings > System > About or with winver.
Advice for IT Administrators
Optional non-security previews are useful in validation rings because they expose upcoming non-security changes before broad security rollout. They should still be deployed according to organizational policy rather than pushed immediately to every production device.
- Use pilot rings before broad deployment.
- Monitor Windows Release Health and current known issues.
- Test ARM devices separately if Teams/new Outlook are business-critical.
- Validate personalization and application-compatibility policies.
- Use Windows Update for Business or Intune for controlled rollout.
- Use WSUS/manual Catalog import only where appropriate.
- Verify backup/recovery readiness before major troubleshooting.
- Plan migration from 24H2 Home/Pro before October 13, 2026.
Final Install Advice
KB5120998 was an important Windows 11 preview because it restored more taskbar flexibility and introduced substantial platform work around Administrator Protection, agentic processes, Execution Containers and update orchestration. Its two clearest user-facing bugs—the mouse-personalization reset and black desktop issue—were fixed by KB5124008.
Today, however, the correct question is not “Should I install KB5120998?” but “Am I on the latest supported cumulative update for my Windows version?” For most users, Windows Update should handle that decision. Keep KB5120998 in mind mainly when diagnosing late-August systems or researching where particular Windows 11 features first appeared.
Frequently Asked Questions
What is KB5120998?
KB5120998 is Microsoft’s August 27, 2026 optional non-security preview cumulative update for Windows 11 24H2 and 25H2.
Is KB5120998 a security update?
No. It was an optional non-security preview.
Does KB5120998 move the taskbar?
Yes. It introduced support for placing the taskbar at the top, bottom, left or right, although some taskbar features are limited by position.
Does KB5120998 cause a black desktop?
Microsoft confirmed a bug in updates released August 27 and later that could reset desktop personalization and show a black background. It was fixed by KB5124008 and later updates.
Does KB5120998 reset mouse settings?
It could reset cursor and animation settings on some non-English Windows installations. Microsoft fixed the issue in KB5124008 and later.
Does KB5120998 break Teams or Outlook on ARM?
The ARM Teams/new Outlook issue appeared on the KB5120998 known-issues page, but Microsoft says it began with August 11 security updates such as KB5121003. It was resolved in KB5124008 and later.
Should I manually install KB5120998 today?
Normally no. Install the latest applicable cumulative update offered by Windows Update instead.
Why can’t I find KB5120998 in Optional Updates?
Newer cumulative updates supersede it, so Windows Update may no longer offer the old August preview.
Can I still download KB5120998 manually?
Yes. Microsoft Update Catalog still lists x64 and Arm64 packages, but manual installation is mainly for administration, offline servicing or troubleshooting.
