Windows 10 gets its August 2026 security patch: what KB5120249 actually fixes
Microsoft's monthly update fixes silent backup failures and widens the Secure Boot certificate rollout. Install it now.
Key points
- Microsoft released Windows 10 cumulative update KB5120249 for versions 22H2 and 21H2 on August 2026 Patch Tuesday.
- The update is mandatory and carries this month's security fixes, bringing systems to OS Builds 19045.7663 and 19044.7663.
- It repairs a File History bug that broke scheduled backups to network shares with a false "invalid credentials" error.
- Microsoft is widening the rollout of new Secure Boot certificates to more eligible devices.
- No new install-breaking issues have been reported by Microsoft at time of writing.
Microsoft has pushed out this month's mandatory Windows 10 security update, KB5120249, for the two versions still receiving fixes: 22H2 and 21H2. The broader August 2026 Patch Tuesday context is worth reading: our report from 11 August covers the 400 fixes and three zero-days in that bundle, including a North Korean-exploited kernel hole.
After KB5120249 installs, your PC will report OS Build 19045.7663 on 22H2, or 19044.7663 on 21H2. That number confirms the patch landed.
How do I install it?
Open Start, then Settings, then Update and Security, then Windows Update, and click "Check for updates". The patch downloads and installs automatically. A restart is required.
The package is also available from the Microsoft Update Catalog if you prefer to grab it manually. Most home users should let Windows Update do the work.
What does the update actually fix?
Two things stand out: a broken backup feature and a broader security rollout.
File History, the built-in Windows tool that copies personal files to another drive on a schedule, was failing when pointed at a shared network folder. Windows threw a misleading "invalid credentials" error and quietly skipped the backup entirely. If you use File History over a network share, a common setup in small offices, your scheduled backups may not have been running at all. KB5120249 fixes that.
The second change concerns Secure Boot, the feature that checks only trusted software loads before Windows starts, blocking low-level malware called bootkits. The digital certificates underpinning Secure Boot are expiring, and Microsoft has been pushing replacements through Windows Update. This month's patch extends that automatic swap to a wider set of devices. The rollout continues over coming months for supported PCs and unmanaged business machines. The certificate question has a longer history: our July story on forgotten boot programs showed what happens when old trusted components linger past their useful life.
Should I be worried about install problems?
Not at this stage. Microsoft says it knows of no new issues from this month's update. That can change once millions of machines apply it, so watch Windows Update if your PC behaves oddly after the reboot.
Fix summary
| Area | What changed | Who it affects |
|---|---|---|
| File History | Backups to network shares no longer fail with a false credentials error | Home and small-business users backing up to a NAS or shared folder |
| Secure Boot | Wider automatic rollout of replacement certificates | Consumer PCs and unmanaged business devices |
| Security | August 2026 Patch Tuesday fixes bundled in | All Windows 10 22H2 and 21H2 users |
| Build number | Machines move to 19045.7663 / 19044.7663 | All patched devices |
What should ordinary users do?
Install it. Windows 10 is close to end of life, and every skipped Patch Tuesday leaves your PC exposed to bugs criminals already know how to exploit. Click through, let it reboot.
Check afterwards that File History is running again on its schedule. That's the one behaviour change most users will actually notice. And if you're still on Windows 10, the deadline pressure is real: Microsoft has set 13 October 2026 as the cut-off for patches on Windows 11 24H2 Home and Pro, a signal of where the support window is heading across the board.
