Windows 10 gets its August 2026 security patch: what KB5120249 actually fixes

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, tracked as KB5120249, for the two versions of Windows 10 still receiving fixes: 22H2 and 21H2.

The update was first reported by BleepingComputer. It contains the August 2026 Patch Tuesday security fixes, which is Microsoft's monthly bundle of security patches released on the second Tuesday of each month.

After it installs, your PC will report one of two build numbers: 19045.7663 (on 22H2) or 19044.7663 (on 21H2). That number is how you confirm the patch actually landed.

How do I install it?

Open Start, then Settings, then Update & Security, then Windows Update, and click "Check for updates". The patch will download and install on its own. A restart is required.

If you prefer to grab the installer by hand, the package is also available from the Microsoft Update Catalog. Most home users should just let Windows Update do the work.

What does the update actually fix?

Two things stand out this month: a broken backup feature and a broader security rollout. Neither is dramatic, but the backup fix will matter to small businesses that rely on File History.

File History, the built-in Windows tool that copies your personal files to another drive on a schedule, was failing when it tried to save to a shared folder on the network. Windows was throwing a misleading "invalid credentials" error and quietly skipping the backup. If you use File History over the network (a common setup in small offices), your scheduled backups may not have been running. KB5120249 fixes that.

The second change is about Secure Boot. Secure Boot is the feature that checks your PC boots only trusted software before Windows even loads, blocking a class of low-level malware called bootkits. The digital certificates that make Secure Boot work are expiring, and Microsoft has been quietly pushing replacements out through Windows Update. This month's patch expands the list of devices that qualify for the automatic swap. The rollout will continue over the coming months for supported PCs and unmanaged business machines.

Should I be worried about install problems?

Not at this stage. Microsoft says it is not aware of any new issues caused by this month's update. That can change once millions of machines start applying it, so keep an eye on Windows Update if your PC starts behaving oddly after the reboot.

Fix summary

What should ordinary users do?

Install it. Windows 10 is close to the end of its supported life, and every skipped Patch Tuesday leaves your PC exposed to bugs that criminals already know how to exploit. If you have been ignoring the update notification for a few weeks, this is a good moment to click through, let it reboot, and move on with your day.

Check afterwards that File History (if you use it) is running again on its schedule. That's the one behaviour change most users will actually notice.