Microsoft Fixes Windows 11 KB5089549 Update Install Failure

Microsoft has closed the book on KB5089549, the May 2026 Windows 11 security update that refused to install on some PCs and rolled back with error code 0x800f0922. The repair shipped on May 26 inside the KB5089573 preview update, and a server-side mitigation has already reached most home and unmanaged business machines.

That fix is the good news. The bad news is older. The cramped system partition that triggered the failure is the same kind of bottleneck that stranded Windows 10 users two years ago, and Microsoft is still patching the symptom rather than the layout that causes it.

Microsoft Closes the KB5089549 Install Failure

The company logged the resolution on its Windows 11 release health status page, marking the known issue as resolved on May 26 after opening it on May 15. The patch arrived in the KB5089573 preview cumulative update for Windows 11 versions 25H2 and 24H2, which moves those builds to 26200.8524 and 26100.8524 respectively.

Most people will not have to lift a finger. Microsoft pushed the repair through Known Issue Rollback (KIR), a server-side feature that reverses a faulty change without a fresh download, and it says the resolution has already propagated to consumer and non-managed business devices. A reboot helps it land faster. The same fix folds into the regular monthly security update later in June, so anyone who installs that rollup is covered automatically.

Here is how the episode played out over two weeks:

1. May 12, 2026: KB5089549 ships as the May Patch Tuesday security update, on OS Build 26100.8457.
2. May 15: Microsoft opens a known issue after reports of installs failing and rolling back.
3. May 26, 2026: KB5089573 ships with the fix, and the Known Issue Rollback begins reaching unmanaged devices.
4. June Patch Tuesday: the correction merges into the monthly security update for the broad install base.

Why the Update Stalled at 35% on Reboot

The failure did not show up right away. Affected machines moved through the early stages of the install, then choked during the restart at roughly 35 to 36 percent before backing out and posting the message, “Something didn’t go as planned. Undoing changes.”

The root cause sits in a small, rarely seen slice of the disk. Every modern Windows PC keeps an EFI System Partition (ESP, the tiny FAT-formatted area that stores boot loader files), and the update needed room there to stage new boot binaries. When that space ran out, the servicing engine gave up and reverted.

This issue affects devices with limited free space on the EFI System Partition (ESP), especially when the device has 10 MB or less space available.

That description comes straight from Microsoft’s known-issue writeup. The threshold is brutal in its specificity: PCs with 10 MB or less free on the ESP were the ones that broke. For users digging into the cause, the evidence lands in CBS.log (the Component Based Servicing log), where three entries point at the squeeze:

• “SpaceCheck: Insufficient free space”
• “ServicingBootFiles failed. Error = 0x70”
• “SpaceCheck: used by third-party/OEM files outside of Microsoft boot directories”

That last line is the tell. On many machines the ESP is partly filled by boot files from other vendors, and the way Windows lays out UEFI and GPT partitions leaves little headroom once an original equipment manufacturer (OEM) has parked its own loaders there.

The KB5034441 Episode Microsoft Already Lived Through

If the shape of this problem feels familiar, that is because Windows hit a near-identical wall in January 2024. The KB5034441 update patched a flaw that let attackers bypass BitLocker drive encryption through the Windows Recovery Environment, but it kept failing with error code 0x80070643 on machines whose recovery partition, often around 500 MB on older Windows 10 layouts, was too small to hold the new recovery image.

The difference is in the response. Two years ago, Microsoft did not ship an automatic fix. It published step-by-step recovery-partition resize instructions that asked everyday users to shrink the OS volume and rebuild a partition using diskpart and reagentc, command-line tools that intimidate most people who just want a security update to install.

So the KB5089549 handling is a genuine upgrade. A server-side rollback that reaches consumer PCs without a manual partition rebuild is exactly what the 2024 episode lacked. The trouble is that a smoother cleanup does not change why the mess keeps happening.

Why Windows Keeps Tripping Over Cramped Partitions

The common thread across both failures is space the user never sees and cannot easily manage. The ESP defaults to 100 MB on a large share of OEM systems, even though Microsoft’s own guidance now recommends 260 MB or more for newer hardware. Once vendor boot files and accumulated padding eat into that allowance, a single update that needs a few extra megabytes can tip the whole install into a rollback.

The same constraint has dogged the move to Windows 11 version 25H2. Devices with a 100 MB ESP have reported feature-upgrade failures carrying the very same 0x800f0922 stop, because the partition cannot absorb the larger boot payload. It is a layout decision baked in years ago, surfacing again every time the boot files grow.

What is missing is a pre-flight check. Windows Update still tends to discover the space problem mid-install, during the boot-file servicing phase, rather than flagging a tight partition before it starts and resizing or warning the user in advance. The reactive design is why a 10 MB shortfall becomes a failed reboot and a confusing rollback message instead of a quiet, handled adjustment.

Fixing a PC That Still Won’t Take the Update

For anyone still stuck on a build from before May 26, the cleanest move is to install KB5089573 or wait for the June security rollup, both of which carry the correction. If the update has to go in immediately on an older build, Microsoft offers two stopgaps.

The first is a registry toggle: adding an EspPaddingPercent DWORD set to 0 under the Bfsvc key, then rebooting and retrying. The second, aimed at IT departments, is the official Known Issue Rollback Group Policy, with Microsoft’s guidance on deploying a Known Issue Rollback through Group Policy and a fuller explanation of how Known Issue Rollback reverses a buggy update. The patch month was a busy one besides: Microsoft also addressed April security updates that broke third-party backup apps using a vulnerable driver, and a Windows Autopatch bug that pushed restricted driver updates to some managed devices across the European Union. Until the June rollup lands everywhere, the reboot and the registry toggle are the only levers users on older builds have.

Frequently Asked Questions

What Causes the KB5089549 Install Error 0x800f0922?

Insufficient free space on the EFI System Partition, the small boot area on the disk. Devices with 10 MB or less free could not stage the update’s new boot files, so the install failed during the reboot phase at roughly 35 to 36 percent and rolled back.

Do I Need to Do Anything to Get the Fix?

Probably not. Most home and unmanaged business PCs already received the fix through a server-side Known Issue Rollback, and restarting the device helps it apply. Installing KB5089573 or the June 2026 security update removes the issue entirely.

How Can I Check My EFI System Partition Free Space?

It is not visible in File Explorer because the ESP carries no drive letter. You can inspect it from an elevated command prompt using the diskpart utility, selecting the EFI volume to view its size and usage before attempting the update again.

What Is the Registry Workaround for KB5089549?

Open Command Prompt as administrator and add a DWORD named EspPaddingPercent set to 0 under HKLM\SYSTEM\CurrentControlSet\Control\Bfsvc, then restart and retry the install. Back up the registry first, since incorrect edits can cause system problems.

Does KB5089573 Do Anything Besides Fix the Error?

Yes. The preview update ships with about 30 changes, including faster app launches and shell experiences such as Start menu and Search, plus NPU usage metrics in Task Manager for devices with a neural processing unit.

Is This the Same Problem as the Windows 10 KB5034441 Failure?

It is closely related but not identical. KB5034441 failed with error 0x80070643 on an undersized Windows Recovery Environment partition, while KB5089549 failed with 0x800f0922 on a full EFI System Partition. The 2024 issue was never auto-fixed; this one was.