Microsoft: Windows 11 boot failures tied to failed December 2025 security update

Microsoft says a Windows 11 boot failure bug is linked to devices that failed to install the December 2025 security update and were left in an improper state, a condition that can trigger problems when installing the January 2026 update (KB5074109).

PC Gamer previously reported users seeing an UNMOUNTABLE_BOOT_VOLUME stop code after installing the January update; Microsoft described the issue as showing up in a "limited number of reports."

Microsoft has issued a partial resolution, but warned: "This partial resolution will not prevent devices from getting into the improper state in the first place, nor will it repair devices that are already unable to boot."

PC Gamer noted the problem can arise when a failed December update leaves a device vulnerable to the subsequent January update, and added that machines kept regularly updated are more likely to already have the newer update without encountering the issue.

What happens next remains unclear: Microsoft’s partial fix does not repair already unbootable systems, and PC Gamer said that, absent a fuller solution, some users may ultimately need to reinstall Windows to recover.

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