Security

Windows 11 Boot Failures Linked to Failed December 2025 Update: What You Need to Know

Published

2 months ago

January 30, 2026

Windows 11 Boot Failures Linked to December 2025 Security Update Issues

Recent reports of Windows 11 boot failures following the installation of the January 2026 updates have been linked to previous issues with the December 2025 security update. Microsoft has identified a connection between the two incidents, with systems being left in an “improper state” after failed attempts to install the December 2025 update.

The boot failures were initially reported after users updated to the January 2026 Patch Tuesday cumulative update, KB5074109, on Windows 11 versions 25H2 and 24H2. Impacted systems failed to start and displayed a BSOD crash screen with a stop error code of “UNMOUNTABLE_BOOT_VOLUME.”

According to a recent update, Microsoft’s investigation revealed that devices experiencing boot errors had previously struggled to install the December 2025 security update. Rolling back the failed update resulted in Windows being left in an unstable or “improper state.”

The updated advisory states, “Recent investigations have determined this issue can occur on devices that failed to install the December 2025 security update and were left in an improper state after rolling back the update.”

Microsoft is working on a partial resolution to prevent devices from entering a no-boot scenario when attempting to install updates while in this improper state. However, this solution will not prevent devices from entering the improper state initially nor repair devices already unable to boot.

The company continues to investigate why some devices fail to install updates or end up in an improper state after a rollback. The issue currently affects physical devices, with no reported impacts on virtual machines.

BleepingComputer reached out to Microsoft for further clarification on the issue and will provide updates as necessary.

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