Microsoft Identifies Windows 11 Boot Failures from December 2025 Update

Recent reports indicate that Windows 11 users are facing boot failures following the installation of the January 2026 updates. These issues appear to stem from earlier difficulties in installing the December 2025 security update, which left some systems in an unstable condition.

Details of the Issue

The boot failure issue surfaced after users upgraded to the January 2026 Patch Tuesday cumulative update, designated as KB5074109. Affected versions of Windows 11 include 25H2 and 24H2. Users reported that their devices failed to start, displaying a Blue Screen of Death (BSOD) crash with the error code “UNMOUNTABLE_BOOT_VOLUME.”

Investigation Findings

According to information released by Microsoft, the devices encountering boot failures had previously attempted and failed to install the December 2025 security update. Upon rolling back this failed update, the affected systems were left in what Microsoft describes as an “improper state.”

Microsoft’s Advisory

Microsoft’s updated advisory outlines the situation clearly. The company states that those devices mistakenly left in this improper state could face severe boot issues if they try to install new updates. The advisory notes:

• The boot failure can occur on devices that failed to install the December 2025 security update.
• Rolling back this update may lead to an unstable system state.
• A partial resolution is underway to prevent additional no-boot scenarios during future updates.

Limitations of the Resolution

While Microsoft’s forthcoming partial resolution aims to reduce the failures, it won’t rectify systems already caught in this improper state. Additionally, it may not prevent future instances of devices entering this problematic condition.

Current Scope of the Problem

At this stage, Microsoft confirms that the issue mainly affects physical devices. There have been no reports indicating that virtual machines are experiencing similar boot failures. The company continues its investigation into why these complications arise.

For users impacted by these issues, monitoring updates from Microsoft is essential. BleepingComputer has reached out to the company for further clarification and will provide updates as they become available.