Microsoft has officially confirmed that the security update KB5074109 for Windows 11 issued in January 2026 does not randomly crash healthy devices, but rather causes boot problems on systems that were already running in an unstable state as a result of the failure of the December 2025 update.
Devices that tried to install the December update, then rolled back and continued to run on an affected system, are more likely to receive an UNMOUNTABLE_BOOT_VOLUME (0xED) error with a black screen after installing the January update.
In the same context, the company explained that the scope of the impact remains limited, as it affects a small number of Windows 11 devices in the 24H2 and 25H2 versions, and focuses more on commercial computers, while it does not appear to affect virtual machines.
Microsoft confirmed that the January update actually builds on top of a damaged system state that originated in December, which explains why some devices enter into bootup failure episodes, while the update passes smoothly on other devices.
In terms of treatment, Microsoft announced that it is working on a partial solution that prevents additional devices from entering in the event of non-booting if they are already operating in an incorrect mode. However, this solution will not be able to fix already broken devices, nor will it address the root of the issue associated with the December update.
Therefore, affected devices still have to resort to manual recovery via WinRE or use external media to remove updates or repair the system.
For now, the recommendations remain the same: system administrators are advised to review the update log for failed installation attempts in December 2025, avoid pushing KB5074109 to vulnerable devices, and rely on WinRE tools to cancel the update or reinstall the entire system for devices stuck on the UNMOUNTABLE_BOOT_VOLUME error screen.
Although this clarification does not put an end to the problems with Windows 11 updates, it provides for the first time a clearer picture showing that the January boot failures were the result of a series of faulty updates, and not due to one isolated patch.
