Microsoft September updates break File History backups

Microsoft has confirmed a regression in its September 2026 cumulative security updates that breaks File History, the built-in backup feature in Windows 10 and Windows 11. The bug causes local and network backups to fail without a clear error, leaving users with a false sense that their files are protected.

Silent failures and misleading errors

Microsoft's known-issues documentation, updated on September 19, says affected systems can log application crashes referencing FileHistory.exe and KERNELBASE.dll in Event Viewer, the kind of entry Windows typically files as a generic application error. Because the backup process crashes before it reaches the destination drive, Windows shows confusing symptoms rather than a straightforward failure. 

Affected users may see repeated "Reconnect your drive" prompts even when an external USB drive or network-attached storage location is connected and working normally. The "Last Backup" date in Control Panel may also stop updating without flagging a failed job, and restoring a file modified after the update may return a "No previous version available" message instead of the expected version history.

Which versions are affected

The bug affects Windows 11 versions 23H2 through 26H1 and Windows 10 22H2, 21H2, and Enterprise LTSC 2016 and 2019, tied to this month's respective security updates KB5124012, KB5124008, KB5122880, KB5122878, KB5122876, and KB5123099. Microsoft says Windows Server editions are not affected.

What to do in the meantime

September's update also closed two actively exploited zero-days and dozens of critical vulnerabilities, so Microsoft and security researchers are advising against uninstalling the affected updates to work around the bug. Microsoft says it is developing a fix for a future release but has not given a timeline or offered an official workaround. 

Until then, users and administrators who rely on File History should check backup logs and Control Panel timestamps directly rather than assuming a connected drive means backups are running and should consider a secondary backup method, such as OneDrive-based Windows Backup or third-party imaging software, in the meantime.