Microsoft confirmed that recent September 2026 cumulative security updates contain a regression that breaks File History backups on Windows systems. Users relying on this feature for local or network data protection will find their backups failing silently or triggering confusing error messages. This issue affects a wide range of supported Windows versions, leaving many users without their automated safety nets until a fix arrives.
Users must keep security patches installed despite broken backups
The regression impacts File History.exe and KERNELBASE.dll components, causing the backup process to fail with misleading prompts such as 'Reconnect your drive.' Affected systems include Windows 11 versions 23H2 through 26H1, as well as Windows 10 versions 22H2, 21H2, and Enterprise LTSC 2016 and 2019. Windows Server editions are not affected by this specific bug, isolating the issue to consumer and standard enterprise desktop environments.
The problematic updates are identified as KB5124012, KB5124008, KB5122880, KB5122878, KB5122876, and KB5123099. Microsoft explicitly advises users against uninstalling these security patches because they address critical zero-day vulnerabilities. Removing the updates would expose systems to serious security risks, forcing users to accept the backup failure as a temporary trade-off for system protection.
Microsoft has not provided a timeline for a fix or offered an official workaround for the broken backups. Users must wait for a subsequent patch to restore File History functionality while keeping their systems secure. We looked at Windows update regressions earlier while tracking Microsoft patch cycles, noting that such trade-offs between security and stability are not uncommon.
Source: NotebookCheck
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