Windows 11 April 2026 Update Breaks Third‑Party Backup Tools

Overview Microsoft’s April 2026 cumulative security update (KB5083769) for Windows 11 has introduced a serious regression in the Volume Shadow Copy Service (VSS), disrupting third‑party backup applications. The issue impacts Windows 11 versions 24H2 and 25H2, leaving many users without reliable scheduled backups — a significant risk for both individuals and enterprises.

What Went Wrong

Component Affected: VSS, the Windows subsystem that enables snapshot‑based backups.
Symptoms: Backup jobs stall or fail with timeout errors.
Scope: Impacts third‑party backup vendors; Microsoft’s native cloud‑based Backup feature remains unaffected.
MS‑DEFCON Advisory: Security patch analyst Susan Bradley at AskWoody raised the alert to DEFCON Level 3, advising administrators to pause deployment in production environments.

Vendors Reporting Failures

UrBackup: File backups failing; recommends uninstalling KB5083769.
Macrium Reflect: VSS failures widely reported in community forums; investigation ongoing.
Acronis Cyber Protect Cloud: Jobs failing with “Microsoft VSS has timed out during snapshot creation”; recommends uninstalling and rebooting.

Mitigation Steps

For users experiencing failures:

Go to Settings → Windows Update → Update History.
Select Uninstall Updates under Related Settings.
Locate KB5083769 and uninstall.
Reboot the system.
Pause updates to prevent automatic reinstallation.

Note: Verify with your backup vendor before uninstalling, as not all solutions are affected.

Broader Context

Point‑in‑Time Restore (PITR): VSS is foundational to Microsoft’s upcoming PITR feature, previewed at Ignite 2025.
Server Issues: April 2026 updates also caused restart loops and installation failures on Windows Server, prompting out‑of‑band fixes.

Final Thought

This incident highlights the fragility of patch cycles when core subsystems like VSS are impacted. Backup reliability is non‑negotiable — organizations should delay KB5083769 deployment until Microsoft issues a fix, expected in an upcoming out‑of‑band or May 2026 cumulative update. In the meantime, rollback and monitoring are essential to avoid gaps in data protection.