Microsoft Confirms Windows Update Issues Cause Backup Failures
Windows Updates Cause Backup Failures in Third-Party Applications
Microsoft has acknowledged that the recent April security updates have resulted in failures in third-party backup software utilizing the psmounterex.sys driver.
- This issue affects various applications, including those from Macrium (Reflect), Acronis (Cyber Protect Cloud), UrBackup Server, and NinjaOne Backup, operating on Windows 11, Windows Server, and Windows 10 devices.
- The problem arises from a Volume Shadow Copy Service (VSS) service timeout, leading to failures in creating and restoring backup images.
- Users may encounter errors such as “The backup has failed because Microsoft VSS has timed out during the snapshot creation” or VSS_E_BAD_STATE.
- Additionally, Event Viewer may display Code Integrity errors indicating that psmounterex.sys was blocked from loading.
Microsoft has addressed this issue by adding the vulnerable kernel driver psmounterex.sys to the Vulnerable Driver Blocklist in the April security updates.
- The company advises users to update their applications to use newer drivers, which include the required protections.
- Affected users can verify if the Microsoft Vulnerable Driver Blocklist blocks a driver by checking the ‘CodeIntegrity’ event logs.
According to Microsoft, “In the April 2026 Windows security update, we added known vulnerable kernel driver psmounterex.sys to the Vulnerable Driver Blocklist. Backup applications that rely on this driver may experience failures when attempting to mount or manage disk images.”
Microsoft also stated, “We do not recommend uninstalling or pausing this update. Customers with an impacted driver should install the latest application versions and validate against the driver blocklist to remain protected.”
Timeline:
- April 2026: Microsoft releases security updates containing the vulnerable kernel driver psmounterex.sys.
- May 4, 2026: Microsoft acknowledges the issue and provides guidance on resolving it.
- Ongoing: Microsoft continues to release updates and patches to address emerging issues and improve security.
Affected Products:
- Macrium Reflect
- Acronis Cyber Protect Cloud
- UrBackup Server
- NinjaOne Backup
- Windows 11
- Windows Server
- Windows 10
Indicators of Compromise:
- Failures in creating and restoring backup images
- Errors such as “The backup has failed because Microsoft VSS has timed out during the snapshot creation”
- VSS_E_BAD_STATE error
- Code Integrity errors indicating that psmounterex.sys was blocked from loading
Financial Losses:
The exact financial impact of this issue is unknown, but it has the potential to cause significant disruptions to businesses relying on affected backup software.
Law Enforcement Actions:
None mentioned in the provided article.