243/69 Tuesday, May 5, 2026
Microsoft has confirmed that its April 2026 Windows security updates may cause certain backup applications to malfunction. The issue primarily affects software that relies on the psmounterex.sys driver for mounting or managing backup image files. It may impact some devices running Windows 10, Windows 11, and Windows Server, resulting in errors or timeouts when attempting to open or restore data from backup images. While full image backups may still complete successfully, mounting those images may fail.
The issue stems from security enhancements to Microsoft’s Vulnerable Driver Blocklist, a mechanism designed to prevent Bring Your Own Vulnerable Driver (BYOVD) attacks. Microsoft has added the psmounterex.sys driver to the blocklist due to its association with CVE-2023-43896. As a result, backup software that depends on this vulnerable driver may no longer function correctly. Microsoft emphasized that this is an intentional security measure, not a flaw in the update.
Users are advised to update their backup software to versions that use compliant and secure drivers. If unsure whether a particular application is affected, users should contact the software vendor directly. Additionally, users can verify whether the driver has been blocked by checking Event ID 3077 in the Applications and Services Logs\Microsoft\Windows\CodeIntegrity\Operational log. Microsoft does not recommend uninstalling or disabling the security update.