Microsoft has confirmed that its April 2026 security updates are causing problems for some third-party backup applications on Windows devices.
The issue affects backup software that uses the psmounterex.sys driver and VSS, or Volume Shadow Copy Service, snapshots. Because of the problem, some backup apps may fail with VSS timeout errors when users try to mount, browse, or restore backup image files.
Affected products include apps from Macrium Reflect, Acronis Cyber Protect Cloud, UrBackup Server, and NinjaOne Backup. The issue can appear on Windows 11, Windows 10, and Windows Server systems.
Microsoft said the April 2026 update added psmounterex.sys to its vulnerable driver blocklist. The company made the change to protect users from attacks linked to a high-severity vulnerability tracked as CVE-2023-43896, which could allow attackers to gain higher privileges or run malicious code.
On affected devices, Windows Code Integrity blocks the vulnerable driver from loading. As a result, backup apps that depend on the driver may fail to mount backup images as virtual drives. Users may also see errors such as VSS timeout messages or VSS_E_BAD_STATE.
Microsoft said full image backups may still work in some cases, but mounting or managing backup images can fail. Event Viewer may also show Code Integrity errors, confirming that psmounterex.sys was blocked.
The company advised users not to uninstall or pause the April security update. Instead, Microsoft recommends updating the affected backup application to the latest version that uses newer, protected drivers.
Users and IT admins can check whether the driver was blocked by opening Event Viewer and going to Applications and Services Logs, then Microsoft, Windows, CodeIntegrity, and Operational. Microsoft said Event ID 3077 with the related policy ID shows that the driver was blocked in enforcement mode.
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The warning comes after Microsoft also confirmed other April 2026 update problems, including BitLocker recovery prompts on some Windows Server 2025 devices and update installation failures that required out-of-band fixes.
