Microsoft’s April 2026 cumulative update KB5083769 has introduced a defect in the Windows Volume Shadow Copy Service (VSS) that breaks snapshot creation on Windows 11 24H2 and 25H2, silently disabling backup pipelines across multiple enterprise backup platforms including Acronis, Macrium Reflect, NinjaOne, and UrBackup. The failure is not a cyberattack, but its operational consequence is identical to one: affected organizations lose the ability to create reliable recovery points, eliminating a primary ransomware mitigation control until the update is uninstalled. Microsoft has not released a fix; the only confirmed remediation is rolling back the patch, forcing security teams to choose between patch currency and backup continuity.