[REVISION HISTORY]

Microsoft Defender false security alert bug

Updated 2 times since CLSTR started tracking revisions of this situation.

What changed

2026-09-19 14:05 UTC → 2026-09-20 11:02 UTC · added ~~removed~~

A software bug in Microsoft Defender caused Windows users to receive false security alerts claiming that antivirus protection had been disabled. These intrusive notifications appeared during system startup and at random intervals, often bypassing ‘Do Not Disturb’ settings. Microsoft confirmed that while the alerts were erroneous, the antivirus service remained fully functional and active. Microsoft addressed the issue by releasing version 4.18.26080.4 of the software, which contains a critical fix to resolve the incorrect reporting of antivirus deactivation. The bug affected multiple platforms, including Windows 11 and Windows Server 2025. The discrepancy between the actual service status and the interface messages caused significant confusion for IT departments, as the notifications prompted users to attempt manual restorations of a service that was already operational. The corrective update aims to restore consistency between the protection services and the notification center.

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