Microsoft has fixed a bug that caused Windows Security to incorrectly report that Microsoft Defender Antivirus was turned off. The fix was delivered through Defender platform update 4.18.26080.4, released on September 17. No manual action is required for the update to install, unless users have blocked mandatory security updates via Group Policy or the Registry Editor.

The false alert typically appeared during Windows startup and reappeared randomly, even with notifications disabled. Clicking the alert only opened the Windows Security home tab, where all protections showed as active. Microsoft acknowledged the issue on August 28 and confirmed it affected all supported versions of Windows and Windows Server running the latest Defender updates.

Since Defender updates are distributed daily, Microsoft did not identify a single KB article to roll back. Windows Insider users reportedly encountered similar warnings as early as June. TechPowerUp covered the alert when it first surfaced, and multiple staff members experienced the issue firsthand.

Notably, nearly three weeks elapsed between Microsoft's acknowledgment and the release of the fix, which appears unusually slow for such a visible bug.