Microsoft fixes Windows Defender false antivirus off alerts

For nearly two months, Windows 11, Windows 10, and Windows Server showed false warnings that Microsoft Defender was off. The issue was fixed in Defender Antivirus update 4.18.26080.4 on September 17, 2026, with no user action required.

• False alerts appeared since early August 2026 on all Windows 11 and Windows 10 versions
• Cause was a bug in a daily Defender Antivirus update
• Fix in version 4.18.26080.4 from September 17, 2026
• Notifications ignored settings and popped up at system startup