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Microsoft Resolves False Alerts in Defender Antivirus

Microsoft Resolves False Alerts in Defender Antivirus

First seen 21 Sep 2026, 17:53 UTC • •
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ThreatCluster AI
ThreatCluster •September 21, 2026 at 18:54 UTC
  • •Microsoft fixed a bug causing false alerts in Defender Antivirus.
  • •The issue affected all supported Windows client and server versions.
  • •An update was released on September 17 to resolve the problem.

Microsoft has fixed a bug that caused incorrect alerts indicating that Defender Antivirus was turned off after recent updates. This issue affected users across all supported Windows client and server versions, including Windows 11 26H1 and Windows Server 2025. The bug was first acknowledged in late August and had been impacting users since at least June, particularly those in the Windows Insider program. The erroneous notifications prompted users to activate Defender Antivirus despite it functioning correctly. Microsoft released an update (version 4.18.26080.4) on September 17 to address this issue. This is not the first instance of Microsoft dealing with incorrect alerts following updates, as similar issues have occurred with BitLocker, Windows Firewall, and certificate enrollment errors. Additionally, emergency updates were released to fix other issues related to Remote Desktop Services and USB audio. Users are advised to apply the latest updates to avoid confusion from these alerts.

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Timeline

2026-06-01
Bug identified in Windows Insider program
Users in the Release Preview Channel began experiencing false alerts about Defender Antivirus being turned off.
BleepingComputer
2026-08-25
Microsoft acknowledges the bug
Microsoft confirmed the existence of the bug causing incorrect alerts in Defender Antivirus.
BleepingComputer
2026-09-17
Update released to fix the issue
Microsoft released version 4.18.26080.4 of Defender Antivirus to resolve the false alerts.
BleepingComputer
2026-09-18
BleepingComputer reports on the fix
BleepingComputer published an article detailing the resolution of the bug affecting Defender Antivirus alerts.
BleepingComputer
2026-09-21
Scworld publishes brief on the fix
Scworld reported on Microsoft's resolution of the false alerts issue, confirming the update and its impact.
Scworld

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