If a notification says “Microsoft Defender Antivirus is turned off” but Windows Security shows protection is active, Microsoft has released a fix for that mismatch. Its 17 September 2026 Defender update, version 4.18.26080.4, addresses the false warning.
The practical response is to check your protection and install official updates. Do not assume every antivirus warning is this bug, and do not disable protection to silence it.
What changed, and who is affected?
Microsoft opened the issue on 28 August and marked it resolved on 17 September. Its release-health notice describes alerts at startup or later even while Defender continues working. It lists several Windows 11 and Windows 10 releases, as well as Windows Server. This article focuses on home PCs.
Check your PC in this order
1. Open Windows Security yourself. Search for it in the Start menu, then open its virus and threat protection area. Check the protection status rather than relying only on the notification. Microsoft explains the app’s status indicators in its Windows Security overview.
2. Check which antivirus is protecting you. If you deliberately installed another compatible antivirus and it is active, Defender may switch off automatically. That is different from the false-notification problem. Confirm that your intended antivirus is running; do not try to force two products into real-time protection simply because of this headline.
3. Install official updates. Open Settings and use Windows Update to check for available updates. Follow any restart prompt, then check the protection status again. Use Microsoft’s Windows update instructions, not a third-party “Defender repair” download.
4. Check protection settings and scan if needed. If Defender is your chosen antivirus, its real-time protection setting is under the virus and threat protection area’s Manage settings option. Microsoft’s threat-protection guide also explains scan options and security-intelligence updates. Updating threat definitions is not the same as confirming the platform fix is installed.
When not to dismiss the warning
If no antivirus is active, protection will not stay on, or Windows Security reports a threat, treat that as a separate problem requiring attention. A notification bug does not prove that an individual PC is safe. Use the Windows Get Help app or official support if you cannot establish your protection status; ask your IT team on a managed device.
There is no reason to buy another security product solely to address this notification bug. If you are comparing protection types for other reasons, our VPN versus antivirus guide explains why a privacy VPN does not replace device-level malware protection.
What remains uncertain
Microsoft’s notice does not give an affected-device count. It also cannot confirm whether your particular PC has received the fix. This is source-based reporting, not a hands-on test or a diagnosis of your device.
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Stuart Kerr Spindlow is Impulsec’s Publisher, Chief Editor and Lead Reviewer. He writes and maintains coverage of consumer cybersecurity and digital-safety software, with a focus on practical suitability, limitations and the evidence behind product claims. Read Stuart’s full profile.