Microsoft has resolved a known issue that causes incorrect alerts warning that Defender Antivirus was turned off after installing recent updates.
In a Windows release health dashboard update on Thursday, Microsoft said the issue was fixed in the Microsoft Defender Antivirus update (version 4.18.26080.4) released on September 17.
The company acknowledged the bug in late August, even though the issue had affected users in the Release Preview Channel of the Windows Insider program since at least June.
As Microsoft explained, this affects all supported Windows client and server versions, including the latest Windows 11 26H1 and Windows Server 2025 releases, and it triggers erroneous alerts in the Windows Security app that prompt users to "Tap or click to turn on Microsoft Defender Antivirus."
"After installing the latest updates for Microsoft Defender Antivirus, notifications might appear stating that "Microsoft Defender Antivirus is turned off," even though the antivirus is functioning correctly and all settings show it as active," it said at the time.
"These notifications can appear when Windows starts and intermittently afterward. They persist even if notification settings are turned off."