Microsoft Defender Bug Triggers False Antivirus Alerts on Windows

A recently discovered bug in Microsoft Defender Antivirus has been causing false notifications on Windows systems, prompting concern among administrators that the protection is turned off.

The issue was confirmed by Microsoft and affects both client and server operating systems, including Windows 11 versions 26H1, 25H2, 24H2, and 23H2, as well as Windows 10 versions 22H2 and 21H2.

These notifications may appear at startup and can recur intermittently after signing in, even when notification settings are disabled. Microsoft has stated that these alerts do not indicate a service failure or deactivated configuration, but rather a defect in the Defender engine.

The company is preparing a fix to be delivered through a future Microsoft Defender Antivirus update, but no resolution date has been provided yet. In the meantime, organizations should document this advisory in their service-desk guidance to distinguish it from legitimate protection warnings.