Microsoft has fixed the issue where Windows Security Center incorrectly reported that Microsoft Defender Antivirus was turned off. The fix is included in the Defender platform update 4.18.26080.4 released on September 17, and users typically do not need to take any manual action.
This update will be pushed automatically by the system. However, if users have blocked mandatory security updates via Group Policy or the Registry Editor, they may not receive the fix properly.
False positives usually appear when Windows starts and may pop up randomly afterward. Even disabling notifications cannot completely avoid them. Clicking the warning only opens the Windows Security home page, where all protection features still show as normally enabled.
Microsoft confirmed this issue on August 28, affecting all supported versions of Windows and Windows Server running the latest Defender updates. Microsoft did not disclose the specific update that caused the glitch, and since Defender updates are released daily, there is no separate KB patch for users to roll back.
Some Windows Insider members reportedly encountered similar warnings as early as June. It took nearly three weeks from Microsoft's confirmation of the issue to the release of the fix. Currently, users just need to wait for the Defender platform update 4.18.26080.4 to install automatically.