If a user is notified upon starting the computer that Microsoft Defender is deactivated, they should not panic, as the security tool remains active and protects the system. This alert is actually caused by a programming error (bug) that affects both Windows 11 and Windows 10.
Microsoft Defender constitutes the security mechanism inherent to the operating system. Although it may not possess the level of sophistication offered by specialized cybersecurity companies, its activation is crucial to shield the computer against threats such as viruses and other malware, especially in the absence of more robust protections.
In this specific scenario, the user receives a warning indicating that the Microsoft Defender antivirus is inactive (and not Defender as a whole). Upon accessing the tool's settings, it is verified that this is an incorrect alert, since protection continues to operate normally. Another proof that this is a bug is the fact that this warning appears even on machines where Windows Defender notifications have been disabled.
According to Microsoft itself, this false alarm from Windows Defender occurs in Windows 10 users on versions 22H2, 21H2, Enterprise LTSC 2019, and Enterprise LTSC 2016. Server versions are also affected, including Windows Server 2022, Windows Server 2019, Windows Server 2016, Windows Server 2012 R2, and Windows Server 2012.
For Windows 11, the affected editions are versions 26H1, 25H2, 24H2, and 23H2. In all cases, the problem appears to originate from recent updates to Windows Defender itself, which explains the large number of Windows versions being affected.
There is a peculiarity in the case of Windows 11: the problem arises concurrently with reports of failures after installing the KB5120998 package. This optional update for Windows 11 introduces a new Start Menu and signals the return of the mobile Taskbar. However, some users who applied the KB5120998 package observed issues such as the loss of customized mouse settings. An inconvenience noted was the replacement of the wallpaper with a black background, although this latter issue is easily resolved by simply reactivating the wallpaper.
Regarding the Microsoft Defender issue, the company has stated that it is already developing a fix and will make it available in a future update.