Microsoft Defender Won't Open on Windows 11 – Error 0xC0000005 (MicrosoftSecurityApp.exe)

Hello everyone,

I'm experiencing a persistent issue with the Microsoft Defender app included with my Microsoft 365 subscription, running on Windows 11, version 26H2.

Initially, the application displayed an error when I tried to sign in with my Microsoft account. Shortly afterward, it stopped launching altogether. Now, whenever I try to open Microsoft Defender from the Start menu or Microsoft Store, absolutely nothing happens. No window appears, and no error message is displayed.

Errors recorded in Windows Event Viewer:

• Faulting application: MicrosoftSecurityApp.exe

Application version: 1.2.2608.18002

Exception code: 0xC0000005 (Access Violation)

Event ID 1000: Application Error

Event ID 1026: .NET Runtime – Unhandled Exception

Troubleshooting steps I have already tried:

Uninstalled and reinstalled Microsoft Defender from the Microsoft Store.

Reset the application using PowerShell (Reset-AppxPackage).

Re-registered the application package using Add-AppxPackage.

Ran DISM /Online /Cleanup-Image /RestoreHealth and sfc /scannow. Both completed successfully, and SFC reported no integrity violations.

Performed a clean boot of Windows, temporarily disabling third-party services and startup applications.

Created a new Windows user account and attempted to launch Microsoft Defender from that account. The application still would not open.

Initiated a Windows 11 repair installation through Windows Update. The computer subsequently restarted, but the issue persisted. I have not been able to confirm whether the repair installation completed successfully.

Verified through PowerShell that the Microsoft Defender application package is installed and reports a status of Ok.

Microsoft Defender Antivirus status:

I also ran Get-MpComputerStatus in PowerShell and confirmed the following:

AntivirusEnabled: True

RealTimeProtectionEnabled: True

Therefore, the built-in Microsoft Defender Antivirus engine and real-time protection are working correctly. The issue appears to affect only the separate Microsoft Defender app included with Microsoft 365.

My questions are:

Has anyone experienced this specific 0xC0000005 access violation involving MicrosoftSecurityApp.exe?

Is there a known compatibility issue, dependency problem, or application-specific fix that could explain this behavior?

Most importantly, is there a way to resolve the issue without performing a full Windows reinstall or losing my installed applications, personal files, and settings?

I would particularly appreciate suggestions beyond the troubleshooting steps listed above, as none of them has resolved the problem.

I can provide Event Viewer screenshots and additional diagnostic information if needed.

Thank you in advance for your help.Hello everyone,

I'm experiencing a persistent issue with the Microsoft Defender app included with my Microsoft 365 subscription, running on Windows 11, version 26H2.

Initially, the application displayed an error when I tried to sign in with my Microsoft account. Shortly afterward, it stopped launching altogether. Now, whenever I try to open Microsoft Defender from the Start menu or Microsoft Store, absolutely nothing happens. No window appears, and no error message is displayed.

Errors recorded in Windows Event Viewer:

Faulting application: MicrosoftSecurityApp.exe

Application version: 1.2.2608.18002

Exception code: 0xC0000005 (Access Violation)

Event ID 1000: Application Error

Event ID 1026: .NET Runtime – Unhandled Exception

Troubleshooting steps I have already tried:

Uninstalled and reinstalled Microsoft Defender from the Microsoft Store.

Reset the application using PowerShell (Reset-AppxPackage).

Re-registered the application package using Add-AppxPackage.

Ran DISM /Online /Cleanup-Image /RestoreHealth and sfc /scannow. Both completed successfully, and SFC reported no integrity violations.

Performed a clean boot of Windows, temporarily disabling third-party services and startup applications.

Created a new Windows user account and attempted to launch Microsoft Defender from that account. The application still would not open.

Initiated a Windows 11 repair installation through Windows Update. The computer subsequently restarted, but the issue persisted. I have not been able to confirm whether the repair installation completed successfully.

Verified through PowerShell that the Microsoft Defender application package is installed and reports a status of Ok.

Microsoft Defender Antivirus status:

I also ran Get-MpComputerStatus in PowerShell and confirmed the following:

AntivirusEnabled: True

RealTimeProtectionEnabled: True

Therefore, the built-in Microsoft Defender Antivirus engine and real-time protection are working correctly. The issue appears to affect only the separate Microsoft Defender app included with Microsoft 365.

My questions are:

Has anyone experienced this specific 0xC0000005 access violation involving MicrosoftSecurityApp.exe?

Is there a known compatibility issue, dependency problem, or application-specific fix that could explain this behavior?

Most importantly, is there a way to resolve the issue without performing a full Windows reinstall or losing my installed applications, personal files, and settings?

I would particularly appreciate suggestions beyond the troubleshooting steps listed above, as none of them has resolved the problem.

I can provide Event Viewer screenshots and additional diagnostic information if needed.

Thank you in advance for your help.