Microsoft Defender Error 0x80070643, how do I fix it?

Hi Gnarley-one,

If you're facing Error 0x80070643 in Microsoft Defender on Server 2016, there are several potential solutions you can try to get it working again. I found this information in a Microsoft Answers forum post and a blog article, they are using these steps:

Windows Defender liefert Update-Fehler 0x80070643 | Borns IT- und Windows-Blog (borncity.com)

Windows update failed to install error 0x80070643 - Microsoft Community

1. Check for Conflicting Antivirus Software:
It's possible that if you had a third-party antivirus product installed before, it might be causing the error. In that case, you should uninstall the third-party antivirus and run a clean-up tool provided by the antivirus vendor to completely remove any remnants.

2. Repair Defender by Deleting Definitions:
One approach you can try is deleting the Defender definitions. To do this, open an administrative Command Prompt by searching for "cmd" and selecting "Run as administrator." Then, execute the following commands:

cd %ProgramFiles%
cd "Windows Defender"
mpcmdrun.exe -removedefinitions –all

These commands will remove the definitions and trigger a notification from Windows Defender that the definitions are outdated.

3. Reset Update Store:
Another possible solution is to reset the Windows Update Store. Open an administrative Command Prompt and enter the following commands:

net stop wuauserv
net stop bits
rd /s /q %windir%\SoftwareDistribution
net start wuauserv
net start bits

After running these commands, you can consider restarting your server to allow Windows Update to search for Defender signature updates again.

4. Repair .NET Framework:
Someone on a Microsoft Answers forum thread mentioned that the error could be related to a corrupted .NET Framework. If the previous solutions don't work, consider repairing the .NET Framework using tools available in this Microsoft blog post:

.NET Framework Cleanup Tool User's Guide | Microsoft Learn

5. Delete Windows Defender Cache Data:
In the registry, Windows Defender stores cache data that might cause issues. If the previous approaches didn't help, you can try deleting the cache files by following these steps:

- Open the Registry Editor with administrative privileges (search for "regedit" and select "Run as administrator").
- Navigate to the registry key: HKEY_LOCAL_MACHINE/SOFTWARE/Microsoft/MpSigStub.
- Delete the entry using the context menu (right-click and choose "Delete").

Alternatively, you can use the following command in an administrative Command Prompt:

reg delete HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\MpSigStub

You can consider combining these solutions and trying them step by step to find the best approach to get Microsoft Defender working on your Server 2016.