Windows Sandbox – unable to run .NET apps

I have just updated my daily used computer to Windows 10 1903 May 2019 Update. The update took a while but after all finished with some nice new cool features on board.

I gave it a try and started the new Windows Sandbox mode, which is an out-of-the-box sandbox built in Windows 10 Pro and Enterprise. Launching of sandbox takes just a few seconds, and then the fully functional dummy environment is available, really well done!

One thing that I found interesting was that it was not possible to start a PowerShell window, or any .NET app. Each try finished with an error, for example:

This application requires one of the following versions of the .NET Framework:.NETFramework,Version=v4.5.2 

The error dialog offers a choice to download the runtime, but the framework installer (correctly) prevents it from being installed, because that or a newer version is already there.

It turns out, that not everyone is affected by this problem. The problem is caused by update KB4495620 which is installed on the host machine. This is a cumulative update for .NET Framework fixing a few vulnerabilities. In order to be able to run .NET apps on Windows 10 Sandbox, uninstall this update and reboot your machine (again to repeat myself – this has to be done on your host and not the sandbox!). After that, powershell.exe and .NET apps will run without issues:

Hello World, RayPack 6.1 (.NET 4.5 app) is running on Windows 10 Sandbox mode.

I hope the problem is going to be fixed by Microsoft soon, until then use this workaround to test Windows 10 Sandbox out.


Hi Marcin,
How did you uninstall KB4495620? Per philberndt’s instructions here (, I ran the command “wusa /uninstall /kb:4495620”, but Windows tells me the update cannot be uninstalled. Do you know another way to do this that actually works?


Marcin Otorowski

Well it was pretty easy on my machine, just using appwiz.cpl to uninstall the update via GUI, no restrictions applied. There were some suggestions in the internet about hacking the .mum files, but I find it kind of weird that the same update on a similar system would be marked differently and suspect the problem may lie elsewhere. In any case, based on your answer in the linked topic you would rather not take the risk now.

I have just checked my two other computers which have just been updated to 1903, and the problematic KB was not even there so I could not verify it. I guess the best for now is to wait for some official fix for Microsoft, but if I encounter this problem on some other machines I will for sure try to look for some alternative solution,

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