Process.Start() and PowerShell Start-Process hang on newer mcr.microsoft.com/windows/servercore:ltsc2019 / .NET Framework 4.8 LTSC2019 images

Description of the bug

We are seeing a regression in newer Windows Server Core LTSC 2019 container images.

Starting a child process from inside the container hangs indefinitely. This affects both:

• .NET System.Diagnostics.Process.Start()
• PowerShell Start-Process

This used to work a few weeks ago and still works on an older pinned .NET Framework SDK image.

Known-good image

The following images still works correctly:

• mcr.microsoft.com/dotnet/framework/sdk:4.8-20260512-windowsservercore-ltsc2019
• mcr.microsoft.com/windows/servercore:10.0.17763.8880

On this image, both:

• .NET Process.Start()
• PowerShell Start-Process

return normally.

Regressed images

The problem occurs on newer images based on updated LTSC2019 Server Core, including:

• mcr.microsoft.com/windows/servercore:ltsc2019
• current floating mcr.microsoft.com/dotnet/framework/sdk:4.8
• current floating mcr.microsoft.com/dotnet/framework/sdk:4.8-windowsservercore-ltsc2019

Expected behavior

Starting cmd.exe /c echo hello should return immediately.

Expected output for the .NET repro:

Before
After

Expected output for the PowerShell repro:

Before
After

Actual behavior

The process-start call hangs indefinitely.

For the .NET repro, output is:

Before

and Process.Start() never returns.

For the PowerShell repro, output is:

Before

and Start-Process never returns.

Container OS versions observed

Known-good older image lineage:

• 10.0.17763.8880

Failing newer image lineage:

• 10.0.17763.9020

Minimal PowerShell repro

Run:

docker run -ti --rm mcr.microsoft.com/windows/servercore:ltsc2019 powershell

Inside the container:

Write-Host "Before"
Start-Process cmd.exe -ArgumentList '/c echo hello' -Wait
Write-Host "After"

Actual result on the failing image:

Before

and then it hangs.

Minimal .NET repro

Run:

docker run -ti --rm mcr.microsoft.com/windows/servercore:ltsc2019 powershell

Inside the container:

Add-Type @"
using System;
using System.Diagnostics;

public class Test {
    public static void Run() {
        var p = new Process();
        p.StartInfo.FileName = "cmd.exe";
        p.StartInfo.Arguments = "/c echo hello";
        Console.WriteLine("Before");
        p.Start();
        Console.WriteLine("After");
    }
}
"@

[Test]::Run()

Actual result on the failing image:

Before

and then it hangs.

Notes

• This does not appear to be specific to .NET Framework, because the same behavior is reproducible on plain mcr.microsoft.com/windows/servercore:ltsc2019.
• The regression appears to have been introduced after the 2026-05-12 .NET Framework SDK LTSC2019 image.
• We are currently pinning to mcr.microsoft.com/dotnet/framework/sdk:4.8-20260512-windowsservercore-ltsc2019 as a temporary workaround.

Environment

• Host OS: Windows Server 2019
• Container base: mcr.microsoft.com/windows/servercore:ltsc2019
• Affected higher-level image: mcr.microsoft.com/dotnet/framework/sdk:4.8
• Working higher-level image: mcr.microsoft.com/windows/servercore:10.0.17763.8880

Question

Is this a known regression in the updated LTSC2019 Server Core image, or an intentional change in process creation behavior inside Windows containers?

Run the minimal PowerShell and .NET repros with the known-good and failing Server Core images, comparing the 10.0.17763.8880 and 10.0.17763.9020 lineages. Check whether process creation behavior changed in the updated LTSC2019 image; done means identifying the regression or confirming the documented pinned-image workaround.