How to Configure Windows Sandbox Local Traefik TLS Dynamic Middlewares PassTLSClientCert via XML in Windows 11

In mutual TLS (mTLS) architectures, verifying that downstream microservices receive validated client certificate headers—such as subject information, serial numbers, and SAN extensions—is critical for zero-trust security pipelines. In Windows 11, Windows Sandbox provides a lightweight, disposable virtual machine that boots in seconds without affecting host system stability. By pairing custom Windows Sandbox XML definitions (.wsb) with Traefik’s dynamic PassTLSClientCert middleware, engineers can evaluate mTLS certificate forwarding and header extraction in an isolated environment.

Architecture of Sandboxed mTLS Header Forwarding Testing

The configuration mounts a directory from the Windows 11 host into Windows Sandbox. Traefik runs inside the sandbox with its dynamic file provider watching a dedicated middleware YAML configuration file. Adjusting certificate parsing options or client CA verification on the host system triggers immediate hot reloads inside the sandbox without restarting the container.

Authoring the Windows Sandbox WSB Blueprint

Create a sandbox definition file named TraefikPassTLSClientCertSandbox.wsb on your Windows 11 workstation:

<Configuration>
  <VGpu>Default</VGpu>
  <Networking>Default</Networking>
  <MappedFolders>
    <MappedFolder>
      <HostFolder>C:SandboxDevTraefikPassCert</HostFolder>
      <SandboxFolder>C:TraefikPassCert</SandboxFolder>
      <ReadOnly>false</ReadOnly>
    </MappedFolder>
  </MappedFolders>
  <LogonCommand>
    <Command>powershell.exe -ExecutionPolicy Bypass -NoExit -File "C:TraefikPassCertstart-cert-forward.ps1"</Command>
  </LogonCommand>
</Configuration>

Defining Dynamic PassTLSClientCert Middleware in Traefik

Inside C:SandboxDevTraefikPassCertdynamicpasscert.yml, specify the certificate forwarding rules:

http:
  middlewares:
    forward-client-cert:
      passTLSClientCert:
        pem: true
        info:
          notAfter: true
          notBefore: true
          sans: true
          serialNumber: true
          subject:
            country: true
            province: true
            locality: true
            organization: true
            commonName: true
            serialNumber: true
            domainComponent: true

  routers:
    mtls-router:
      rule: "Host(`mtls.sandbox.local`)"
      service: mock-cert-backend
      entryPoints:
        - websecure
      middlewares:
        - forward-client-cert
      tls:
        options: mtls-strict

  services:
    mock-cert-backend:
      loadBalancer:
        servers:
          - url: "http://127.0.0.1:8110"

tls:
  options:
    mtls-strict:
      clientAuth:
        caFiles:
          - "C:/TraefikPassCert/certs/client-ca.crt"
        clientAuthType: RequireAndVerifyClientCert

Automated Sandbox Provisioning via PowerShell

Create C:SandboxDevTraefikPassCertstart-cert-forward.ps1 to launch the services upon container initialization:

Write-Host "Provisioning Traefik Dynamic Client Certificate Forwarding Sandbox..." -ForegroundColor Green
Set-Location "C:TraefikPassCert"

# Start mock HTTP backend echoing received TLS client certificate headers
$listener = New-Object System.Net.HttpListener
$listener.Prefixes.Add("http://localhost:8110/")
$listener.Start()

$asyncJob = Start-Job -ScriptBlock {
    param($listener)
    while ($listener.IsListening) {
        $context = $listener.GetContext()
        $req = $context.Request
        $certHeader = $req.Headers["X-Forwarded-Tls-Client-Cert-Info"]
        $payload = "Client Certificate Info Header: " + $certHeader
        $buffer = [System.Text.Encoding]::UTF8.GetBytes($payload)
        $context.Response.ContentType = "text/plain"
        $context.Response.ContentLength64 = $buffer.Length
        $context.Response.OutputStream.Write($buffer, 0, $buffer.Length)
        $context.Response.Close()
    }
} -ArgumentList $listener

# Launch Traefik with live configuration reload
Start-Process "traefik.exe" -ArgumentList "--configfile=traefik.yml" -NoNewWindow
Write-Host "Traefik mTLS router active on port 443." -ForegroundColor Yellow

Validation and Teardown Advantages

Double-click the .wsb file to boot the testing environment. You can test mTLS using curl -k --cert client.crt --key client.key https://mtls.sandbox.local/ to verify presence of forwarded client headers. Because Windows Sandbox is completely disposable, closing the window purges all ephemeral processes, leaving zero residual configuration files on your host system.