When debugging ASP.NET applications that use client certificate authentication, IIS Express needs proper configuration to mimic production behavior. Unlike full IIS, IIS Express requires manual configuration in both applicationhost.config and your project settings.

Locate your IIS Express configuration file (typically in %USERPROFILE%\Documents\IISExpress\config\applicationhost.config). Find your site's configuration section and modify it:

<site name="YourSiteName" id="1">
  <application path="/" applicationPool="Clr4IntegratedAppPool">
    <virtualDirectory path="/" physicalPath="C:\YourProjectPath" />
  </application>
  <bindings>
    <binding protocol="https" bindingInformation="*:44300:localhost" />
  </bindings>
</site>

Add these settings within the <location> tag for your site:

<access sslFlags="Ssl, SslNegotiateCert, SslRequireCert" />

Update your project's launchSettings.json to ensure SSL is enabled:

{
  "iisSettings": {
    "windowsAuthentication": false,
    "anonymousAuthentication": true,
    "iisExpress": {
      "applicationUrl": "https://localhost:44300",
      "sslPort": 44300
    }
  }
}

In your ASP.NET application, you can access the client certificate through the HttpContext:

public IActionResult SecureEndpoint()
{
    var clientCert = HttpContext.Connection.ClientCertificate;
    if (clientCert == null || !clientCert.Verify())
    {
        return Unauthorized("Valid client certificate required");
    }
    
    // Certificate is valid - process request
    return Ok("Authenticated successfully");
}

• Ensure your development certificate is trusted (check with mmc.exe)
• Use Fiddler or Wireshark to inspect SSL handshake
• Test with different browsers as they handle client certs differently
• Check Windows Event Viewer for schannel errors

If you're having persistent issues, consider using a reverse proxy setup with nginx or Caddy that handles the client certificate authentication before forwarding to IIS Express.

When debugging ASP.NET applications that use client certificate authentication, IIS Express requires specific configuration that differs from full IIS. The key settings involve both the applicationhost.config file and your project properties.

First, locate your applicationhost.config file (typically in Documents\IISExpress\config). You'll need to modify the section:

<security>
    <access sslFlags="Ssl, SslNegotiateCert, SslRequireCert" />
</security>

In your Visual Studio project properties (Debug tab), ensure SSL is enabled and verify the port number matches what's in applicationhost.config. The SSL port should be configured for HTTPS:

"iisSettings": {
    "iisExpress": {
        "applicationUrl": "https://localhost:44300",
        "sslPort": 44300
    }
}

For development, you'll need to:

1. Install the client certificate in your personal certificate store
2. Ensure the certificate's root CA is trusted
3. Configure IIS Express to trust the client certificate

If you encounter HTTP 403.7 errors, verify:

• The certificate is properly installed and not expired
• The certificate's CN matches the expected value
• No intermediate certificates are missing from the chain

For more complex scenarios, you can implement certificate mapping in your web.config:

<system.webServer>
    <security>
        <authentication>
            <clientCertificateMappingAuthentication enabled="true" />
        </authentication>
    </security>
</system.webServer>

Enable failed request tracing in IIS Express by adding this to your applicationhost.config:

<traceFailedRequests>
    <add path="*">
        <traceAreas>
            <add provider="ASP" verbosity="Verbose" />
            <add provider="WWW Server" areas="Authentication,Security" verbosity="Verbose" />
        </traceAreas>
        <failureDefinitions timeTaken="00:00:00" statusCodes="403" />
    </add>
</traceFailedRequests>