ASP.NET - Client Certificate Authentication

Client certificate authentication is a security method where the client proves its identity using a digital certificate. Instead of usernames or tokens, the client presents a certificate that the server verifies before allowing access. This approach is commonly used in high-security systems and service-to-service communication.

How It Works
When a client connects over HTTPS, it sends its certificate to the server during the TLS handshake. The server checks whether the certificate is valid, trusted and not expired. If the certificate passes validation, the request is allowed to continue to the application.

When Client Certificates Are Used
This method is often used for internal APIs, banking systems, government services and microservices. It is especially useful when machines or services communicate with each other and human login credentials are not suitable.

Certificate Validation Process
The server validates the certificate by checking its issuer, expiration date and trust chain. Optional checks can include revocation status or matching specific certificate fields. Only certificates issued by trusted authorities are accepted.

Security Benefits
Client certificates provide strong authentication because they are difficult to forge. They also remove the need to send passwords or tokens with each request, reducing the risk of credential theft.

Limitations to Consider
Managing certificates requires extra setup. Certificates must be issued, renewed and securely stored on the client side. This adds operational complexity compared to token-based authentication.

Example

var builder = WebApplication.CreateBuilder(args);

// Configure certificate authentication
builder.Services.AddAuthentication(
    CertificateAuthenticationDefaults.AuthenticationScheme)
.AddCertificate(options =>
{
    options.AllowedCertificateTypes = CertificateTypes.All;
});

var app = builder.Build();

app.UseAuthentication();
app.UseAuthorization();

app.MapGet("/secure", () => "Client certificate accepted");

app.Run();

Result

  • Client must present a valid certificate

  • Unauthorized clients are rejected

  • Suitable for high-security ASP.NET Core applications