Information Disclosure in Aspnet with Mutual Tls

Information disclosure in ASP.NET applications using Mutual TLS occurs when sensitive data is unintentionally exposed during the TLS handshake and subsequent request processing. Mutual TLS (mTLS) requires both the client and server to present valid certificates, which adds authentication and helps prevent man-in-the-middle attacks. However, when implemented or configured incorrectly, mTLS can inadvertently contribute to information disclosure by revealing details about the application, its configuration, or its users through logs, error messages, or metadata.

ASP.NET applications that terminate TLS at the server (e.g., via Kestrel or IIS with HTTPS bindings) and request client certificates can expose information if they log certificate details, such as the Subject Distinguished Name (DN), thumbprint, or certificate serial number. These logs may be stored in application logs, diagnostic traces, or error responses, and can be accessed by unauthorized parties if log management practices are weak. For example, including the client certificate subject in an HTTP 403 response message can reveal which certificate was used, aiding an attacker in mapping identities to users or systems.

Another vector involves exception handling and stack traces. If an ASP.NET application does not properly suppress detailed errors in production, a failed mTLS handshake or a rejected client certificate may produce stack traces that include file paths, method names, or configuration details. These details can expose the underlying framework version, server structure, or certificate validation logic. Attackers can use this information to tailor exploits against known vulnerabilities in specific ASP.NET versions or libraries.

Protocol-level information disclosure can also occur if the server inadvertently signals support for insecure or deprecated TLS versions or cipher suites during the negotiation process, even when mTLS is enforced. This can be observed through error responses or handshake failures that hint at the server’s capabilities. Additionally, if the application reflects certificate metadata in JSON or XML responses (for example, in health check or status endpoints), sensitive data such as issuer details or certificate validity periods may be exposed to unauthenticated or low-privilege attackers.

In the context of OWASP API Top 10, these issues align with '2023-A1: Broken Object Level Authorization' and '2023-A5: Security Misconfiguration', as improper handling of certificate data and verbose error messages constitute a security misconfiguration that leads to information leakage. Real-world examples include CVE-2021-26767 in Microsoft Exchange, where improperly handled certificate validation contributed to information exposure, and CVE-2022-21446, where verbose error messages revealed stack traces. middleBrick’s LLM/AI Security checks can detect instances where system prompts or error messages inadvertently expose sensitive patterns, while its 12 security checks—including Input Validation, Data Exposure, and Authentication—help identify such leakage in unauthenticated scans.

To detect these risks without relying on internal implementation details, scanners simulate unauthenticated interactions and analyze responses for sensitive content. For instance, a scan may send a request with an invalid client certificate and inspect the response for certificate-related data or stack traces. middleBrick’s OpenAPI/Swagger spec analysis, which resolves full $ref chains across OpenAPI 2.0, 3.0, and 3.1, can further identify endpoints that reference certificate metadata in schemas or documentation, flagging potential exposure points.

Remediation focuses on minimizing information exposure while maintaining strong client authentication. In ASP.NET Core, configure Kestrel to request client certificates without logging or reflecting sensitive certificate details. Use the ConfigureKestrel method to enforce client certificate validation and avoid including certificate metadata in responses or logs.

// Program.cs (ASP.NET Core 6+)
var builder = WebApplication.CreateBuilder(args);

builder.WebHost.ConfigureKestrel(serverOptions =>
{
    serverOptions.ConfigureHttpsDefaults(httpsOptions =>
    {
        httpsOptions.ClientCertificateMode = ClientCertificateMode.RequireCertificate;
        httpsOptions.AllowedCipherSuites = new List
        {
            // Example: restrict to strong cipher suites
            0xC02F, // TLS_AES_128_GCM_SHA256
            0xC030  // TLS_AES_256_GCM_SHA384
        };
    });
});

// Enforce certificate validation without echoing details
builder.Services.Configure<CertificateAuthenticationOptions>(options =>
{
    options.AllowedCertificateTypes = CertificateTypes.All;
    options.RevocationMode = X509RevocationMode.NoCheck; // or Online/Offline based on policy
});

var app = builder.Build();
app.UseHttpsRedirection();
app.UseAuthentication();
app.UseAuthorization();
app.MapGet("/", () => "Secure endpoint");
app.Run();

Ensure that exceptions and validation errors do not leak certificate information. Avoid logging certificate details and sanitize error responses. In production, disable detailed developer exception pages and use generic error messages.

// Example: safe exception handling in middleware
app.UseExceptionHandler(appBuilder =>
{
    appBuilder.Run(async context =>
    {
        context.Response.StatusCode = 500;
        await context.Response.WriteAsync("An error occurred.");
        // Do not log or expose exception.StackTrace or certificate details
    });
});

For IIS-hosted applications, configure SSL settings in web.config to require client certificates and disable verbose errors. Do not include certificate metadata in custom headers or response bodies.

<!-- web.config -->
<configuration>
  <system.webServer>
    <security>
      <access sslFlags="Ssl, SslNegotiateCert" />
      <requestFiltering>
        <hiddenSegments>
          <add segment="certificates" />
        </hiddenSegments>
      </requestFiltering>
    </security>
  </system.webServer>
</configuration>

Validate client certificates against a trusted store and avoid accepting self-signed or weak certificates. Use revocation checks where appropriate, but balance availability and security. Do not reflect certificate properties such as Subject or Issuer in API responses, including error payloads or status endpoints.

middleBrick’s CLI tool (middlebrick scan <url>) can validate these configurations by checking for information leakage in responses and verifying that error messages and headers do not disclose certificate or framework details. Its GitHub Action integration allows you to add API security checks to your CI/CD pipeline, failing builds if risk scores exceed your threshold, while the Web Dashboard helps track findings and scores over time.