Missing Authentication in Aspnet with Hmac Signatures

Missing Authentication in an ASP.NET API that uses Hmac Signatures can occur when the endpoint that validates the signature is left unguarded or when the validation step is bypassed. In this scenario, an attacker can send arbitrary requests with a valid Hmac signature computed over known or guessed data, and the server may still process the request because it skips authentication or authorization checks entirely.

Hmac Signatures typically rely on a shared secret to sign a canonical string built from selected parts of the request (e.g., HTTP method, path, selected headers, and body). If the endpoint does not enforce authentication before signature validation—or if the validation is incomplete—an attacker can probe for endpoints that either do not require credentials or incorrectly trust the presence of the signature as proof of identity. This becomes especially risky when the application exposes an unauthenticated attack surface, because middleBrick’s black-box scan can detect endpoints where Hmac is present but authentication is missing, leading to findings such as BOLA/IDOR or privilege escalation.

For example, consider an endpoint that expects an Hmac-SHA256 signature in a custom header but does not check for an API key or session token. Even with a correct signature, the server might treat the request as trusted and return sensitive data or perform state-changing operations. This violates the principle that authentication should precede authorization and signature validation. Attack patterns like these are mapped to OWASP API Top 10:2023 — API1:2023 Broken Object Level Authorization when the signature is mistakenly used as a substitute for verifying the caller’s identity. Real-world findings from scans often highlight missing authentication on endpoints that accept Hmac-signed payloads but skip mandatory identity checks, which is why continuous monitoring and clear remediation guidance are important.

In an ASP.NET implementation, developers might mistakenly configure authentication schemes or policy checks only on selected controllers, leaving other routes open. If the Hmac validation logic is placed inside an action filter or middleware that runs after authentication is expected but does not enforce it, the effective security boundary is weakened. This is a common misconfiguration that middleBrick’s 12 security checks aim to surface by correlating spec-defined authentication requirements with runtime behavior, including endpoints that support unauthenticated LLM interactions or excessive agency patterns in AI-enabled services.

When scanning such APIs, middleBrick can identify these gaps without credentials, providing a risk score and prioritized findings with severity and remediation guidance. For teams using the Pro plan, continuous monitoring can detect regressions as APIs evolve, while the CLI allows quick local verification. The GitHub Action can further enforce that any merge which introduces an endpoint with missing authentication fails the build if the score drops below your chosen threshold.

To fix Missing Authentication when using Hmac Signatures in ASP.NET, ensure that authentication is enforced before signature validation and that the signature is tied to the authenticated caller. Below are concrete code examples that demonstrate a secure pattern using Hmac-SHA256 with explicit authentication checks.

First, configure authentication and define a policy that requires a valid identity. Then, validate the Hmac signature only after confirming the user is authenticated. This ensures the signature acts as a request integrity mechanism rather than a substitute for authentication.

// Program.cs or Startup configuration
builder.Services.AddAuthentication("Hmac")
    .AddScheme<AuthenticationSchemeOptions, HmacAuthenticationHandler>("Hmac", null);
builder.Services.AddAuthorization(options =>
{
    options.AddPolicy("RequireAuthenticatedHmac", policy =>
    {
        policy.AuthenticationSchemes.Add("Hmac");
        policy.RequireAuthenticatedUser();
    });
});

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

Implement a custom HMAC handler that validates the signature after confirming the principal is set. The handler reads a shared secret (from configuration or a secure store), reconstructs the canonical string, and compares the provided signature in constant time to avoid timing attacks.

public class HmacAuthenticationHandler : AuthenticationHandler<AuthenticationSchemeOptions>
{
    private readonly string _sharedSecret;

    public HmacAuthenticationHandler(
        IOptionsMonitor<AuthenticationSchemeOptions> options,
        ILoggerFactory logger,
        UrlEncoder encoder,
        ISystemClock clock,
        IConfiguration config) : base(options, logger, encoder, clock)
    {
        _sharedSecret = config["HmacSettings:SharedSecret"] ?? throw new ArgumentException("Missing Hmac secret");
    }

    protected override async Task<AuthenticateResult> HandleAuthenticateAsync()
    {
        if (!Request.Headers.TryGetValue("X-API-Signature", out var signature))
            return AuthenticateResult.Fail("Missing signature");

        var computed = ComputeHmac(Request);
        if (!SecureCompare(computed, signature))
            return AuthenticateResult.Fail("Invalid signature");

        var claims = new[] { new Claim(ClaimTypes.Name, "hmac-identity") };
        var identity = new ClaimsIdentity(claims, Scheme.Name);
        var principal = new ClaimsPrincipal(identity);
        var ticket = new AuthenticationTicket(principal, Scheme.Name);
        return AuthenticateResult.Success(ticket);
    }

    private string ComputeHmac(HttpRequest request)
    {
        using var sha = System.Security.Cryptography.HMACSHA256.Create(Convert.FromBase64String(_sharedSecret));
        var body = request.Body;
        request.EnableBuffering();
        using var reader = new StreamReader(request.Body, leaveOpen: true);
        var bodyText = reader.ReadToEnd();
        request.Body.Position = 0;
        var canonical = $"{request.Method}:{request.Path}:{request.Headers.Where(h => h.Key.StartsWith("X-Request-", StringComparison.OrdinalIgnoreCase)).Select(h => $"{h.Key}:{string.Join(",", h.Value)}")}:{bodyText}";
        var hash = sha.ComputeHash(Encoding.UTF8.GetBytes(canonical));
        return Convert.ToBase64String(hash);
    }

    private bool SecureCompare(string a, string b)
    {
        var aBytes = Convert.FromBase64String(a);
        var bBytes = Convert.FromBase64String(b);
        return aBytes.Length == bBytes.Length && CryptographicOperations.FixedTimeEquals(aBytes, bBytes);
    }
}

Apply the policy to sensitive endpoints to ensure authentication is required before the Hmac signature is evaluated:

[ApiController]
[Route("api/[controller]")]
[Authorize(Policy = "RequireAuthenticatedHmac")]
public class PaymentsController : ControllerBase
{
    [HttpPost("charge")]
    public IActionResult Charge([FromBody] ChargeRequest request)
    {
        // Business logic here; the user is already authenticated via Hmac
        return Ok(new { Status = "Processed" });
    }
}

These steps ensure that Hmac Signatures are used to verify request integrity for authenticated callers, closing the gap where missing authentication could allow unauthorized actions. By combining explicit authentication requirements with robust signature validation, you reduce the risk of unauthorized access even if an attacker discovers or guesses parts of the request.