Password Spraying in Aspnet with Hmac Signatures

Password spraying is an authentication attack that attempts a small number of common passwords against many accounts to avoid account lockout. In ASP.NET applications that rely on HMAC signatures for request authentication, password spraying can be relevant when the HMAC process is coupled with user credentials or API keys that are verified using a shared secret.

HMAC (Hash-based Message Authentication Code) uses a secret key to sign requests, typically to ensure integrity and origin authentication. In ASP.NET, HMAC is often implemented in custom authentication handlers or filters that validate a signature sent in a request header. If the server-side validation logic first retrieves a user or API key based on a username or identifier and then derives or looks up the expected HMAC secret, an attacker can perform password spraying against the user identifier without triggering per-account lockouts.

For example, consider an endpoint that accepts a username and an HMAC-signed payload. An attacker can enumerate usernames or use known test accounts and then attempt a few common passwords or API keys, signing requests with each attempt. Because the attack rate can be kept low and the requests appear as valid HMAC-signed traffic, standard rate limiting based on IP or session may not trigger. If the server responds differently—such as returning a 401 for bad signature versus bad credentials—the attacker can infer whether the username exists, enabling targeted follow-up attacks.

OWASP API Top 10 categories such as Broken Object Level Authorization (BOLA) and Authentication weaknesses align with this risk. The attack surface is especially relevant when HMAC is used as the sole or primary authentication mechanism without additional protections like nonce/timestamp validation or strict request throttling per user or key.

middleBrick’s authentication and BOLA/IDOR checks can surface these patterns by analyzing unauthenticated endpoints that accept HMAC-signed requests and inspecting whether username enumeration or weak key management is exposed. The scanner also runs input validation and unsafe consumption checks to detect whether HMAC validation is performed in a way that can be bypassed or abused during credential guessing.

To reduce risk when using HMAC signatures in ASP.NET, enforce strict validation order, constant-time comparison, and rate limiting per key or user. Avoid deriving or selecting secrets based on user input in a way that reveals existence of accounts.

Example: Secure HMAC validation in ASP.NET Core middleware

The following example demonstrates a middleware approach that validates HMAC signatures early and uses a constant-time comparison to avoid timing leaks. It also avoids branching logic based on credential validity before signature verification.

using System;using System.Linq;using System.Security.Cryptography;using System.Text;using System.Threading.Tasks;using Microsoft.AspNetCore.Http;

public class HmacValidationMiddleware{    private readonly RequestDelegate _next;
    private readonly byte[] _globalApiKey; // Ideally stored securely, e.g., Azure Key Vault

    public HmacValidationMiddleware(RequestDelegate next)    {        _next = next;        // In production, load key securely and rotate regularly        _globalApiKey = Encoding.UTF8.GetBytes("SUPERSECRETKEY_PLACEHOLDER_REPLACE_WITH_SECURE_KEY");    }

    public async Task Invoke(HttpContext context)    {        // Expect headers: X-API-Key, X-Signature, X-Timestamp        if (!context.Request.Headers.TryGetValue("X-API-Key", out var apiKeyHeader) ||            !context.Request.Headers.TryGetValue("X-Signature", out var signatureHeader) ||            !context.Request.Headers.TryGetValue("X-Timestamp", out var timestampHeader))        {            context.Response.StatusCode = 401;            await context.Response.WriteAsync("Missing authentication headers");            return;        }

        // Validate timestamp to prevent replay (e.g., within 5 minutes)
        if (!long.TryParse(timestampHeader, out var timestamp) ||            Math.Abs(DateTimeOffset.UtcNow.ToUnixTimeSeconds() - timestamp) > 300)        {            context.Response.StatusCode = 401;            await context.Response.WriteAsync("Request expired");            return;        }

        // Reconstruct the payload that was signed (example: raw body or selected headers)
        context.Request.EnableBuffering();        var bodyStream = new System.IO.MemoryStream();        await context.Request.Body.CopyToAsync(bodyStream);        var payload = bodyStream.ToArray();        context.Request.Body.Position = 0;

        // Compute HMAC over key + timestamp + payload to bind key usage
        using var hmac = new HMACSHA256(_globalApiKey);        string computedSignature = Convert.ToBase64String(hmac.ComputeHash(Encoding.UTF8.GetBytes($"{apiKeyHeader}{timestamp}{Convert.ToBase64String(payload)}")));

        // Constant-time comparison to avoid timing attacks
        if (!ConstantTimeEquals(computedSignature, signatureHeader))        {            context.Response.StatusCode = 401;            await context.Response.WriteAsync("Invalid signature");            return;        }

        // At this point, signature is valid; proceed to route or enforce per-key rate limits
        await _next(context);    }

    private bool ConstantTimeEquals(string a, string b)    {        if (a.Length != b.Length) return false;        int result = 0;        for (int i = 0; i < a.Length; i++)        {            result |= a[i] ^ b[i];        }        return result == 0;    }}

In this pattern, the server does not reveal whether an API key or username is valid before performing the HMAC verification. The timestamp header helps mitigate replay attacks, and constant-time comparison prevents attackers from using timing differences to learn about the signature.

Additionally, avoid using user-supplied values to directly select secrets. If per-user keys are required, maintain a secure mapping (e.g., hashed key identifiers stored server-side) and apply rate limiting at the key level rather than relying on account lockout mechanisms that can be bypassed by changing credentials slightly.

middleBrick’s Pro plan supports continuous monitoring and can integrate with CI/CD via the GitHub Action to ensure HMAC validation logic remains secure across deployments. The scanner checks for missing nonce/timestamp handling and insecure HMAC construction that could enable password spraying or signature manipulation.