Password Spraying in Fiber with Hmac Signatures

Password spraying is an authentication abuse pattern where an attacker uses a small list of commonly used passwords against many accounts to avoid account lockouts. When an API built with Fiber uses Hmac Signatures for request authentication, misconfiguration or weak implementation can make the endpoint more susceptible to password spraying during the signature verification phase.

In a typical Fiber route protected by Hmac Signatures, the client computes a hash-based message authentication code using a shared secret and includes it in a header (e.g., x-hmac-signature). The server recomputes the Hmac and compares it in constant time. If the server does not enforce strict rate limiting or does not uniformly return errors for invalid credentials across users, an attacker can probe multiple accounts with the same password by iterating over usernames and observing timing differences or error message variations. This can reveal whether a given username exists and allow low-and-slow password spraying that bypasses simple per-IP rate controls.

Because Hmac Signatures rely on a shared secret, if that secret is weak, leaked, or reused across services, an attacker who observes or guesses a signature may attempt to recover the secret and forge requests. In a password spraying context, this can allow an attacker to test credential pairs more efficiently if the signature generation is deterministic and tied to a predictable user identifier. Moreover, if the API surface that validates Hmac Signatures also exposes account enumeration through timing or error messages, the combination of Hmac Signatures with password spraying becomes a practical attack vector. Attackers may use tools like Burp Suite intruder or custom scripts to cycle through passwords while keeping the Hmac signature valid per request, especially when the signing key is static across a session or per user.

These risks map to common OWASP API Top 10 categories such as Broken Authentication (API2:2023) and can be surfaced by middleBrick’s Authentication and BOLA/IDOR checks, which test for weak authentication flows and insecure direct object references. middleBrick’s scans run 12 security checks in parallel, including Authentication and Rate Limiting, to detect whether your Fiber endpoints leak account existence or allow credential abuse via Hmac-based flows.

To mitigate password spraying risks when using Hmac Signatures in Fiber, apply strong operational and coding practices. Use constant-time comparison, enforce per-user rate limits, avoid exposing user existence through timing or error messages, and rotate secrets regularly. Below are concrete code examples for a secure setup.

1. Constant-time Hmac verification in Fiber

Use crypto.timingSafeEqual to compare signatures and avoid early-exit timing leaks.

import { createHmac, timingSafeEqual } from 'crypto';
import { Request, Response } from 'express';

function verifyHmac(req: Request, res: Response, secret: string): boolean {
  const signature = req.get('x-hmac-signature');
  const payload = req.rawBody; // ensure raw body is used for signing
  if (!signature || !payload) return false;
  const hmac = createHmac('sha256', secret);
  hmac.update(payload);
  const expected = hmac.digest('hex');
  const given = Buffer.from(signature, 'hex');
  // Ensure lengths match to avoid length-based leaks
  if (given.length !== expected.length) {
    // Use a dummy comparison to keep timing consistent
    const dummy = createHmac('sha256', secret).update('dummy').digest('hex');
    const dummyBuf = Buffer.from(dummy, 'hex');
    timingSafeEqual(given, dummyBuf);
    return false;
  }
  const isValid = timingSafeEqual(given, Buffer.from(expected, 'hex'));
  return isValid;
}

2. Rate limiting per user and globally

Apply rate limits that consider the user or API key derived from the Hmac identity to prevent spraying across accounts. Example using @fastify/rate-limit with a custom key function.

import Fastify from 'fastify';
import rateLimit from '@fastify/rate-limit';

const app = Fastify();
app.register(rateLimit, {
  rateLimit: {
    max: 100,
    timeWindow: '15 minutes',
    keyGenerator: (request) => {
      // Use user identifier from Hmac payload or a header; fallback to IP
      return request.headers['x-user-id'] || request.ip;
    },
  },
});

app.get('/protected', async (request, reply) => {
  // Verify Hmac here; if invalid, reply.code(401).send({ error: 'Unauthorized' })
  return { ok: true };
});

3. Avoid account enumeration in error responses

Return uniform error messages and status codes for invalid or missing credentials, regardless of whether the username exists.

app.post('/login', (request, reply) => {
  const isValid = verifyHmac(request, reply, process.env.HMAC_SECRET);
  if (!isValid) {
    reply.code(401).send({ error: 'Invalid credentials' });
    return;
  }
  // proceed with authentication
});

4. Rotate secrets and monitor usage

Rotate your Hmac secret periodically and monitor for anomalous request volumes across user accounts. middleBrick’s Pro plan includes continuous monitoring and can alert you when risk scores degrade or unusual patterns appear across endpoints using Hmac Signatures.

By combining constant-time verification, per-user rate limiting, and uniform error handling, you reduce the effectiveness of password spraying against Fiber APIs that rely on Hmac Signatures. For ongoing assurance, integrate the middleBrick CLI to scan from terminal with middlebrick scan <url> or add the GitHub Action to your CI/CD pipeline to fail builds if security scores drop below your chosen threshold.