Credential Stuffing in Koa with Hmac Signatures

Credential stuffing is an automated attack where adversaries use lists of breached username and password pairs to gain unauthorized access. In Koa applications that rely on Hmac signatures for request authentication, a common misconfiguration can weaken the protection that Hmac is intended to provide and expose endpoints to credential stuffing.

Hmac signatures are designed to ensure integrity and authenticity: a client computes a hash-based message authentication code using a shared secret and includes it in headers. If the server validates the signature before enforcing strong per-request protections, an attacker can flood the endpoint with many credential guesses without triggering rate limits or other anti-automation controls tied to the request body or session state. This happens because the signature itself does not inherently prevent enumeration or brute-force attempts; it only proves the request was authorized by someone who knows the secret.

In Koa, if routes that verify Hmac signatures do not apply rate limiting or strict authentication checks before signature validation, attackers can iterate over many credentials quickly. The server may respond with different status codes or timing differences for valid versus invalid users, enabling online guessing. Moreover, if the Hmac is computed over a subset of request data and does not include a nonce or timestamp, replay of intercepted signed requests becomes feasible, increasing the risk of successful credential reuse.

Another subtlety is how the secret is stored and derived. Hardcoding or leaking the Hmac secret in client-side code or logs allows attackers to forge valid signatures and mount more sophisticated stuffing or token replay attacks. Even with a correct signature scheme, weak account lockout, predictable user identifiers, or missing multi-factor authentication amplify the impact of credential stuffing against Hmac-protected endpoints.

To detect this risk, middleBrick scans for unauthenticated attack surface and checks whether rate limiting and proper authentication are enforced alongside Hmac signature validation. It also tests for timing differences and status code variations that can aid enumeration, and flags missing nonce/timestamp usage in signed payloads.

Remediation focuses on ensuring that authentication, rate limiting, and signature validation are tightly coupled so that credential stuffing attempts are throttled and forged requests are rejected. Below are concrete, working examples for a Koa server using Hmac signatures.

Example 1: Hmac signature verification middleware in Koa

const Koa = require('koa');
const crypto = require('crypto');
const app = new Koa();

const SHARED_SECRET = process.env.HMAC_SHARED_SECRET; // store securely, e.g., via secrets manager

function verifyHmac(ctx, next) {
  const signature = ctx.request.header['x-hmac-signature'];
  const timestamp = ctx.request.header['x-timestamp'];
  const nonce = ctx.request.header['x-nonce'];

  if (!signature || !timestamp || !nonce) {
    ctx.status = 400;
    ctx.body = { error: 'Missing required headers' };
    return;
  }

  // Prevent replay: reject if timestamp older than 5 minutes
  const now = Math.floor(Date.now() / 1000);
  if (Math.abs(now - parseInt(timestamp, 10)) > 300) {
    ctx.status = 401;
    ctx.body = { error: 'Request expired' };
    return;
  }

  const payload = `${timestamp}${nonce}${ctx.request.method}${ctx.path}${ctx.request.rawBody}`;
  const expected = crypto.createHmac('sha256', SHARED_SECRET)
                         .update(payload)
                         .digest('hex');

  if (!crypto.timingSafeEqual(Buffer.from(signature), Buffer.from(expected))) {
    ctx.status = 401;
    ctx.body = { error: 'Invalid signature' };
    return;
  }

  await next;
}

app.use(verifyHmac);

app.use(async (ctx) => {
  ctx.body = { ok: true };
});

app.listen(3000);

Example 2: Combine Hmac verification with rate limiting in Koa

const Koa = require('koa');
const Router = require('@koa/router');
const rateLimit = require('koa-rate-limit');
const crypto = require('crypto');

const app = new Koa();
const router = new Router();
const SHARED_SECRET = process.env.HMAC_SHARED_SECRET;

const apiLimiter = rateLimit({
  duration: 60 * 1000, // 1 minute
  max: 10, // max 10 requests per window per IP
  keyGenerator: (ctx) => {
    // use a stable identifier; for authenticated endpoints, prefer user ID
    return ctx.ip;
  },
  onLimitReached: (ctx) => {
    // optional: emit alert or custom logic
  }
});

function verifyHmac(ctx, next) {
  const signature = ctx.request.header['x-hmac-signature'];
  const timestamp = ctx.request.header['x-timestamp'];
  const nonce = ctx.request.header['x-nonce'];

  if (!signature || !timestamp || !nonce) {
    ctx.status = 400;
    ctx.body = { error: 'Missing required headers' };
    return;
  }

  const now = Math.floor(Date.now() / 1000);
  if (Math.abs(now - parseInt(timestamp, 10)) > 300) {
    ctx.status = 401;
    ctx.body = { error: 'Request expired' };
    return;
  }

  const payload = `${timestamp}${nonce}${ctx.request.method}${ctx.path}${ctx.request.rawBody}`;
  const expected = crypto.createHmac('sha256', SHARED_SECRET)
                         .update(payload)
                         .digest('hex');

  if (!crypto.timingSafeEqual(Buffer.from(signature), Buffer.from(expected))) {
    ctx.status = 401;
    ctx.body = { error: 'Invalid signature' };
    return;
  }

  return next;
}

// Apply rate limiting to sensitive routes, e.g., login
router.post('/login', apiLimiter, verifyHmac, async (ctx) => {
  const { username, password } = ctx.request.body;
  // Perform credential validation; ensure constant-time comparison where applicable
  // and avoid revealing whether username exists to mitigate enumeration.
  ctx.body = { authenticated: false };
});

app.use(router.routes()).use(router.allowedMethods());
app.listen(3000);

Best practices summary

• Include a nonce and timestamp in the Hmac payload and reject reused or stale requests to prevent replay.
• Apply rate limiting on authentication endpoints regardless of signature validity to blunt stuffing attempts.
• Use crypto.timingSafeEqual to compare signatures to avoid timing attacks.
• Ensure the shared secret is stored securely and rotated periodically; avoid logging raw signatures or secrets.
• Return uniform error responses and status codes for authentication failures to prevent user enumeration.