Stack Overflow in Feathersjs with Hmac Signatures

A Stack Overflow in the context of FeathersJS with Hmac Signatures typically occurs when an endpoint that validates Hmac signatures does not enforce sufficient rate limiting or request‑validation constraints, allowing an attacker to flood the service with signed requests. Because FeathersJS applications often use hooks to verify Hmac signatures on incoming payloads and headers, an unauthenticated attacker can exploit missing rate limits to exhaust server resources or trigger excessive computation in the signature verification logic.

Consider an unprotected FeathersJS service that uses Hmac signatures for integrity but lacks per‑client or global rate limits. An attacker can rapidly generate valid Hmac signatures (e.g., by replaying recent requests with nonced timestamps or by brute‑forcing short nonces) and send a high volume of requests. Even though each request is individually authenticated via Hmac, the absence of rate limiting enables a denial‑of‑service condition through resource exhaustion. This pattern is commonly seen in integrations where the signature scheme relies on a shared secret known to both client and server, and the server does not enforce request quotas.

Another vector arises when query parameters or headers used to compute the Hmac are not canonicalized consistently. If an attacker can manipulate non‑canonicalized inputs (e.g., varying parameter order or whitespace), they may generate many distinct but valid signed requests that bypass caching or idempotency checks, amplifying the effect of the flood. Because the server must recompute the Hmac for each request, CPU usage increases, contributing to the Stack Overflow risk in high‑concurrency scenarios.

middleBrick detects this class of issue under the Rate Limiting and Authentication checks, flagging endpoints that accept authenticated requests without sufficient request‑volume controls. In scans of FeathersJS APIs, findings often highlight missing rate limits on Hmac‑protected routes, alongside recommendations to enforce per‑client quotas and request throttling.

To remediate Stack Overflow risks when using Hmac Signatures in FeathersJS, enforce rate limiting at the service or global level and ensure Hmac verification is efficient and side‑channel resistant. Below are concrete code examples that integrate rate limiting and robust Hmac validation within a FeathersJS application.

Example 1: Hmac signature verification hook

This hook computes the expected Hmac using the shared secret and the request payload/headers, then compares it to the client-supplied signature in constant time to avoid timing attacks.

const crypto = require('node:crypto');

function verifyHmac(secret) {
  return context => {
    const { headers, body } = context.params;
    const receivedSignature = headers['x-hmac-signature'];
    if (!receivedSignature) {
      throw new Error('Missing Hmac signature');
    }
    const payload = typeof body === 'string' ? body : JSON.stringify(body);
    const expected = crypto.createHmac('sha256', secret)
      .update(payload)
      .digest('hex');
    // Constant‑time comparison to prevent timing leaks
    const isValid = crypto.timingSafeEqual(
      Buffer.from(expected, 'utf8'),
      Buffer.from(receivedSignature, 'utf8')
    );
    if (!isValid) {
      throw new Error('Invalid Hmac signature');
    }
    return context;
  };
}

// Usage in a FeathersJS app
const app = require('@feathersjs/feathers')();
app.use('/secure', {
  before: {
    all: [verifyHmac('your-256-bit-secret')]
  }
});

Example 2: Adding rate limiting with express-rate-limit

Apply a rate limiter to the FeathersJS app or specific service to limit the number of signed requests per window per client identifier (e.g., API key or IP). This prevents a single client from triggering a Stack Overflow via excessive Hmac verification requests.

const rateLimit = require('express-rate-limit');

// Global rate limit for signed endpoints
const hmacLimiter = rateLimit({
  windowMs: 60 * 1000, // 1 minute
  max: 100, // limit each IP to 100 requests per window
  keyGenerator: (req) => {
    // Use a stable identifier, e.g., API key from headers
    return req.headers['x-api-key'] || req.ip;
  },
  standardHeaders: true,
  legacyHeaders: false,
});

// Apply to the FeathersJS app before Hmac verification
const app = require('@feathersjs/feathers')();
app.use(hmacLimiter);
app.use('/secure', {
  before: {
    all: [verifyHmac('your-256-bit-secret')]
  }
});

Example 3: Canonicalizing inputs before Hmac computation

Ensure consistent input formatting to avoid signature variability that can be exploited for request flooding. Sort query parameters and normalize JSON serialization before computing the Hmac.

function canonicalizeForHmac(body) {
  if (typeof body !== 'object' || body === null) return body;
  // Deterministic JSON serialization
  return JSON.stringify(body, Object.keys(body).sort());
}

function verifyHmacCanonical(secret) {
  return context => {
    const { headers, body } = context.params;
    const receivedSignature = headers['x-hmac-signature'];
    const canonicalBody = canonicalizeForHmac(body);
    const expected = crypto.createHmac('sha256', secret)
      .update(canonicalBody)
      .digest('hex');
    const isValid = crypto.timingSafeEqual(
      Buffer.from(expected, 'utf8'),
      Buffer.from(receivedSignature, 'utf8')
    );
    if (!isValid) throw new Error('Invalid Hmac signature');
    return context;
  };
}

By combining constant‑time verification, deterministic input handling, and rate limiting, you reduce the risk of Stack Overflow conditions in FeathersJS services that rely on Hmac Signatures. These patterns align with secure authentication practices and help ensure that authenticated endpoints remain resilient under high request volumes.