Stack Overflow in Express with Hmac Signatures

When an Express API uses Hmac Signatures to verify request integrity but does not enforce strict size and complexity limits on incoming data, it can expose endpoints to resource exhaustion and denial-of-service conditions. A Stack Overflow style attack in this context refers to an adversary sending many requests or very large payloads that cause the server to perform expensive signature verification and body parsing, leading to high CPU or memory usage.

Express by default parses bodies using body-parser or similar middleware. If the body size limit is not explicitly constrained, an attacker can POST large JSON or form payloads that require processing before the Hmac signature is even checked. Because Hmac verification requires reading the full body to recompute the signature, large payloads amplify the work per request. Combine this with a permissive route that accepts untrusted input, and the attack surface grows: the endpoint may also perform additional operations (parsing, transformation, or lookup) that further increase resource consumption.

The interaction with the 12 security checks run by middleBrick is relevant here. For example, the Rate Limiting check may flag missing or weak request throttling, while the Input Validation check may detect missing size constraints or lack of rejection for overly large payloads. The Authentication and BOLA/IDOR checks are less directly related, but if signature verification is implemented inconsistently across routes, it can expose authorization gaps. Data Exposure and Encryption checks may surface if large error messages or stack traces are returned under stress, revealing internal details.

Using middleBrick’s OpenAPI/Swagger analysis, the scanner resolves $ref definitions and cross-references declared size limits and security schemes with runtime behavior. If the spec declares a small maximum content length but the implementation does not enforce it, middleBrick will highlight the mismatch as a potential risk. Similarly, if the Hmac verification logic is only applied to a subset of routes, this inconsistency will be surfaced as a finding under Authentication and Property Authorization.

Real-world examples include endpoints that accept JSON objects with deeply nested structures or file uploads without explicit limits. In such cases, an attacker can craft payloads that trigger recursive parsing or large in-memory buffers. Even with Hmac signatures ensuring integrity, the server’s resource usage can degrade before the signature is deemed invalid, effectively achieving a denial of state rather than a breach of integrity.

To secure Express APIs that use Hmac Signatures, enforce strict request size limits, validate input early, and ensure consistent verification logic. Below are concrete, working examples that demonstrate these practices.

1. Enforce body size limits before Hmac verification

Configure Express to reject payloads that exceed a reasonable threshold before any processing. This reduces CPU and memory pressure during signature verification.

import express from 'express';
import crypto from 'crypto';

const app = express();
const MAX_BODY_SIZE = '10kb'; // limit before parsing

// Apply size limit to specific routes or globally
app.use(express.json({ limit: MAX_BODY_SIZE }));
app.use(express.urlencoded({ extended: true, limit: MAX_BODY_SIZE }));

function verifyHmac(req, res, next) {
  const signature = req.headers['x-hmac-signature'];
  if (!signature) {
    return res.status(401).json({ error: 'Missing Hmac signature' });
  }
  const secret = process.env.HMAC_SECRET;
  const hmac = crypto.createHmac('sha256', secret);
  hmac.update(JSON.stringify(req.body));
  const expected = hmac.digest('hex');
  if (!crypto.timingSafeEqual(Buffer.from(signature), Buffer.from(expected))) {
    return res.status(401).json({ error: 'Invalid Hmac signature' });
  }
  next();
}

app.post('/api/action', verifyHmac, (req, res) => {
  res.json({ status: 'ok' });
});

app.listen(3000, () => console.log('Server running on port 3000'));

2. Reject oversized and deeply nested payloads explicitly

Combine body-parser limits with runtime validation to ensure incoming data conforms to expected structure and avoids resource-intensive parsing patterns.

import { body, validationResult } from 'express-validator';

app.post('/api/submit',
  body('data').isObject().custom((value) => {
    const depth = (obj) => {
      if (obj === null || typeof obj !== 'object') return 0;
      return 1 + Math.max(0, ...Object.values(obj).map(depth));
    };
    if (depth(value) > 4) {
      throw new Error('Payload nesting too deep');
    }
    return true;
  }),
  (req, res) => {
    const errors = validationResult(req);
    if (!errors.isEmpty()) {
      return res.status(400).json({ errors: errors.array() });
    }
    res.json({ received: req.body });
  }
);

3. Apply consistent Hmac verification across all sensitive routes

Use a centralized middleware and route definitions that explicitly include the verification step. Avoid omitting it on any route that processes untrusted input.

const securedRoutes = express.Router();
securedRoutes.use(verifyHmac);

securedRoutes.post('/transfer', (req, res) => {
  // Business logic here
  res.json({ success: true });
});

securedRoutes.post('/admin/config', (req, res) => {
  res.json({ updated: true });
});

app.use('/api', securedRoutes);

By combining size constraints, early validation, and consistent Hmac verification, you reduce the impact of Stack Overflow style resource abuse and ensure the integrity checks remain effective under load. middleBrick’s checks for Rate Limiting, Input Validation, and Authentication help confirm that these controls are correctly implemented across your API surface.