Out Of Bounds Read in Express with Hmac Signatures

An Out Of Bounds Read occurs when an application reads memory beyond the intended buffer or data structure. In Express, combining Hmac Signatures with unsafe handling of input length or signature comparison can expose this class of vulnerability. Hmac Signatures are commonly used to verify integrity of tokens, webhook payloads, or query parameters, and if the application reads beyond the expected byte length during parsing or comparison, it may read sensitive data or cause unstable behavior.

Consider a route that validates a query parameter token containing a signature generated with a shared secret. If the developer uses a custom buffer-based comparison that iterates over bytes without validating the input length, an attacker can supply a token longer than expected, causing the read to step outside the allocated buffer. This is not necessarily a remote code execution flaw, but it can leak stack contents or internal memory to the attacker, leading to information disclosure.

Example vulnerable pattern in Express:

const crypto = require('crypto');
const express = require('express');
const app = express();

function verifySignature(data, signature, secret) {
  const expected = crypto.createHmac('sha256', secret).update(data).digest('hex');
  // Vulnerable: naive byte-by-byte comparison without length guard
  let mismatch = 0;
  for (let i = 0; i < signature.length; i++) {
    mismatch |= signature.charCodeAt(i) ^ expected.charCodeAt(i);
  }
  return mismatch === 0;
}

app.get('/validate', (req, res) => {
  const { token, data } = req.query;
  if (!token || !data) return res.status(400).send('missing');
  if (verifySignature(data, token, 'super-secret-key')) {
    res.json({ valid: true });
  } else {
    res.status(401).send('invalid');
  }
});

app.listen(3000);

In this example, if token is longer than the length of expected, the loop in verifySignature can read beyond the bounds of expected (a string), which in JavaScript may result in undefined coerced to NaN or unexpected behavior. While JavaScript’s memory safety reduces direct memory leaks, such logic errors can still lead to incorrect validation and information leakage through timing or error paths. The risk is higher when integrating with native addons or when the framework exposes raw buffers.

An attacker can probe the endpoint with crafted tokens to observe timing differences or error behavior, potentially inferring internal state. This aligns with common web vulnerabilities categorized under OWASP API Top 10 (2023) — including invalid authentication and excessive data exposure — and may intersect with BOLA/IDOR when the signature is tied to resource identifiers.

middleBrick detects such patterns during its 12 parallel security checks, including Input Validation and Property Authorization, and flags risky signature handling alongside findings from its Active prompt injection testing and System prompt leakage detection. For compliance, these findings map to OWASP API Top 10, PCI-DSS, and SOC2 controls.

Remediation focuses on using constant-time comparison and avoiding manual byte-by-byte loops. Use built-in cryptographic APIs that guarantee safe length handling and avoid exposing differences in timing. In Express, prefer crypto.timingSafeEqual for comparing buffers of equal length, and always validate input lengths before processing.

Corrected example with Hmac Signatures in Express:

const crypto = require('crypto');
const express = require('express');
const app = express();

function verifySignature(data, token, secret) {
  const expected = crypto.createHmac('sha256', secret).update(data).digest('hex');
  if (token.length !== expected.length) {
    return false;
  }
  // Convert hex strings to buffers for constant-time comparison
  const tokenBuf = Buffer.from(token, 'utf8');
  const expectedBuf = Buffer.from(expected, 'utf8');
  return crypto.timingSafeEqual(tokenBuf, expectedBuf);
}

app.get('/validate', (req, res) => {
  const { token, data } = req.query;
  if (!token || !data || typeof token !== 'string' || typeof data !== 'string') {
    return res.status(400).send('missing');
  }
  if (verifySignature(data, token, 'super-secret-key')) {
    res.json({ valid: true });
  } else {
    res.status(401).send('invalid');
  }
});

app.listen(3000);

This approach ensures that the length check prevents out-of-bounds reads, and crypto.timingSafeEqual performs comparison in constant time, mitigating timing attacks. When integrating with OpenAPI/Swagger specs (2.0, 3.0, 3.1), ensure that the schema for token includes minLength and maxLength where applicable, enabling runtime validation that aligns with spec-driven testing performed by middleBrick’s scanner.

For production, consider using environment variables for secrets and rotating keys. middleBrick’s Pro plan supports continuous monitoring and GitHub Action integration to fail builds if insecure signature handling is detected in your CI/CD pipeline. Its MCP Server allows you to scan APIs directly from your AI coding assistant, helping catch such issues before deployment.