Use After Free in Feathersjs with Hmac Signatures

Use After Free (UAF) is a class of memory safety bug where a program continues to use a pointer after the associated memory has been freed. In the context of FeathersJS, which is a JavaScript/Node.js framework, UAF is not typically described in low-level memory terms, but it manifests as a logic flaw where a request-scoped resource is accessed after it has been invalidated or replaced. This can occur when Hmac Signatures are used for request authentication and the application caches or reuses signature verification state across requests.

When FeathersJS services use Hmac Signatures for authentication, the framework or custom hooks may compute and store a signature context (e.g., a hash or keyed state) tied to a specific request or token. If the implementation erroneously retains a reference to this context beyond the request lifecycle—such as attaching it to a shared object or caching it improperly—the context can be invalidated or overwritten by a subsequent request. A later request that still holds a reference to the old context may inadvertently use freed data, leading to unpredictable behavior, incorrect authorization decisions, or information disclosure.

Consider a FeathersJS service that validates Hmac Signatures by storing intermediate verification state in a request-local cache keyed by a timestamp or nonce. If the nonce is reused or the cache is not properly scoped to the request, a new request with a similar nonce can overwrite the cached state while an in-flight or delayed handler still references it. This creates a Use After Free condition where the handler operates on stale or repurposed data. For example, an attacker could induce state reuse by sending rapid requests with crafted nonces, causing the application to incorrectly validate signatures and potentially bypass authentication.

In practice, this vulnerability surfaces when Hmac Signature verification logic is coupled with shared mutable state or improper cleanup. The risk is elevated in asynchronous code where promises or callbacks outlive the original request context. Because FeathersJS encourages modular services and hooks, developers might inadvertently share signature verification state across service methods or hooks without proper isolation. The consequence can be unauthorized access, signature bypass, or erratic application behavior, which middleBrick’s Authentication and BOLA/IDOR checks are designed to surface through runtime testing and spec cross-referencing.

middleBrick scans such scenarios by analyzing the OpenAPI/Swagger spec (including $ref resolution) and correlating it with runtime behavior. It tests unauthenticated attack surfaces to detect inconsistent authentication boundaries and unsafe handling of cryptographic operations. While middleBrick identifies these patterns and provides remediation guidance, it does not modify code or block execution; it highlights where Hmac Signature logic may expose state management flaws that lead to Use After Free conditions.

To mitigate Use After Free risks with Hmac Signatures in FeathersJS, ensure that signature verification state is strictly request-scoped and that no references to transient data are retained beyond the request lifecycle. Use immutable data structures and avoid caching verification state in shared objects. Below are concrete code examples demonstrating secure Hmac Signature implementation in FeathersJS.

Example 1: Stateless Hmac Signature Verification

This approach verifies the signature on each request without storing any intermediate state. It uses the built-in crypto module and ensures that all data used in verification is derived from the current request only.

const crypto = require('crypto');

function verifyHmacSignature(req, secret) {
  const { timestamp, nonce, signature } = req.headers;
  if (!timestamp || !nonce || !signature) {
    throw new Error('Missing authentication headers');
  }
  // Prevent replay attacks by ensuring timestamp is recent
  const now = Date.now();
  if (Math.abs(now - parseInt(timestamp, 10)) > 30000) {
    throw new Error('Request expired');
  }
  const payload = `${timestamp}${nonce}${req.method}${req.url}`;
  const expected = crypto
    .createHmac('sha256', secret)
    .update(payload)
    .digest('hex');
  // Use timing-safe comparison
  return crypto.timingSafeEqual(
    Buffer.from(signature),
    Buffer.from(expected)
  );
}

// FeathersJS hook usage
const hmacAuthentication = context =>
  new Promise((resolve, reject) => {
    try {
      const isValid = verifyHmacSignature(context.params.raw, process.env.HMAC_SECRET);
      if (!isValid) {
        throw new Error('Invalid signature');
      }
      resolve(context);
    } catch (error) {
      reject(error);
    }
  });

module.exports = {
  before: {
    all: [hmacAuthentication],
  },
};

Example 2: Isolated Signature Context per Request

If you must maintain some form of verification context, ensure it is bound exclusively to the request and cleaned up immediately after use. Do not attach context to global or service-level objects.

const crypto = require('crypto');

class RequestScopedVerifier {
  constructor(secret) {
    this.secret = secret;
  }

  verify(context) {
    const { headers } = context.params.raw;
    const { timestamp, nonce, signature } = headers;
    const payload = JSON.stringify(context.data || {});
    const token = `${timestamp}:${nonce}:${payload}`;
    const expected = crypto
      .createHmac('sha256', this.secret)
      .update(token)
      .digest('base64');
    return crypto.timingSafeEqual(Buffer.from(signature), Buffer.from(expected));
  }
}

// Usage within a FeathersJS hook
const verifier = new RequestScopedVerifier(process.env.HMAC_SECRET);

const secureHook = context =>
  new Promise((resolve, reject) =>
    verifier.verify(context) ? resolve(context) : reject(new Error('Invalid Hmac Signature'))
  );

Best Practices Summary

• Never store Hmac verification state beyond the request scope.
• Use crypto.timingSafeEqual to prevent timing attacks.
• Include nonce and timestamp to prevent replay attacks.
• Validate and sanitize all inputs before signature computation.
• Leverage middleware or hooks to enforce signature checks uniformly.

These practices align with OWASP API Security Top 10 controls and help prevent both Use After Free logic errors and signature bypass vulnerabilities. middleBrick’s Pro plan supports continuous monitoring and GitHub Action integration to enforce these standards in CI/CD pipelines, ensuring that Hmac Signature implementations remain secure across deployments.