Double Free in Fiber with Hmac Signatures

A Double Free occurs when a program attempts to free the same dynamically allocated memory twice. In the context of a Fiber-based API service that uses Hmac Signatures for request integrity, the interaction between signature verification and request body handling can create conditions where memory is mismanaged. When a Fiber application parses incoming requests and validates Hmac Signatures, it typically reads the raw body to compute or compare the signature. If the body buffer is managed incorrectly—such as being freed after signature validation and then freed again during request cleanup—a Double Free can occur.

In Go, memory safety is largely enforced by the runtime; however, when integrating with C via CGO or using certain low-level memory optimizations, improper handling can surface. For example, a developer might compute an Hmac Signature over the request body and later release the body buffer to save memory, but if the same pointer is passed to a final cleanup routine, the double deallocation is triggered. This is especially risky in high-throughput Fiber endpoints where multiple middleware layers inspect the body and signature. The vulnerability is not in the Hmac algorithm itself but in how the runtime or C dependencies handle the memory lifecycle after the signature check completes.

An attacker could exploit this by sending specially crafted requests that cause the server to process the same memory region repeatedly, potentially leading to undefined behavior, crashes, or information disclosure. Because the attack surface involves unauthenticated request parsing and signature verification, it maps to the BOLA/IDOR and Input Validation checks performed by middleBrick. The scanner would flag this as a security risk if the endpoint processes untrusted input without strict validation, highlighting the interaction between signature logic and memory handling as a concern.

Real-world scenarios include endpoints that use streaming bodies and perform on-the-fly Hmac verification. If the streaming buffer is reused and freed incorrectly across goroutines, the Double Free may manifest intermittently. This aligns with the kind of runtime behavior that middleBrick detects through its parallel security checks, including Property Authorization and Unsafe Consumption, ensuring that such patterns are surfaced before deployment.

Using the CLI tool, you can scan such endpoints with middlebrick scan <url> to identify risky request handling patterns. The report will include findings related to input validation and memory safety considerations, even though the tool does not directly trace internal memory management. This helps developers understand where architectural safeguards are needed to prevent Double Free conditions in Fiber services that rely on Hmac Signatures.

To mitigate Double Free and similar issues in Fiber when using Hmac Signatures, ensure that request body buffers are managed with clear ownership and lifecycle boundaries. Avoid reusing or freeing the same memory region across multiple stages of request processing. Below are concrete code examples demonstrating secure handling.

Example 1: Validating Hmac Signature Safely

Read the body once, compute the Hmac, and avoid additional frees or mutations of the original buffer.

const crypto = require('crypto');
const express = require('express'); // Fiber-like pattern in Node.js
const app = express();

const SECRET = 'my-secret-key';

app.use(express.json({ limit: '10kb' }));

app.post('/secure', (req, res) => {
  const receivedSignature = req.headers['x-signature'];
  if (!receivedSignature) {
    return res.status(400).send('Missing signature');
  }

  const body = JSON.stringify(req.body);
  const hmac = crypto.createHmac('sha256', SECRET);
  const computedSignature = hmac.update(body).digest('hex');

  // Constant-time comparison to avoid timing attacks
  const isValid = crypto.timingSafeEqual(
    Buffer.from(receivedSignature),
    Buffer.from(computedSignature)
  );

  if (!isValid) {
    return res.status(401).send('Invalid signature');
  }

  res.status(200).send('Valid request');
});

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

Example 2: Using Middleware to Centralize Hmac Verification

Centralize signature validation to ensure consistent buffer handling and reduce the risk of double processing.

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

const SECRET = 'my-secret-key';

const hmacMiddleware = (req, res, next) => {
  const receivedSignature = req.headers['x-signature'];
  if (!receivedSignature) {
    return res.status(400).send('Missing signature');
  }

  const body = JSON.stringify(req.body);
  const hmac = crypto.createHmac('sha256', SECRET);
  const computedSignature = hmac.update(body).digest('hex');

  const isValid = crypto.timingSafeEqual(
    Buffer.from(receivedSignature),
    Buffer.from(computedSignature)
  );

  if (!isValid) {
    return res.status(401).send('Invalid signature');
  }

  next();
};

app.use(express.json(), hmacMiddleware);

app.post('/data', (req, res) => {
  res.status(200).send('Request processed securely');
});

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

Best Practices for Fiber-like Environments

• Always compute Hmac over a canonical representation of the body (e.g., stringified JSON) to ensure consistency.
• Use constant-time comparison functions to prevent timing attacks, as shown with crypto.timingSafeEqual.
• Avoid manual memory management in Go or C integrations; rely on the runtime unless CGO is necessary, and then ensure clear ownership rules.
• Leverage the GitHub Action to enforce these patterns in CI/CD, failing builds if insecure handling is detected.

By following these patterns, developers can prevent Double Free issues and ensure robust Hmac Signature validation in Fiber services. The MCP Server can also be used to integrate these checks directly into AI coding assistants for real-time guidance during development.