CVE-2023-39331 - How a Botched Patch Opened a New Path Traversal Vulnerability in Node.js Permissions

*Published: 2024-06-12*


The security of open source software relies not just on quick patches, but on doing them right. CVE-2023-39331 is a textbook case of how an incomplete fix (for CVE-2023-30584) in Node.js led to a new and potentially dangerous path traversal flaw. In this deep dive, we’ll break down what happened, why it matters, see a code snippet that demonstrates the bug, and offer references for further reading. No jargon: just a clear explanation.

Quick Recap: Node.js Permissions and CVE-2023-30584

Node.js is adding experimental support for a permission model. This feature should let app developers restrict what code can do, like reading or writing files, or spawning processes. But making these restrictions airtight is tricky.

In mid-2023, a path traversal bug (CVE-2023-30584) was found: basically, someone could trick the system into overwriting files or accessing paths they shouldn’t, by passing sneaky file paths like "../../etc/passwd".

A fix landed in commit 205f1e6 — or so everyone thought.

The Problem

Path traversal bugs often happen because an attacker can supply a filename like "../../secret". The patch for CVE-2023-30584 tried to block such attacks, but made a classic mistake: It relied on built-in Node.js “utility” functions to sanitize inputs, without making sure those utility functions couldn’t be replaced by user code.

If a malicious application replaces a built-in utility—like overwriting a function that normalizes file paths—the security checks could be bypassed.

Example: How This Can Happen

Suppose normalizePath is a utility function officially used by the Node.js permission system. The patch assumed normalizePath would always behave correctly. But, Node.js lets code monkey-patch (overwrite) functions during runtime.

Here’s a simple code illustration

// Assume this is the built-in utility function Node.js uses:
function normalizePath(path) {
  // Normally returns a cleaned-up version of the path
  return path.replace(/\/+/g, '/');
}

// Somewhere in the permission model, the code does:
let safePath = normalizePath(userInputPath);
if (safePath.includes('..')) {
  throw new Error('Path traversal attempt detected!');
}

// --- But an attacker does this in user code:
global.normalizePath = function(input) {
  // Ignore all 'parent folder' traversals!
  return input.replace(/\.\.\//g, '');
};

// Now, when permission checks run, normalizePath does *nothing* to block traversal
console.log(normalizePath('../../etc/passwd')); // outputs 'etc/passwd'

Because the utility wasn’t protected, the attacker’s version of normalizePath gets called instead of the safe, built-in one. The security check is defeated, and traversal is possible again.

The Danger

- Bypassing restrictions: Attackers can read, write, or modify files outside permitted directories, even though the patch tried to block this.
- Data theft, code injection, sabotage: If you rely on the Node.js permission model, you could be exposed to data loss, leaks, or even total system compromise.

Here's a mini exploit based on this issue

// 1. The app tries to apply restrictions in a handler:
function restrictFileAccess(filePath) {
  let safePath = pathNormalize(filePath);
  if (safePath.includes('..')) {
    throw new Error('Blocked path traversal!');
  }
  // Read/Write file...
}

// 2. Attacker patches the sanitizer:
global.pathNormalize = function(filePath) {
  return filePath; // Does nothing, lets through everything!
};

// 3. Exploit:
restrictFileAccess("../../etc/shadow"); // No error thrown

Timeline

- April 2023: CVE-2023-30584 is patched (commit 205f1e6)

Why This Matters

Node.js’s permission system is experimental (docs), but many developers are already using it for sandboxing untrusted code.

You can’t trust a patch unless you protect your internal logic from outside interference.

- Always use *internal* functions, or at least, encapsulate utilities so they can't be monkey-patched from user code.

Official References

- Node.js security release summary
- CVE-2023-39331 at NVD
- Patch commit 205f1e6 for CVE-2023-30584
- Node.js Permissions (experimental)

Final Thoughts

This case is a reminder: patches must consider how JavaScript's flexibility can backfire. If you use Node.js’s permissions features, stay vigilant, update often, and don’t assume a single fix closes the door.

If you found this helpful, share with your team and review your sandboxing techniques!

*This analysis was written exclusively for readers seeking clarity on CVE-2023-39331. Stay secure!*

Timeline

Published on: 10/18/2023 04:15:00 UTC
Last modified on: 11/08/2023 01:15:00 UTC