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If you're using Mongoose—a popular MongoDB object modeling tool for Node.js—you should know about a serious security flaw: CVE-2023-3696. This vulnerability, called a prototype pollution, affected versions of Mongoose before 7.3.4 and could let attackers manipulate your application data and behavior in unpredictable and potentially dangerous ways.

This writeup will help you understand what prototype pollution is, how CVE-2023-3696 works, and what you need to do to protect your software. We'll show some simple examples, link you to official resources, and highlight real risks. This guide is written for developers, sysadmins, or anyone who wants a no-nonsense, exclusive explanation.

What is Prototype Pollution?

JavaScript lets you add properties to an object's prototype. That means changing every object that inherits from it. If a library, like Mongoose, merges user-supplied data without enough checks, a hacker may inject data like __proto__ or constructor and alter the behavior for every object in your app!

In plain English: Imagine every car on the road suddenly gets the same, dangerous software update—no matter the model or owner—because someone hacked your car manufacturer’s blueprint.

About Mongoose and How CVE-2023-3696 Happened

Mongoose handles your Mongo data as JavaScript objects. Before version 7.3.4, its query/merge logic didn't fully prevent users from passing fields like __proto__ or constructor. When Mongoose tried to merge or update documents, it could overwrite the prototype, making built-in objects behave dangerously.

Here’s a simplified code snippet showing what could go wrong

// Simulated vulnerable merge (not real Mongoose source code)
function merge(target, source) {
    for (let key in source) {
        target[key] = source[key];
    }
}

If a user sends JSON like

{
    "__proto__": {
        "isAdmin": true
    }
}

That pollutes the prototype, so all objects in your app will have obj.isAdmin === true. Imagine the implications for privilege checks!

1. Attacker Sends Polluted Data

// POST /api/update-user
{
    "name": "Mallory",
    "__proto__": {
        "isAdmin": true
    }
}

If the app merges this data naively, like

Object.assign(userObject, req.body); // Don't do this!

Or, the vulnerable Mongoose version does similar merging under the hood.

Now, every new object in your app thinks isAdmin is true

let check = {};
console.log(check.isAdmin); // true! Uh oh...

This breaks everything from permission checks to feature toggles—giving attackers admin access or crashing the app.

Read the official advisories

- GitHub Security Advisory – GHSA-8vx5-fp9r-xc46 (Mongoose)
- NIST NVD Entry for CVE-2023-3696

How Did Mongoose Fix It?

Starting in 7.3.4, Mongoose checks for keys like __proto__, prototype, and constructor before merging, and ignores or throws on them. That means malicious input won't infect every object in your process.

Patched pseudo code

function mergeSecure(target, source) {
    for (let key in source) {
        if (['__proto__', 'constructor', 'prototype'].includes(key)) continue;
        target[key] = source[key];
    }
}

Update Mongoose (and dependencies): npm install mongoose@latest

2. Sanitize user input: Never trust client data blindly. Consider libraries like lodash's _.mergeWith protection or validator.js.

Add tests to catch polluted prototypes or unexpected keys.

5. Read advisories, subscribe to security mailing lists—GitHub Security Advisories is a good start.

Here’s a quick demo script (safe for local test)

// test-proto.js
let userInput = JSON.parse('{"__proto__":{"admin":true}}');
let victim = {};
Object.assign(victim, userInput); // Simulates old merge
console.log(victim.admin);        // undefined
console.log(({}).admin);          // true! Uh oh

Run this, and see how EVERY {} gets admin: true after merge.

Final Thoughts & Recommendations

Prototype pollution bugs are subtle, but dangerous. Even if you trust your own code, a dependency (like Mongoose) can open a door for attackers. Update your Mongoose to 7.3.4 or later immediately. Sanitize all inputs!

Not sure you're safe? Audit your dependencies

npm audit

And remember: prototype pollution is a common class of bugs across the Node.js ecosystem. Stay alert and patch fast.

References

- CVE-2023-3696 – NVD
- Mongoose Security Advisory
- OWASP – Prototype Pollution
- How to Fix Prototype Pollution in Node.js

Timeline

Published on: 07/17/2023 01:15:00 UTC
Last modified on: 08/02/2023 17:30:00 UTC