Outdated jQuery Library Allows Malicious Tampering with Web Page Behaviour

Vulnerability Explanation

The vulnerability exists in jQuery's deep merge function, jQuery.extend(true, {}, ...). When the true flag is passed to enable recursive (deep) merging, jQuery does not sanitize property names in the source object. If an attacker can supply an object containing an enumerable __proto__ property — for example, via JSON parsed from user input, URL parameters, or a form field — that property is recursively merged into Object.prototype. Because all JavaScript objects inherit from Object.prototype, this pollution propagates globally across the page, allowing the attacker to inject or overwrite properties on every object in the application.

Root Cause

jQuery.extend() performs a deep merge without checking whether the property being copied is __proto__, constructor, or prototype. The fix introduced in 3.4.0 adds an explicit guard that skips any property named __proto__ during the merge, preventing it from being applied to the base object prototype.

Technical Impact

An attacker who controls data that is deep-merged via jQuery.extend() can inject arbitrary properties into Object.prototype. This can lead to: property injection (adding unexpected properties to all objects), denial of service via JavaScript exceptions, logic bypass (overriding security-relevant properties checked elsewhere in the app), or in severe cases, remote code execution if the polluted property is used in a code execution path.

Severity Justification

CVSS 3.1 vector AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:L/A:N. Exploitation requires the attacker to control data that reaches the vulnerable jQuery.extend() call, which is an application-specific condition. Impact is limited to client-side logic tampering rather than direct server compromise.

Affected Components

• jQuery >= 1.1.4 and < 3.4.0

Remediation Steps

1. Upgrade jQuery to version 3.4.0 or later. This is the primary and recommended fix: `npm install jquery@latest` or update your CDN/script tag to reference 3.7.x or the current stable release.
2. If you cannot upgrade to jQuery 3.x (e.g., due to jQuery 1.x/2.x compatibility constraints), audit all usages of jQuery.extend(true, ...) in your codebase and ensure no user-controlled data reaches those calls unsanitized.
3. If using a CMS such as WordPress or Drupal, apply the latest CMS core update — both platforms have shipped patched versions of their bundled jQuery.
4. After upgrading, run the browser console verification snippet to confirm the fix is in place (see verification steps).
5. Update your package-lock.json or yarn.lock and audit transitive dependencies that may bundle their own copy of jQuery using `npm audit`.

Verification Steps

1. Open your site in a browser, open the Developer Tools console, and run: `console.log(jQuery.fn.jquery)` — confirm the output is 3.4.0 or higher.
2. Run the prototype pollution check in the browser console: `var t = jQuery.extend(true, {}, JSON.parse('{"__proto__":{"polluted":true}}')); console.log({}.polluted);` — the result should be `undefined` if the fix is applied.
3. Run `npm audit` in your project directory and confirm CVE-2019-11358 no longer appears in the output.
4. If using a CDN, inspect the page source or Network tab to confirm the jQuery script URL references version 3.4.0 or later.

Code Examples (html)

<script src="https://code.jquery.com/jquery-3.3.1.min.js"></script>

// Vulnerable usage — attacker-controlled JSON reaches jQuery.extend
var userInput = JSON.parse('{"__proto__": {"isAdmin": true}}');
jQuery.extend(true, {}, userInput);
console.log({}.isAdmin); // true — Object.prototype is now polluted

<script src="https://code.jquery.com/jquery-3.7.1.min.js"></script>

// Same call is now safe in jQuery >= 3.4.0
// The __proto__ property is silently skipped during deep merge
var userInput = JSON.parse('{"__proto__": {"isAdmin": true}}');
jQuery.extend(true, {}, userInput);
console.log({}.isAdmin); // undefined — prototype is clean

Best Practices

• Pin your jQuery version explicitly in package.json and review it during each dependency audit cycle.
• Never pass unsanitized user input (from URL params, form fields, or API responses) directly into jQuery.extend() or any deep-merge function.
• Consider using Object.freeze(Object.prototype) in your application bootstrap to make prototype pollution attempts fail loudly during development.
• Use `npm audit` or a software composition analysis (SCA) tool in your CI pipeline to catch known vulnerable library versions before they reach production.

References

• nvd.nist.gov/vuln/detail/CVE-2019-11358
• security.snyk.io/vuln/SNYK-JS-JQUERY-174006
• blog.jquery.com/2019/04/10/jquery-3-4-0-released/
• www.cvedetails.com/cve/CVE-2019-11358/