CVE-2026-33916: Handlebars.js Prototype Pollution Leading to XSS through Partial Template Injection
Summary
CVE-2026-33916: Handlebars.js has Prototype Pollution Leading to XSS through Partial Template Injection. Affects handlebars versions >= 4.0.0, < 4.7.9. Fixed in 4.7.9. How to fix ↓
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Background
Handlebars.js is a popular templating engine for JavaScript that allows developers to create dynamic HTML content by embedding expressions within templates. It is widely used in web applications to separate HTML generation from JavaScript logic. However, it has a vulnerability that allows for prototype pollution, which can lead to cross-site scripting (XSS) attacks if not addressed.
Vulnerability Detail
The vulnerability arises from the resolvePartial() function in the Handlebars runtime, which resolves partial names through a direct property lookup on options.partials without checking for prototype-chain traversal. If an attacker can manipulate Object.prototype to include a key that matches a partial reference in a template, the application may render a polluted string as the partial body without HTML escaping. This can result in reflected or stored XSS, allowing attackers to execute arbitrary JavaScript in the context of the user's browser.
How To Fix
Latest published version: 4.7.10 as of 22 Oct 2026
npm install handlebars@4.7.10Upgrading to it may include breaking changes; the version below is the minimum fixed release for this vulnerability.
If you need the minimum change
4.7.9
This is the first release that contains the fix.
npm install handlebars@4.7.9Tip — Not sure which version your app runs? Vulert can match this fix to it. Scan free →
Workaround
As a temporary workaround, you can apply Object.freeze(Object.prototype) early in your application startup to prevent prototype pollution. However, be aware that this may break compatibility with other libraries that rely on modifying the prototype.
Conclusion
The Handlebars.js vulnerability (CVE-2026-33916) poses a significant risk due to its potential for prototype pollution and subsequent XSS attacks. It is crucial for developers using Handlebars to update to the patched version 4.7.9 or later and consider implementing the provided workarounds to mitigate the risk until they can update.
See every vulnerability recorded for handlebars →
FAQ
Prototype pollution is a security vulnerability that allows an attacker to modify the prototype of built-in objects in JavaScript, potentially leading to unexpected behavior or security issues in applications.
An attacker can exploit this vulnerability by injecting a key into Object.prototype that matches a partial reference in a Handlebars template, allowing them to execute arbitrary JavaScript code.
The consequences include the potential for cross-site scripting (XSS) attacks, where an attacker can execute malicious scripts in the context of a user's browser, leading to data theft or session hijacking.
You can check if your application is affected by using Vulert, which monitors open-source vulnerabilities in your software. Visit Vulert Scanner to analyze your package-lock.json without needing to sign up.
The CVSS score of this vulnerability is 4.7, indicating a moderate severity level.
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