# CVE-2026-33916: Handlebars.js prototype pollution XSS via partial template injection

Source: https://junglewise.ai/threats/cve-2026-33916-handlebars-js-prototype-pollution-in-resolvepartial-leads-to-xss  
Publisher: Junglewise Threat Intelligence, https://junglewise.ai/threats  
Published: 2026-03-27. Updated: 2026-09-25  
License: CC BY 4.0 with a link back

CVE-2026-33916 · Severity: medium · CVSS 4.7 · Published 2026-03-27

Handlebars.js is a widely-used template engine for rendering dynamic content in web applications. A flaw in how it resolves partial templates allows an attacker to inject arbitrary HTML/JavaScript into rendered pages when the application is vulnerable to prototype pollution. An attacker can pollute JavaScript's Object.prototype and cause Handlebars to use a malicious string as a template partial, leading to cross-site scripting (XSS) attacks that steal user data or perform actions on their behalf.

## Quick answers

**Is CVE-2026-33916 exploited in the wild?** No. Junglewise Threat Intelligence has no report of CVE-2026-33916 being exploited in the wild.

**How severe is CVE-2026-33916?** Medium severity, with a CVSS score of 4.7, published 27 March 2026.

**Which products and versions are affected?** handlebars-lang handlebars 4.0.0 to 4.7.8.

**Is there a fix for CVE-2026-33916?** A patch was published on 2026-03-26: Version 4.7.9 released with security fixes Junglewise finds every place you run it, drafts the upgrade and applies it once you approve.

**How is CVE-2026-33916 attacked?** It is attacked over the network, in a software library.

**Am I affected by CVE-2026-33916?** Check whether you run Handlebarsjs Handlebars. Junglewise does that check for you around the clock: connect your code, cloud and domains, and it matches new vulnerabilities like this one against what you actually run and tells you when one reaches you.

## Technical details

The vulnerability exists in lib/handlebars/runtime.js within resolvePartial() and invokePartial() functions. The code performs unguarded bracket-access property lookup (partial = options.partials[options.name]) that traverses the prototype chain without checking hasOwnProperty(). When Object.prototype has been seeded with a key matching a partial reference in a template (e.g., "widget"), the lookup succeeds and returns the polluted string value. Critically, this polluted content is rendered without HTML escaping, resulting in unescaped XSS. The attack requires two preconditions: (1) the target application must be vulnerable to prototype pollution via a separate vector such as qs, minimist, or JSON merge operations, and (2) the attacker must know or guess a partial name referenced in the template. Although the runtime prints a prototype access warning, the partial is still resolved and rendered, contradicting the documented security model. A patch is available in version 4.7.9.

## Affected products

- handlebars-lang handlebars 4.0.0 to 4.7.8

## Identifiers

- CVE-2026-33916
- GHSA-2qvq-rjwj-gvw9

## Timeline

- 2026-03-26: disclosed: GitHub Security Advisory GHSA-2qvq-rjwj-gvw9 published
- 2026-03-26: patched: Version 4.7.9 released with security fixes

## References

- [https://github.com/handlebars-lang/handlebars.js/security/advisories/GHSA-2qvq-rjwj-gvw9](https://github.com/handlebars-lang/handlebars.js/security/advisories/GHSA-2qvq-rjwj-gvw9)
- [https://github.com/handlebars-lang/handlebars.js/commit/68d8df5a88e0a26fe9e6084c5c6aaebe67b07da2](https://github.com/handlebars-lang/handlebars.js/commit/68d8df5a88e0a26fe9e6084c5c6aaebe67b07da2)
- [https://github.com/handlebars-lang/handlebars.js](https://github.com/handlebars-lang/handlebars.js)
- [https://github.com/handlebars-lang/handlebars.js/releases/tag/v4.7.9](https://github.com/handlebars-lang/handlebars.js/releases/tag/v4.7.9)