Handlebars.js has Prototype Pollution Leading to XSS through Partial Template Injection
Summary
resolvePartial() in the Handlebars runtime resolves partial names via a plain property lookup on options.partials without guarding against prototype-chain traversal. When Object.prototype has been polluted with a string value whose key matches a partial reference in a template, the polluted string is used as the partial body and rendered without HTML escaping, resulting in reflected or stored XSS.
Description
The root cause is in lib/handlebars/runtime.js inside resolvePartial() and invokePartial():
// Vulnerable: plain bracket access traverses Object.prototype
partial = options.partials[options.name];hasOwnProperty is never checked, so if Object.prototype has been seeded with a key whose name matches a partial reference in the template (e.g. widget), the lookup succeeds and the polluted string is returned. The runtime emits a prototype-access warning, but the partial is still resolved and its content is inserted into the rendered output unescaped. This contradicts the documented security model and is distinct from CVE-2021-23369 and CVE-2021-23383, which addressed data property access rather than partial template resolution.
Prerequisites for exploitation:
- The target application must be vulnerable to prototype pollution (e.g. via
qs,minimist, or any querystring/JSON merge sink). - The attacker must know or guess the name of a partial reference used in a template.
Proof of Concept
const Handlebars = require('handlebars');
// Step 1: Prototype pollution (via qs, minimist, or another vector)
Object.prototype.widget = '<img src=x onerror="alert(document.domain)">';
// Step 2: Normal template that references a partial
const template = Handlebars.compile('<div>Welcome! {{> widget}}</div>');
// Step 3: Render — XSS payload injected unescaped
const output = template({});
// Output: <div>Welcome! <img src=x onerror="alert(document.domain)"></div>The runtime prints a prototype access warning claiming "access has been denied," but the partial still resolves and returns the polluted value.
Workarounds
- Apply
Object.freeze(Object.prototype)early in application startup to prevent prototype pollution. Note: this may break other libraries. - Use the Handlebars runtime-only build (
handlebars/runtime), which does not compile templates and reduces the attack surface.
Czego dotyczy
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| Platforma | Paczka | Podatne wersje | Poprawka w wersji |
|---|---|---|---|
| Paczka npm | handlebars | >= 4.0.0, < 4.7.9 | 4.7.9 |
Odnośniki
- https://github.com/handlebars-lang/handlebars.js/security/advisories/GHSA-2qvq-rjwj-gvw9
- https://nvd.nist.gov/vuln/detail/CVE-2021-23369
- https://nvd.nist.gov/vuln/detail/CVE-2021-23383
- https://nvd.nist.gov/vuln/detail/CVE-2026-33916
- https://github.com/handlebars-lang/handlebars.js/commit/68d8df5a88e0a26fe9e6084c5c6aaebe67b07da2
- https://github.com/handlebars-lang/handlebars.js
- https://github.com/handlebars-lang/handlebars.js/releases/tag/v4.7.9
- Wektor CVSS
CVSS:3.1/AV:N/AC:H/PR:N/UI:R/S:C/C:L/I:L/A:N- Słabość (CWE)
CWE-1321, CWE-79- Platforma
- npm
- Paczka
handlebars- Ujawniono
- 26.03.2026
- Aktualizacja
- 11.10.2026 10:22
Źródła i licencje
OSV / GitHub Advisory Database - GHSA-2qvq-rjwj-gvw9 - CC BY 4.0
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