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:

1. The target application must be vulnerable to prototype pollution (e.g. via qs, minimist, or any querystring/JSON merge sink).
2. 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.