Affected: npm: handlebars

## 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()`:

```javascript // 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

```javascript…