Handlebars.js has Prototype Pollution Leading to XSS through Partial Template Injection
Date: 2026-03
Affected: npm/handlebars
Attack vector: xss
Category: vulnerability-disclosure
Severity: Medium
CVE: CVE-2026-33916
CWE: CWE-1321
CWE-79
CVSS: 4.7 (CVSS:3.1/AV:N/AC:H/PR:N/UI:R/S:C/C:L/I:L/A:N)

## 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


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