{"id":"CVE-2026-33916","aliases":["GHSA-2qvq-rjwj-gvw9"],"url":"https://o3.security/vulnerability/CVE-2026-33916","summary":"Handlebars.js has Prototype Pollution Leading to XSS through Partial Template Injection","details":"## Summary\n\n`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.\n\n## Description\n\nThe root cause is in `lib/handlebars/runtime.js` inside `resolvePartial()` and `invokePartial()`:\n\n```javascript\n// Vulnerable: plain bracket access traverses Object.prototype\npartial = options.partials[options.name];\n```\n\n`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.\n\n**Prerequisites for exploitation:**\n1. The target application must be vulnerable to prototype pollution (e.g. via `qs`, `minimist`, or\n any querystring/JSON merge sink).\n2. The attacker must know or guess the name of a partial reference used in a template.\n\n## Proof of Concept\n\n```javascript\nconst Handlebars = require('handlebars');\n\n// Step 1: Prototype pollution (via qs, minimist, or another vector)\nObject.prototype.widget = '
';\n\n// Step 2: Normal template that references a partial\nconst template = Handlebars.compile('Welcome! {{> widget}}
');\n\n// Step 3: Render — XSS payload injected unescaped\nconst output = template({});\n// Output: Welcome!