{"id":"GHSA-2qvq-rjwj-gvw9","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 = '\u003cimg src=x onerror=\"alert(document.domain)\"\u003e';\n\n// Step 2: Normal template that references a partial\nconst template = Handlebars.compile('\u003cdiv\u003eWelcome! {{\u003e widget}}\u003c/div\u003e');\n\n// Step 3: Render — XSS payload injected unescaped\nconst output = template({});\n// Output: \u003cdiv\u003eWelcome! \u003cimg src=x onerror=\"alert(document.domain)\"\u003e\u003c/div\u003e\n```\n\n\u003e The runtime prints a prototype access warning claiming \"access has been denied,\" but the partial still resolves and returns the polluted value.\n\n## Workarounds\n\n- Apply `Object.freeze(Object.prototype)` early in application startup to prevent prototype  pollution. Note: this may break other libraries.\n- Use the Handlebars runtime-only build (`handlebars/runtime`), which does not compile templates  and reduces the attack surface.","aliases":["CVE-2026-33916"],"modified":"2026-07-17T21:08:03.314535124Z","published":"2026-03-26T22:20:51Z","database_specific":{"github_reviewed_at":"2026-03-26T22:20:51Z","nvd_published_at":"2026-03-27T21:17:27Z","cwe_ids":["CWE-1321","CWE-79"],"severity":"MODERATE","github_reviewed":true},"references":[{"type":"WEB","url":"https://github.com/handlebars-lang/handlebars.js/security/advisories/GHSA-2qvq-rjwj-gvw9"},{"type":"ADVISORY","url":"https://nvd.nist.gov/vuln/detail/CVE-2021-23369"},{"type":"ADVISORY","url":"https://nvd.nist.gov/vuln/detail/CVE-2021-23383"},{"type":"ADVISORY","url":"https://nvd.nist.gov/vuln/detail/CVE-2026-33916"},{"type":"WEB","url":"https://github.com/handlebars-lang/handlebars.js/commit/68d8df5a88e0a26fe9e6084c5c6aaebe67b07da2"},{"type":"PACKAGE","url":"https://github.com/handlebars-lang/handlebars.js"},{"type":"WEB","url":"https://github.com/handlebars-lang/handlebars.js/releases/tag/v4.7.9"}],"affected":[{"package":{"name":"handlebars","ecosystem":"npm","purl":"pkg:npm/handlebars"},"ranges":[{"type":"SEMVER","events":[{"introduced":"4.0.0"},{"fixed":"4.7.9"}]}],"database_specific":{"source":"https://github.com/github/advisory-database/blob/main/advisories/github-reviewed/2026/03/GHSA-2qvq-rjwj-gvw9/GHSA-2qvq-rjwj-gvw9.json"}}],"schema_version":"1.9.0","severity":[{"type":"CVSS_V3","score":"CVSS:3.1/AV:N/AC:H/PR:N/UI:R/S:C/C:L/I:L/A:N"}]}