{"id":"GHSA-2qvq-rjwj-gvw9","revision":"e51da863c38e48fc9b1f94dfc308d3af","records":[{"revision":"e51da863c38e48fc9b1f94dfc308d3af","observedAt":"2026-10-24T13:17:17.793+00:00","statement":{"source":"osv","id":"GHSA-2qvq-rjwj-gvw9","aliases":["CVE-2026-33916"],"title":"Handlebars.js has Prototype Pollution Leading to XSS through Partial Template Injection","description":"## 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 = '<img src=x onerror=\"alert(document.domain)\">';\n\n// Step 2: Normal template that references a partial\nconst template = Handlebars.compile('<div>Welcome! {{> widget}}</div>');\n\n// Step 3: Render — XSS payload injected unescaped\nconst output = template({});\n// Output: <div>Welcome! <img src=x onerror=\"alert(document.domain)\"></div>\n```\n\n> 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.","state":0,"publishedAt":"2026-03-26T22:20:51+00:00","modifiedAt":"2026-09-10T03:50:36.3816747+00:00","affected":[{"ecosystem":"npm","name":"handlebars","ranges":[{"type":"SEMVER","events":[{"kind":"introduced","version":"4.0.0"},{"kind":"fixed","version":"4.7.9"}]}],"versions":[],"rawVersionEvidence":null}],"metrics":[{"system":"CVSS_V3","vector":"CVSS:3.1/AV:N/AC:H/PR:N/UI:R/S:C/C:L/I:L/A:N","score":null,"severity":null},{"system":"Publisher rating","vector":null,"score":null,"severity":"MODERATE"}],"references":["https://github.com/handlebars-lang/handlebars.js/security/advisories/GHSA-2qvq-rjwj-gvw9","https://nvd.nist.gov/vuln/detail/CVE-2021-23369","https://nvd.nist.gov/vuln/detail/CVE-2021-23383","https://nvd.nist.gov/vuln/detail/CVE-2026-33916","https://github.com/handlebars-lang/handlebars.js/commit/68d8df5a88e0a26fe9e6084c5c6aaebe67b07da2","https://github.com/handlebars-lang/handlebars.js","https://github.com/handlebars-lang/handlebars.js/releases/tag/v4.7.9"],"sourceUrl":"https://github.com/advisories/GHSA-2qvq-rjwj-gvw9","license":"GitHub Advisory Database · CC-BY-4.0","key":"osv:GHSA-2qvq-rjwj-gvw9","identifiers":["GHSA-2qvq-rjwj-gvw9","CVE-2026-33916"]},"retained":false,"historicalAffected":[]}],"coverage":[]}