CVE-2026-33916: Handlebars Cross-Site Scripting Vulnerability

{
  "schema_version" : "1.4.0",
  "id" : "GHSA-2qvq-rjwj-gvw9",
  "modified" : "2026-03-27T21:52:02Z",
  "published" : "2026-03-26T22:20:51Z",
  "aliases" : [ "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 = '<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.",
  "severity" : [ {
    "type" : "CVSS_V3",
    "score" : "CVSS:3.1/AV:N/AC:H/PR:N/UI:R/S:C/C:L/I:L/A:N"
  } ],
  "affected" : [ {
    "package" : {
      "ecosystem" : "npm",
      "name" : "handlebars"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "4.0.0"
      }, {
        "fixed" : "4.7.9"
      } ]
    } ]
  } ],
  "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"
  } ],
  "database_specific" : {
    "cwe_ids" : [ "CWE-1321", "CWE-79" ],
    "severity" : "MODERATE",
    "github_reviewed" : true,
    "github_reviewed_at" : "2026-03-26T22:20:51Z",
    "nvd_published_at" : "2026-03-27T21:17:27Z"
  }
}