{
  "affected": [
    {
      "package": {
        "ecosystem": "npm",
        "name": "handlebars"
      },
      "ranges": [
        {
          "events": [
            {
              "introduced": "4.0.0"
            },
            {
              "fixed": "4.7.9"
            }
          ],
          "type": "ECOSYSTEM"
        }
      ]
    }
  ],
  "aliases": [
    "CVE-2026-33916"
  ],
  "database_specific": {
    "cwe_ids": [
      "CWE-1321",
      "CWE-79"
    ],
    "github_reviewed": true,
    "github_reviewed_at": "2026-03-26T22:20:51Z",
    "nvd_published_at": "2026-03-27T21:17:27Z",
    "severity": "MODERATE"
  },
  "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(\u0027handlebars\u0027);\n\n// Step 1: Prototype pollution (via qs, minimist, or another vector)\nObject.prototype.widget = \u0027\u003cimg src=x onerror=\"alert(document.domain)\"\u003e\u0027;\n\n// Step 2: Normal template that references a partial\nconst template = Handlebars.compile(\u0027\u003cdiv\u003eWelcome! {{\u003e widget}}\u003c/div\u003e\u0027);\n\n// Step 3: Render \u2014 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.",
  "id": "GHSA-2qvq-rjwj-gvw9",
  "modified": "2026-03-27T21:52:02Z",
  "published": "2026-03-26T22:20:51Z",
  "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"
    }
  ],
  "schema_version": "1.4.0",
  "severity": [
    {
      "score": "CVSS:3.1/AV:N/AC:H/PR:N/UI:R/S:C/C:L/I:L/A:N",
      "type": "CVSS_V3"
    }
  ],
  "summary": "Handlebars.js has Prototype Pollution Leading to XSS through Partial Template Injection"
}

{
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  "descriptions": [
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      "value": "Handlebars provides the power necessary to let users build semantic templates. In versions 4.0.0 through 4.7.8, `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. Version 4.7.9 fixes the issue. Some workarounds are available. Apply `Object.freeze(Object.prototype)` early in application startup to prevent prototype  pollution. Note: this may break other libraries, and/or use the Handlebars runtime-only build (`handlebars/runtime`), which does not compile templates  and reduces the attack surface."
    }
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  "id": "CVE-2026-33916",
  "lastModified": "2026-03-31T17:48:27.727",
  "metrics": {
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          "availabilityImpact": "NONE",
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          "confidentialityImpact": "LOW",
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      "source": "security-advisories@github.com",
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        "text": "To determine the support lifecycle for your software, see the Microsoft Support Lifecycle: https://support.microsoft.com/lifecycle",
        "title": "Additional Resources"
      },
      {
        "category": "legal_disclaimer",
        "text": "The information provided in the Microsoft Knowledge Base is provided \\\"as is\\\" without warranty of any kind. Microsoft disclaims all warranties, either express or implied, including the warranties of merchantability and fitness for a particular purpose. In no event shall Microsoft Corporation or its suppliers be liable for any damages whatsoever including direct, indirect, incidental, consequential, loss of business profits or special damages, even if Microsoft Corporation or its suppliers have been advised of the possibility of such damages. Some states do not allow the exclusion or limitation of liability for consequential or incidental damages so the foregoing limitation may not apply.",
        "title": "Disclaimer"
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        "summary": "Common Vulnerability Scoring System",
        "url": "https://www.first.org/cvss"
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    ],
    "title": "Handlebars.js has Prototype Pollution Leading to XSS through Partial Template Injection",
    "tracking": {
      "current_release_date": "2026-03-31T01:03:13.000Z",
      "generator": {
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          "text": "GitHub_M",
          "title": "Assigning CNA"
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      "title": "Handlebars.js has Prototype Pollution Leading to XSS through Partial Template Injection"
    }
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}