Convict has Prototype Pollution via startsWith() function
Summary
A prototype pollution vulnerability exists in the latest version of the convict npm package (6.2.4). Despite a previous fix that attempted to mitigate prototype pollution by checking whether user input started with a forbidden key, it is still possible to polluteObject.prototype via a crafted input using String.prototype. Details
The vulnerability resides in line 564 of https://github.com/mozilla/node-convict/blob/master/packages/convict/src/main.js wherestartsWith() function is used to check whether user provided input contain forbidden strings. PoC
Steps to reproduce
- Install latest version of convict using
npm installor cloning from git - Run the following code snippet:
String.prototype.startsWith = () => false;
const convict = require('convict');
let obj = {};
const config = convict(obj);
console.log({}.polluted);
config.set('constructor.prototype.polluted', 'yes');
console.log({}.polluted); // prints yes -> the patch is bypassed and prototype pollution occurredExpected behavior
Prototype pollution should be prevented and {} should not gain new properties. This should be printed on the console:undefined
undefined OR throw an ErrorActual behavior
Object.prototype is polluted
This is printed on the console:
undefined
yesImpact
This is a prototype pollution vulnerability, which can have severe security implications depending on how convict is used by downstream applications. Any application that processes attacker-controlled input usingconvict.set may be affected.
It could potentially lead to the following problems:
- Authentication bypass
- Denial of service
- Remote code execution (if polluted property is passed to sinks like eval or child_process)