bourne

Introduction

Consider this:

> const a = '{"__proto__":{ "b":5}}'; '{"__proto__":{ "b":5}}' > const b = JSON.parse(a); { __proto__: { b: 5 } } > b.b; undefined > const c = Object.assign({}, b); {} > c.b 5 

The problem is that JSON.parse() retains the __proto__ property as a plain object key. By
itself, this is not a security issue. However, as soon as that object is assigned to another or
iterated on and values copied, the __proto__ property leaks and becomes the object's prototype.