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16 August 2019

RedpwnCTF - Performing a prototype pollution attack

by Xh4H


Writeup for RedpwnCTF blueprint challenge which involved prototype pollution attack.

First of all, what is a prototype pollution attack? As the name says, it is about polluting the prototype of a base object which can allow us to modify any existing object and get RCE.

In JavaScript, an Array is a base object. These base object have a property which is constructor and inside of this there is prototype. If someone managed to edit this property, any Array would be affected.

Challenge description

All the haxors are using blueprint. You created a blueprint with the flag in it, but the military-grade security of blueprint won’t let you get it!



We are given two files, package.json and blueprint.js. We look at the package:

  "name": "blueprint",
  "version": "0.0.0",
  "private": true,
  "dependencies": {
    "lodash": "4.17.11",
    "mustache": "^3.0.1",
    "raw-body": "^2.4.1"

Here we can see the versions of the dependencies that the blueprint.js is using. Will be useful for the future.

After examining blueprint.js, which is an HTTP server, I find a strange comment in the code:

It is using _.defaultsDeep, and _ is declared to be as lodash const _ = require('lodash')

A google search shows that lodash has some serious vulnerability affecting all the versions prior to 4.17.11 including this version as well. You can read about this finding here: Snyk research team discovers severe prototype pollution security vulnerabilities affecting all versions of lodash. Lodash package maintainer already fixed this as of today, but many projects are using old versions of lodash, making them vulnerable to this attack. Let’s start our attack.

This challenge allows us to create blueprints with a content and they can either be private or public. Every time we access the page without cookies, we get assigned a user_id and a private blueprint is created which contains the flag, but it is set as private. / endpoint allows us to list all existing public blueprints.

makeId function creates a 16 chars long random character, we are not supposed to bruteforce this.

I set up a basic environment with lodash. At my first attempt I couldn’t make this vulnerability work because when setting it up I installed lodash like this: npm i lodash, which downloaded the latest version which had fixed this vulnerability. So I removed it npm remove lodash and installed a vulnerable version, in fact, the one this server is using: npm i [email protected]. Now it worked :D

In the following screenshot you can see a piece of code I wrote as a Proof of Concept (PoC) where I pollute Array’s prototype. As you can see, accessing the public property of a previously declared Array and as well a new Array declared after the prototype pollution will both return true, seeing that our test was successful it is time to get our flag.

In other words, any Array has now the public property set to true.

Time to get the flag

Open up BurpSuite and grab the request of creating a new blueprint, send the payload we previously tested.

Go to / and boom, flag.

First blueprint is the “private” flag and second is the new created blueprint which we used to pollute the prototype :)


Why does this work and mitigations

Lodash module, and many other npm modules (this pdf analyzes deeply this threat and lists vulnerable modules) that perform deep Array copies can allow an attacker to modify object prototypes and take control of that object’s properties.

If you are using lodash, simply update it. If you are performing deep copies or using any affected module, this can be fixed rather easily. The ECMAScript standard version 5 introduced a very interesting set of functionality to the JavaScript language, Object.freeze. When that function is called on an object, any further modification on that object will silently fail. Since the prototype of Object is an object, it’s possible to freeze it. Doing so will mitigate almost all the exploitable case.

1. Object.freeze(Object.prototype);
2. Object.freeze(Object);
3. ({}).__proto__.test = 123;
4. ({}).test; // this will be undefined

Thanks for reading :)





tags: javascript - ctf - redpwnctf - blueprint - prototype - pollution - mitigations