🍪 CookieMonster

CookieMonster is a command-line tool and API for decoding and modifying vulnerable session cookies from several different frameworks. It is designed to run in automation pipelines which must be able to efficiently process a large amount of these cookies to quickly discover vulnerabilities. Additionally, CookieMonster is extensible and can easily support new cookie formats.

It's worth emphasizing that CookieMonster finds vulnerabilities in users of frameworks, usually not in the frameworks themselves. These users can resolve vulnerabilities found via CookieMonster by configuring the framework to use a strong secret key.

Features

• Decodes and unsigns session cookies from Laravel, Django, Flask, Rack, and Express, and also handles raw JWTs.
• Rapidly evaluates cookies; ignores invalid and unsupported cookies, and quickly tests those that it can.
• Takes full advantage of Go's fast, native implementations for hash functions.
• Intelligently decodes URL-encoded and Base64-encoded cookies (i.e. the Base64 of a JWT) when the initial decoding fails.
• Supports many algorithms for HMAC-based decoders, even if the framework typically only uses one.
• Flexible base64-encoded wordlist format allows any sequence of bytes key to be added as an entry; ships with a reasonable default list.

Getting Started

To install CookieMonster, install Go and then install the CLI:

go install github.com/iangcarroll/cookiemonster/cmd/cookiemonster@latest

CookieMonster only needs two essentials: a cookie to try and unsign, and a wordlist to use. If you don't have a wordlist, CookieMonster ships with a default wordlist from the Flask-Unsign project. CookieMonster wordlists are a bit different; each line must be encoded with base64. This is because Python projects are especially liberal with inserting garbage bytes into these keys, and we need to be able to properly handle them.

An example of using the CLI:

% ./cookiemonster -cookie "gAJ9cQFYCgAAAHRlc3Rjb29raWVxAlgGAAAAd29ya2VkcQNzLg:1mgnkC:z5yDxzI06qYVAU3bkLaWYpADT4I"

🍪 CookieMonster 1.0.0
ℹ️ CookieMonster loaded the default wordlist; it has 38921 entries.
✅ Success! I discovered the key for this cookie; it is: changeme

Express support

CookieMonster is capable of supporting cookies signed with cookie-session, which is common with Express. However, it does several strange things that require care in order to use this tool. A common response from a cookie-session application looks like this:

set-cookie: session=eyJhbmltYWxzIjoibGlvbiJ9
set-cookie: session.sig=Vf2INocdJIqKWVfYGhXwPhQZNFI

In order to pass this into CookieMonster, you must include both the cookie name and the signature cookie. In this example, you would call CookieMonster like this: cookiemonster -cookie session=eyJhbmltYWxzIjoibGlvbiJ9^Vf2INocdJIqKWVfYGhXwPhQZNFI (note the delimiting ^ and the prefixed cookie name). The API accepts this same format in monster.NewCookie.

Resigning support

CookieMonster has limited support for resigning a cookie once it has been unsigned, with the -resign flag. This involves modifying the body of the cookie to match your input, and then re-computing the signature with the key we discovered. Currently, you can do this for Django-decoded cookies; ensure you pass the original cookie to -cookie, and pass -resign an unencoded string of text you'd like to be inside the cookie. CookieMonster will correctly encode your input and then resign the cookie.

API usage

CookieMonster exposes pkg/monster, which allows other applications to easily take advantage of it. This is much more performant than booting the CLI if you are testing many cookies. An example usage of it is below.

import (
    "github.com/iangcarroll/cookiemonster/pkg/monster"
)

var (
	//go:embed wordlists/my-wordlist.txt
	monsterWordlist string

	wl = monster.NewWordlist()
)

func init() {
	if err := wl.LoadFromString(monsterWordlist); err != nil {
        panic(err)
    }
}

func MonsterRun(cookie string) (success bool, err error) {
	c := monster.NewCookie(cookie)

	if !c.Decode() {
		return false, errors.New("could not decode")
	}

	if _, success := c.Unsign(wl, 100); !success {
		return false, errors.New("could not unsign")
	}

	return true, nil
}

Credits

CookieMonster is built with inspiration from several sources, and ships with the excellent Flask-Unsign wordlists.

• https://github.com/Paradoxis/Flask-Unsign
• https://github.com/nicksanders/rust-django-signing