Anudeep Reddy

CTF.live - Secret in Claim

Cyber Security
Monday, August 3, 2020

Anudeep Reddy

This post is a walkthrough of a lab from ctflive. You can find this lab here, First give it a try yourself before going through this post. It's a JWT based challenge. For those of you who don't know what a JWT is. don't worry there will be a short introduction about that in this post.

What is JWT🤔?

JSON Web Tokens are an open, industry standard RFC 7519 method for representing claims securely between two parties. (source: https://jwt.io). What does this even mean🤷‍♀️?.
Let us have a look at one of the best explanations i read so far and it's by Kasey Speakman. I am dropping his dev profile here, do drop a heart on his actual comment that is linked below.

Let's get into the actual working now

A JWT is of the format xxxxxxxxx.yyyyyyyyyyy.zzzzzzzzzzzz.
  • x's specify the algorithm used to sign the JWT in base64.
  • y's contain the claims in base64.
  • z's are the signature that is generated with xxxxxxxxx.yyyyyyyyyyy as the data.
The Algorithms that are generally used to sign JWT includes:
  • HS256 (Symmetric)
  • RS256 (Asymmetric)

Where are these used?

As far as our lab is concerned, we use it to authenticate users to a website. So when a user logins to a website the website will issue a JWT with the claims of who the user is and any additional information. This JWT is sent back to the website in the subsequent requests made by the user. The JWT is first verified to check if the signature is legit, it's basically done by again signing the xxxxxxxxx.yyyyyyyyyyy of the token and checking if it matches with the signature that is sent as a part of the JWT. If they are equal then the server can trust the claims that come with the JWT. If any user tries to tamper with the claims in the JWT(the data in yyyyyyyyyyyy part) then the signature won't match and in an ideal condition the website should through an unauthorized message.

Now the Lab

Mission 💻

Retrieve the secret information present in the token payload!
We are given with a CMS to interact with. The Lab also gives you with the username and password to login to the user account.

Let's start

Starting the lab you will be given a virtual environment where the CMS is hosted in your local network and you have a machine with all the tools you will need preinstalled.
The challenge page also gives you few instruction on how to access the CMS that is hosted in your local environment.
It says the CMS runs on port 1337 and since I have already worked with a CMS that works on that port, I knew it was strapi already.
Now let us try to login with the credentials given. Strapi currently manages admin and end users separately. I assume that the credentials given to us are of the end user. So we can't access the strapi admin page with these credentials. All we need is the JWT because out flag is hidden these as the name of the challege suggests.
First we need to find the IP address on which the CMS is hosted. For that run ifconfig in your console to find your IP and then follow the instructions of the challenge to find the IP of the CMS.
In my case my IP was and that of the CMS was It is given in the challenge on what is the auth endpoint and the parameters that it accepts.
Let us use curl to send the request. I ran the following command to send a post request to the CMS and retrieve the JWT from the response.
Now copy that JWT from the console and head over to jwt.io which will decode the JWT for us.

Voila, there's our flag🎉.


Sometimes developers might end up sending critical information in the JWT. Make sure you check for such information during bug bounty or when you are building your own application.