Advent of CTF - Challenge 2



This challenge focuses on the way data is stored in the browser and the way it can be used to change the workings of a backend service.

The things you will learn are:

  • How cookies are stored in a browser
  • Identifying structures with Base64
  • URL Encoding
  • Manipulating JSON structures


Once the challenge has loaded a webpage is presented with a login form.

Figure 1: Login screen

A trained CTF player might try various advanced techniques, however the teaser below the login form says “Do not try too hard”. Entering any combination of username / password will get you into the system.

Figure 2: Logged in as guest

As this is only the 2nd challenge it is not expected that the challenge will be overly complex. After you login you will notice that a cookie has been set. A hint is given by the “Whoop Whoop” - @GevuldeCookie quote. You can view this in the DevTools (F12) or using a proxy like Burp. For now I will use the browser as the tool to examine web traffic.

Figure 3: Authenticated cookie in the devtools

The cookie is called “authenticated” and has a value that starts with eyJ. You will soon get familiar with identifying JSON datastructures in Base64 as they always start with the combination {" followed by some characters. That combination converts to eyJ in Base64.

As it is clear that it is a Base64 encoded string it can be copied over to CyberChef. You might notice, whily copying, that the string ends in %3D. This is the URL encoding of an = sign. We could replace this manually, but CyberChef has decoders for this purpose. Select the URL Decoder and From Base64 and drag them to the recipe column.

Figure 4: Decoding the string in CyberChef

The JSON structure is visible now. It reads {"guest":"true","admin":"false"}. You can try various combinations of true and false. In order to solve the challenge the guest needs to be set to false and admin to true. Remove the previously used decoders from the recipe and add the To Base64 encoder to create the payload.

Figure 5: Encode Base64 payload

Go to your browser, change the value of the cookie (double click on it), and reload the page. You will now be presented with the flag for this challenge.

Figure 6: Challenge flag

Go and grab your points in the CTFd dashboard. Also, don’t forget to grab the beautiful badge that you can share on your social media to promote the Advent of CTF!.

Figure 7: Badge for challenge 2

The challenge is solved.

Go back to the homepage.