Personal-Website/content/writeups/adventofctf/2020/challenge_3.md

+++ author = "Maik de Kruif" title = "Challenge 3" subtitle = "Challenge 3 - AdventOfCTF" date = 2020-12-03T09:29:12+01:00 description = "A writeup for challenge 3 of AdventOfCTF." cover = "img/writeups/adventofctf/2020/4f5cc0afbb9e7ec6a57cdd68a92b9213.png" tags = [ "AdventOfCTF", "challenge", "ctf", "hacking", "writeup", "web", "javascript", ] categories = [ "ctf", "writeups", "hacking", ] +++

• Points: 300

Description

For this challenge you will, again, need to bypass the login mechanism.

Visit https://03.adventofctf.com to start the challenge.

Solution

When opening the website we're provided with a login form. If we fill in the form with random data, nothing happens. Usually a website will do a POST request to a URL when submitting a form, but even that didn't happen. So my guess is that there is some javascript in play.

Let's open the source and take a look at the form. Here we can see that when the form is submitted, a javascript function called checkPass() is called.

<form action="/index.php" onsubmit="checkPass(); return false"></form>

To find this funtion, enter checkPass in the devtools console and click on the three dots at the bottom of the output.

function checkPass() {
  var username = document.getElementById("username").value;
  var password = document.getElementById("password").value;

  var novi = "-NOVI";

  if (password == btoa(username + novi)) {
    window.setTimeout(function () {
      window.location.assign(
        "inde" + "x.php?username=" + username + "&password=" + password
      );
    }, 500);
  }
}

We can see there is a check which checks if password is equal to btoa(username + novi). But what is btoa? According to w3schools:

The btoa() method encodes a string in base-64.

To get the value of what the password should be, we have to know the output of btoa(username + novi). Above this check, we see the novi variable is set to '-NOVI'. Now, we go to the devtools console and generate the password. In the console, enter btoa("a" + "-NOVI"). This returns "YS1OT1ZJ", so lets try that combination. I used the username "a". If we enter this combination in the form, we get redirected to a page with the flag.

This flag is NOVI{javascript_is_not_s@fe}.

This flag can then be submitted for the challenge.