[VulnHub] BlueMoon Walkthrough

A walkthrough for the BlueMoon virtual machine, available from VulnHub.

[VulnHub] BlueMoon Walkthrough

Difficulty level: Easy
Aim: Get root access and capture x3 flags.
Author: Kirthik

VulnHub: http://www.vulnhub.com/entry/bluemoon-2021,679/

Information Gathering

arp-scan can be used to discover the target IP:

sudo arp-scan -l --interface=eth1



Target locked! We can now use nmap to discover open ports and services:

nmap -sC -sV -vvv

nmap has found these ports and services:

  • port 21/tcp - FTP - vsftpd 3.0.3
  • port 22/tcp - SSH - OpenSSH 7.9p1
  • port 80/tcp - HTTP - Apache httpd 2.4.38


The HTTP service seems a good place to start, so let's fire up our browser and take a look:

Checking for robots.txt and viewing the source-code doesn't reveal anything, so let's fire up gobuster to brute-force some directories:

gobuster dir -u -w /usr/share/wordlists/dirbuster/directory-list-2.3-medium.txt

A directory named hidden_text has been found, which contains the following:

The link on this page takes us to a QR code image:

We can download a local copy of this and try decoding it using zbar-tools (run sudo apt-get install zbar-tools if you do not already have this installed):

zbarimg .QR_C0d3.png

Awesome! We've got some FTP credentials, let's see what we can find!

Logging into the FTP service shows there are two files - information.txt and p_lists.txt - which we can download:

[email protected]
ls -la
get information.txt
get p_lists.txt

information.txt contains a possible username of robin.

p_files.txt is a password list.

Gaining Access

We can run this information through Hydra to brute-force the SSH service:

hydra -l robin -P p_lists.txt ssh://

We can now login to the SSH service as robin:

ssh [email protected]

The first flag is located in the home directory of robin:

The project folder within this directory contains a bash script named feedback.sh:

This is a simple script that will execute commands entered into the $feedback variable.

Checking the sudo permissions shows that robin is able to run this script as the user jerry:

Privilege Escalation

By executing this script as the user jerry we can pass in a command that will launch a bash shell as this user:

sudo -u jerry /home/robin/project/feedback.sh

This shell can be upgraded to a fully interactive TTY shell by running:

python -c 'import pty; pty.spawn("/bin/sh")'

The second flag can then be found in the home directory of jerry:

Further enumeration shows that jerry is in the docker group.

The docker page on GTFOBins gives us the information that will allow us to mount the root directory within a docker container and spawn a root shell:

docker run -v /:/mnt --rm -it alpine chroot /mnt sh

From here we can grab the final flag within the /root directory:

Please feel free to contact me via Twitter and thanks for reading.