Hack the Lin.Security VM (Boot to Root)

As we know how some weak misconfiguration sudo rights can lead to root privilege escalation and today I am going to solve the CTF “Lin. Security – Vulnhub” which is a design on weak sudo right permissions for beginners to test their skill set through this VM.

This lab has been designed by the researcher to help us understand, how certain built-in applications and services if misconfigured, may be abused by an attacker. Here an up-to-date Ubuntu distro (18.04 LTS) suffers from a number of vulnerabilities that allow a user to escalate to root on the box.

You can download it from here: https://www.vulnhub.com/entry/linsecurity-1,244/

Task: Get root shell for ALL Users (Bob, Susan, Peter)

Level: Beginner to Advance

Penetrating Methodologies

Escalate root shell for Bob

  • Via sudo right
  • Via wildcard Injection
  • Via SUID Binaries

Escalate root shell for Susan

  • Via SUID Binaries

Escalate root shell for Peter

  • Via Network file sharing (NFS)

Note: Kindly follow the reference link given in the article for details, because this machine vulnerable to a bunch of services and reference link holds detail of each Privilege Escalation Techniques.

Escalate root shell for Bob

1st Procedures

  • Login using ssh credential
  • Check sudoers list for sudo permissions
  • Get root shell via system binaries having sudo permission

Let’s move now to find ways to root this lab!!

The first thing before doing any of the CTF is to read the instructions carefully before starting your vulnerability assessment and save your precious time.

The writer of the lab has given a hint in terms of user-id & password of one of the users.

It is “To get started you can log onto the host with the credentials: bob/secret“ and the IP of my lab is 192.168.1.104.

I used the port 22 (SSH) to login into the machine with given credentials: bob/secret

After ‘logging in’ I tried to check the sudo rights for user bob:

The next prompt asked for the root password and I tried the same credentials “secret” and it worked!! I can see all the permissions which bob has and now I can easily root the machine using any of these permitted commands.

Visit given below link for complete details:

https://gtfobins.github.io/

https://www.hackingarticles.in/linux-privilege-escalation-using-exploiting-sudo-rights/

As you can observe that we had escalated root shell when sudo have rights for all types of the shell such as ksh, zsh, bash and so on or for editors or for other programs such as pico, vi, Perl, scp, find, less and so on. It goes in a privileged environment with elevated privileges to access the file system or elevate root shell if sudo permission is enabled.

You can use the following list of command to get a root shell:

Get root shell via FTP, Expect, Socat

With this, we can spawn the interactive system shell and can get away from low-privilege environments. It goes in a privileged environment with elevated privileges to access the file system or elevate root shell if sudo permission is enabled.

On other you should be in listening mode by using socat listener in your local machine to get privilege shell:

Less

With less, we can spawn the interactive system shell and can get away from low-privilege environments. It goes in a privileged environment with elevated privileges to access the file system or elevate root shell if sudo permission is enabled.

As we know it opens the file in vi mode and since you have run the command with sudo right which means we can get a shell through it. Execute !bash command inside hosts file as shown above and you will get the root shell.

Man

Man can be used to break out from restricted environments by spawning an interactive system shell.

As we know it opens the file in vi mode and since you have run the command with sudo right which means we can get a shell through it. Execute !bash command inside hosts file as shown above and you will get a root shell.

Git

With this, we can spawn the interactive system shell and can get away from low-privilege environments. It goes in a privileged environment with elevated privileges to access the file system or elevate root shell if sudo permission is enabled.

As we know it opens the file in vi mode and since you have run the command with sudo right which means we can get a shell through it. Execute !bash command inside git status file as shown above and you will get a root shell.

 

Pico

Since we know that pico is a text editor such as nano and vi also has sudo permission, which means that now we can also edit system files. Now open /etc/shadow file with pico editor and try to modify the root password.

On other hands, I have generated a new encrypted password: pass123 using OpenSSL passwd

Now paste the above generated encrypted password inside shadow file for user root.

Since we have modified the root password into pass123, therefore, let get root login using it.

SCP

It can be used to break out from restricted environments by spawning an interactive system shell. It runs in privileged context and may be used to access the file system, escalate or maintain access with elevated privileges if enabled on sudo.

SSH

With this, we can spawn the interactive system shell and can get away from low-privilege environments. It goes in a privileged environment with elevated privileges to access the file system or elevate root shell if sudo permission is enabled. Let’s spawn an interactive root shell through Proxy Command option.

Curl

With this, we can spawn the interactive system shell and can get away from low-privilege environments. Since we have a low privilege shell which means, we can access /etc/passswd file.

So, I copied /etc/passwd file in my local machine and removed encrypted password for user “insecurity”.

By running curl with sudo permission we can fetch a remote file via http-get method.

So, as you can observe that now we are having modified passwd file where user insecurity has an entry with UID: GID 0:0 and no password.

Since we have modified the insecurity password into the blank password, therefore, let get root login using it.

Vi Editor

With this, we can spawn the interactive system shell and can get away from low-privilege environments. It goes in a privileged environment with elevated privileges to access the file system or elevate root shell if sudo permission is enabled.

Type following:

All you can observe that I have root shell after this.

Rvim

With rvim, we can spawn the root shell and can get away from low-privilege environments. It goes in a privileged environment with elevated privileges to access the file system or elevate root shell if sudo permission is enabled. This requires that rvim is compiled with Python support. Prep-end: py3 for Python 3. Now enter following command to spawn /bin/sh shell with higher privilege shell.

You will see, it opens the file in vi mode and since you have run the command with sudo right which means we can get a shell through it. Execute !bash command inside it as shown below and you will get a root shell.

2nd Procedures

  • Wildcard Injection
  • Check cron job
  • Generate one-liner for the netcat_reverse shell (msfvenom)
  • Exploit tar wildcard injection
  • Spawn root shell through netcat session

Now open crontab to view if any task is scheduled.

cat /etc/crontab

Here we notice the target has scheduled a bash program script for every 1 minute and we know that cron job runs as root. The minute attacker read the program written inside /etc/cron.daily/backup the script, since it is saving a compressed copy of each user’s home directories to /etc/backups. Therefore we can apply tar wildcard injection.

Visit below link to read complete detail of wildcard injection attack: https://www.hackingarticles.in/exploiting-wildcard-for-privilege-escalation/

On a new terminal in your Kali to generate netcat reverse shell malicious code for achieving netcat reverse connection by using msfvenom and enter the following command for that:

Now copy the generated payload and paste inside the victim’s shell as described below.

Since the tar command is running as root due to crontab, this has the effect of spawning a netcat shell and sending it to the attack platform on port 8888. And if you go back to the terminal window where the listener was on, you will have victim’s reverse connection in after 1 minute.

 

3rd Procedures

  • Identify SUID binaries with a find command
  • Exploit SUID binary

Here xxd and taskset are system binaries file having suid permissions and with this, we can spawn the interactive system shell and can get away from low-privilege environments.

Visit below link to read complete detail of SUID Privilege Escalation:

https://www.hackingarticles.in/linux-privilege-escalation-using-suid-binaries/

It was quite easy to get root shell through taskset but fail to get privilege via xxd, although you can execute below command to get a root shell.

 

Escalate root shell for Susan

Procedures:

  • Identify the SUID Binary with the find command
  • Exploiting SUID binary
  • Read /etc/shadow through xxd
  • Get root password
  • Crack root password
  • Get Root shell

Let’s get into /home directory where we discover 3 users’ directory bob, susan, and peter. Since we have escalated root shell via bob in all possible way but now I’m curious in penetrating /susan directory. So while exploring /susan; I found password “MySuperS3cretValue!” from inside .secret

So we logged as Susan with the help of password we found inside .secret, next we need escalated root privilege because we are again in low privilege shell. As we knew xxd has suid permission, therefore, we try to escalate root privilege with the following command

xxd is a Linux command that creates a hex dump of a given file when it has read permissions and as we know SUID bit is enabled for xxd hence we can use it to read the information of /etc/shadow file.

Once you have shadow file on your screen, copy the encrypted text for user root and crack it either with john the ripper or other resources.  So I have used online hash cracking tools and found the root password “secret123” and logged with it.

Escalate root shell for Peter

Procedures:

  • Exploiting NFS
  • Mount share directory
  • Create SSH pair key
  • Replace your pub_key to users’ authorized_keys
  • Get SSH login session for peter
  • Exploit Sudo right to get a root shell

On scanning its network we found port 2049 is open for a network share, therefore, we run showmount command to identify share directory

But I mount /home/peter in our /tmp/raj directory, I didn’t find anything here, truthfully when I try to create .ssh directory, it gave permission denied error, therefore, I create a fake peter account with a UID of 1001 on my Kali machine and successfully created .ssh folder as shown below steps.

Now create SSH pair key and add your public SSH key to the users’ authorized_keys file and it allows you to log in as the target user, assuming that you have the appropriate private key.

Therefore follow the below steps:

Booomm!! Booomm!! We’re logged in as Peter. There multiple ways to exploit NFS services, it totally depends on the situation. Visit the link below to learn more about NFS Privilege Escalation:

https://www.hackingarticles.in/linux-privilege-escalation-using-misconfigured-nfs/

Now let’s check peter’s sudo permission for user peter:

So, here peter can run strace commands with sudo which means we can spawn the interactive system shell and can get away from low-privilege environments. It goes in a privileged environment with elevated privileges to access the file system or elevate root shell if sudo permission is enabled

AuthorDevender Kumar, An IT security Specialist and Post Graduate in Cyber Law & Cyber Forensics. You can Contact him here 

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