Lin.Security is a Boot to Root CTF available here on Vulnhub. It’s difficulty is rated as Easy/Intermediate. I could not get this VM to work in VMWare, however had success with VirtualBox. Additionally, this VM provides you with a set of credentials to begin with. I only used these credentials to check that my networking was configured correctly, otherwise I forgot they existed and attacked this box externally.

After using the credentials provided to check the networking, I knew that the box’s IP was So as usual, I kicked off with an nmap scan.

We see that port 2049 is exposed, which is typically a NFS share. I use showmount to view what shares are available (note: if your on Kali can don’t have showmount it can easily be installed by using apt-get install nfs-common)

We see that the home directory for the user peter is shared. I go ahead and mount this to a folder titled ‘lin’ and view its contents. There is nothing in here, except for a few hidden files (sadly no .ssh keys as I was hoping). However I do not have access to read some files and I cannot write to the drive.

After reading up on NFS share misconfigurations, I learn that you can get full access to a folder if you share the same UID. I check the UID of the NFS share and see that it is 1001.

I create a new user called ‘test’ and give them the UID of 1001. This now gives me the ability to read all files and write to the share. Originally, I was disappointed in that there were no .ssh keys in the share, but now that I can write to the share I can put my own .ssh keys in and log in as the user peter. so I create a .ssh folder and generate a new set of .ssh keys for the test user and copy these accross to the authorized_keys file in .ssh. I then ssh into the box as peter and I successfully log in!

I start looking to escalate my privileges to root and I see in the /etc/passwd file that the user insecurity has a password hash defined. I copy this hash across to my own machine and then use john to crack the password using the rockyou.txt wordlist.

john takes no time at all to crack the hash and reveal the password to me.

I then use this password to su into the user insecurity. Amazingly this user is actually root! Job Complete!

This a was fun VM that took hardly any time at all to complete. I learnt a bit about NFS share misconfiguration which I will store away for later use. Recommend this VM for anyone just starting out or if your looking for something quick and easy to do on a weeknight