Today we are going to talk about CAT command and learn how helpful the cat command is for Linux penetration testing and how we’ll progress cat to scale the greater privilege shell.

NOTE: “The main objective of publishing the series of “Linux for pentester” is to introduce the circumstances and any kind of hurdles that can be faced by any pentester while solving CTF challenges or OSCP labs which are based on Linux privilege escalations. Here we do not criticizing any kind of misconfiguration that a network or system administrator does for providing higher permissions on any programs/binaries/files & etc.”  

Table of Content

• Introduction to CAT
• Major Functions of CAT command
• Sudo rights Lab setups for Privilege Escalation
• Exploiting Sudo Rights

Introduction to CAT

In Linux, Cat stands for “catenate,” which is one of Unix-like operating system most frequently used commands. It reads file information and displays its content as an output. It enables us to build, view and link files. So, we can not only see the content using CAT command; apart from this we can, copy the content of the file to some other file and view the files with numbers and so on. Not only this we will do such things which is not only new but is what we might have not thought of. We will perform Privilege Escalation using CAT command. That’s sounds interesting. Isn’t it? So, let’s start-

Major Functions of CAT command

At first, we will run cat -h command which means help and which will tell you about all the options which are available in CAT command as we can see in the picture below.

cat --help

Write and Read a file:

Our next step is to create a file using the cat command. And for this, we will use greater than sign (>) after cat command to generate a new file. So, we have created a new file named notes.txt by using (>) this sign after cat command and write the content which you want to keep in the file as in our case I have written “Welcome to Hacking articles” in the file notes.txt

cat > notes.txt

Not only this we can also edit the content of the existing file without opening the file by using greater than sign twice (>>) as you can see in the screenshot that we have added “Join Ignite Technologies”  in notes.txt

cat >> notes.txt
cat notes.txt

Now we can confirm this by reading the file once again.

cat notes.txt

Number all output lines:

Now let’s say if we want to view file contents preceding line numbers or in other words you want to view the output serialized. So first we will create a new text file named dict.txt in which we have written some content which is going to be easily readable number wise with -n command.

cat dict.txt
cat -n dict.txt

As a result, this add a serial number column for every line as shown below:

Overwriting a file

Now we want to copy the content of file dict.txt into notes.txt or in other words we want to overwrite the file notes.txt. So in order to do, this first we write the file name from which the content is to be copied and then we will write the file name whose content we want to replace followed by greater than sign(>).

Syntax: cat [file1] > [file2]
cat dict.txt > notes.txt

As you can observe in the picture below that we have replaced the content of notes.txt with dict.txt

Concatenating files:

Now we want to merge two files together or in other words, we want to combine two files. So, what will we do? Its again very simple; we will use greater than sigh here but now twice (>>) and the content will be replaced successfully. So here we have another new file which is pass.txt and then we will proceed towards merging two files for which we will use (>>) sign again as we have done in the image below. Now again we will use -n to put this content number wise which we have done above.

cat > pass.txt
cat dict.txt >> pass.txt                
cat -n pass.txt

As result, you can observe that we have concatenate dict.txt in the pass.txt file.

Reverse order

As the name suggests and we can reverse all the content using tac command which is just a reverse of cat command and it works for this purpose only.

cat dict.txt

With the help of tac command, we try to reverse the file by making a vertical flip as shown below.

Sudo rights Lab setups for Privilege Escalation

Now here our next step is to set up the lab of Sudo rights or in other words to provide Sudo privileges to a user for cat executable. Here we are going to add a user by the name of the test in the suoders files and here we have given permission to user test to run cat command as root user.

Exploiting Sudo Rights

Now we will connect through ssh in kali and after that, we will run sudo -l which is sudo list and through which we can see that user test has the permission to run cat as root user.

ssh test@192.168.1.108

Now our next step is to exploit sudo rights through cat command. So, we will run cat /etc/shadow command to see all the users and their respective passwords hashes.

sudo -l
sudo cat /etc/shadow

Wonderful! We have got all the user’s list and their passwords’ hash value.

Cracking the Hash Password

Now our next step is to crack the hash value so that we are going to use “John the Ripper” tool to crack this hash value in order to get the password in decrypted form. So first we have taken one user whose password we want to check. So, run the following command in the terminal-

john hash --show

Great! We have cracked the password successfully. Now we will switch user raj to check if we can log in through that password and we can see that we have successfully logged in as raj user.

Now we will run sudo -l command to check if user raj, and found he has all the root permissions.

sudo -l
sudo su

Now, we will again try to switch to user root and we are logged in as root and then we run id command we get to know that we got a root shell.

So, we have performed privilege escalation through cat command successfully.

Author: Geet Madan is a Certified Ethical Hacker, Researcher and Technical Writer at Hacking Articles on Information Security. Contact here