Skip to main content

Linux Command Line CTF Challenge

Welcome, CTF participants! This guide will walk you through the seven challenges you'll face in this Linux Command Line Capture The Flag. Each challenge is designed to test a different aspect of your Linux skills. Remember, all flags are in the format CTF{some_text_here}.

IMPORTANT NOTE​

As you complete each challenge, we encourage you to take note of any concepts or commands you find challenging and to revisit them later for further study. Additionally, we kindly ask that you DO NOT SHARE SOLUTIONS in social media posts. We aim to discourage copying and pasting and request your support in this effort. Instead, share your reflections and unique perspectives along with the announcement that you’ve completed the labs.

Environment Setup​

This CTF challenge requires you to deploy a lab environment in your preferred cloud provider, everything is automated, please refer to individual guides to setup the environment. And then proceed with the CTF Challenges. Start by cloning ltc-linux-challenge repo.

git clone https://github.com/learntocloud/ltc-linux-challenge

Then follow the appropriate guide to deploy the environment.

Finding help and answers​

If you are stuck, leverage man pages

Compare your results with our answer key

Optional but highly recommended books that you can use to learn more Linux and help you with this lab. Get the books, look up the command your stuck on in the appendix, read. These books are also great to go through entirely.

  • Linux Basics for Hackers by OccupyTheWeb
  • The Linux Command Line by William Shotts
  • How Linux Works by Brian Ward

Challenge 1: The Hidden File​

Objective: Find a hidden file in the ctf_challenges directory and read its contents.

Skills tested:

  • Understanding of hidden files in Linux
  • Using ls with appropriate flags
  • Reading file contents

Hint: In Linux, files that start with a dot (.) are considered hidden. How can you list all files, including hidden ones?

Challenge 2: The Secret File​

Objective: Locate a file with the word "secret" in its name anywhere in the /home/ctf_user directory.

Skills tested:

  • Recursive file searching
  • Using grep or find commands

Hint: The find command can search for files based on their names. How can you use it to search for a file containing "secret" in its name?

Challenge 3: The Largest Log​

Objective: Find the largest file in the /var/log directory and retrieve the flag from it.

Skills tested:

  • Navigating directory structures
  • Sorting and filtering files based on size
  • Reading file contents

Hint: The du command can help you find file sizes. How can you combine it with sort to find the largest file?

Challenge 4: The User Detective​

Objective: Identify the user with UID 1001 and find the flag in their home directory.

Skills tested:

  • Understanding user management in Linux
  • Reading the /etc/passwd file or using id command
  • Navigating to other users' home directories

Hint: The /etc/passwd file contains information about user accounts. How can you search for a specific UID in this file?

Challenge 5: The Permissive File​

Objective: Locate the file owned by root with permissions 777 and read its contents.

Skills tested:

  • Understanding Linux file permissions
  • Using find command with permission parameters
  • Reading file contents as a non-root user

Hint: The find command can also search for files based on their permissions. What does 777 mean in terms of file permissions?

Challenge 6: The Hidden Service​

Objective: Find the process running on port 8080 and retrieve the flag from its command.

Skills tested:

  • Using network-related commands (netstat, ss, or lsof)
  • Understanding process information
  • Reading process details

Hint: The netstat or ss command can show you which processes are listening on which ports. How can you use these to find a process on a specific port?

Challenge 7: The Encoded Secret​

Objective: Decode the base64 encoded flag in the 'encoded_flag.txt' file.

Skills tested:

  • Understanding of base64 encoding
  • Using command-line decoding tools

Hint: Linux has a built-in command for base64 decoding. How can you use it to decode the contents of a file?

General Tips:​

  1. Make use of man pages (e.g., man ls) to learn more about command options.
  2. The pipe operator (|) can be very useful for combining commands.
  3. Don't forget about command history! Use the up arrow or Ctrl+R to search for previously used commands.
  4. If you're stuck, try to break down the problem into smaller steps.
  5. Remember, Google is your friend! Looking up commands and their usage is a valuable skill in itself.

Good luck, and happy hunting! Remember, the journey of learning is just as important as capturing the flags. Enjoy exploring the power of the Linux command line!