Phase 1: Linux Command Line CTF Challenge

This set of progressive Capture The Flag (CTF) challenges will test your Linux command line skills. Each challenge builds upon previous concepts while introducing new ones. All flags follow the format CTF{some_text_here}.

Important

Please complete Phase 1 Guide before attempting these challenges. Do not share solutions publicly - focus on sharing your learning journey instead.

Flag Submission

Submit flags using the verify command:

• Check progress: verify progress
• Submit a flag: verify [challenge_number] [flag]
• Test the system: verify 0 CTF{example}

Example: verify 0 CTF{example}

ctf_user@ctf-vm:~$ verify 0 CTF{example}
✓ Example flag verified! Now try finding real flags.

Environment Setup

Follow the setup guide for your preferred cloud provider:

• AWS
• Azure

Challenges

Beginner Level

Challenge 1: The Hidden File

Find and read a hidden file in the ctf_challenges directory.

• Skills: Basic file listing, hidden files concept
• Hint: Hidden files in Linux start with a special character

Challenge 2: The Secret File

Locate a file containing "secret" in its name within /home/ctf_user.

• Skills: File searching, directory navigation
• Hint: Explore commands that can search through directories

Intermediate Level

Challenge 3: The Largest Log

Identify and read the largest file in /var/log.

• Skills: File size analysis, sorting, log navigation
• Hint: Look into commands that display file sizes

Challenge 4: The User Detective

Find a flag in the .profile of the user with UID 1002.

• Skills: User management, system files, permissions
• Hint: System files contain user information

Challenge 5: The Permissive File

Find a root-owned file with 777 permissions. The flag is the contents of this file.

• Skills: Permission understanding, advanced file searching
• Hint: Consider both ownership and permission patterns

Advanced Level

Challenge 6: The Hidden Service

Identify a process on port 8080 and retrieve its flag.

• Skills: Process management, networking tools, service inspection
• Hint: Network diagnostic tools can reveal running services. Port 8080 is often used for HTTP

Challenge 7: The Encoded Secret

Decode a base64-encoded flag.

• Skills: Encoding/decoding, command piping
• Hint: Linux provides built-in encoding tools

Challenge 8: SSH Key Authentication

Configure SSH key authentication and find a hidden flag.

• Skills: SSH configuration, key management, security practices
• Hint: Pay attention to file permissions and hidden directories

Tips for Success

1. Use man pages to understand command options
2. Break down complex problems into smaller steps
3. Understand command combinations using pipes
4. Review basic Linux concepts from Phase 1 Guide
5. Take notes on new commands you discover

Author

• LinkedIn: rishabkumar7
• X/Twitter: @rishabincloud

License