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I'm setting up beginner CTF (Capture the flag) in a Linux environment. I will have questions that will tell the user to run specific commands.

The issue I'm having is restricting the user to a directory and only being able to run certain commands.

For example Players need to change the directory and then use "ls" to find the flag and complete the challenge.

But I don't want a player to see other directories in the server or people able to cd where there not supposed to go.

Any help would be much appreciated.

2 Answers 2

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Investigate the possibility to use a restricted shell for the users. Together with modifying PATH to specify a directory wich contains only commands that you allow your users to run (you may create such a directory and just put symlinks or hardlinks to the original commands there) it may give you what you need.

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I have 2 possible solutions:

  1. Give the ctf's user permissions only to their home directory, you will need a different user for every challenge. every time they will try to cd out of the directory they will be denied.

I found a different post that suggests using the adduser command:

adduser --home/restricted/directory restricted_user

https://superuser.com/questions/149404/create-an-ssh-user-who-only-has-permission-to-access-specific-folders

  1. Use docker and run each level in a different environment.

hope this helps!

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  • Hey Roey, Your first option is exactly what I want but I need help setting it up, any clue?
    – Joe Lynch
    Jan 16, 2021 at 19:24
  • I edited my post to show a possible solution to how you might want to do that.
    – roey
    Jan 16, 2021 at 19:30

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