File permissions

Log in to your Debian VM for the following exercises.

Create a user and a group

Create a new user with sudo adduser NAME - I'm going to be using brian as an example name in these notes. When it asks for a password, you can just use brian or something; it will complain about the password being too short but it will create the user anyway. You can skip the GECOS information asking for a full name and phone number---it's just to help an admin contact you if needed.

Check the user and group files with tail /etc/passwd and tail /etc/group to check that the new user has been created - tail displays the last 10 lines of a file by default; tail -n N FILE would display the last N lines. Your new user brian (or whatever you called them) should appear in both files. Also check with ls -l /home that the home directory for Brian exists and is set to the correct user and group.

Time to change user: su brian and enter the password. Notice that the prompt has changed to brian@debian12:/home/vagrant$ (at least if you started off in that folder). So the user has changed, and because /home/vagrant is no longer the current user's home directory, it gets written out in full. Run cd to go home followed by pwd and check that you are now in /home/brian or whatever you called your new user.

Next, create a user nigel (or some other name) add both your two new users, but not vagrant, to the group users (which already exists) using the command sudo usermod -aG GROUPNAME USERNAME, where group and username are changed accordingly. Note: brian cannot use sudo, so you have to exit his terminal to get back to one running as vagrant for this.

If you want to edit the group memberships directly checkout the /etc/group file (and /etc/passwd for users).

Explore file permissions

As user brian (or whatever you called your first new user), set up your home directory using what you learnt in the videos so that

  • You can do everything (rwx).
  • Members of the users group can list files and change to your home directory, but not add/remove files. You will need to change the group of your home directory to users for this, using the command chgrp -R GROUPNAME DIRECTORY.
  • Everyone else cannot do anything with your home directory.

Create a file in your home directory, e.g. nano readme.txt then add some content.

Check, by using su USERNAME to log in as the different users, that:

  • nigel can view Brian's home directory but not create files there;
  • nigel can view but not edit Brian's readme file;
  • vagrant cannot list files in or enter Brian's home directory at all. What happens when you try?

Of course, vagrant can use sudo to get around all these restrictions. Permissions do not protect you from anyone who can become root.

Also as brian, make a private subdirectory in your home folder that no-one but you can access (read, write or execute). Create a file secret.txt in there with nano private/secret.txt as user brian from Brian's home directory, and put something in it. Do not change any permissions on secret.txt itself.

Check as Nigel that you can see the folder itself, but not cd into it nor list the file. Check that even knowing the file name (cat /home/brian/private/secret.txt) as Nigel doesn't work.

Using ls -l as Brian in both ~ and ~/private, compare the entries for the files ~/readme.txt, ~/private/secret.txt and the folder ~/private. Why do the groups of the two files differ?

Note that, even though the secret file has read permissions for everyone by default, Nigel cannot read it. The rule is that you need permissions on the whole path from / to a file to be able to access it.

This is another reminder that if you want to store private files on a lab machine, then put it in a folder that is only accessible to you. Other students can read your home directory by default, and they would be able to look at your work. This has led to plagiarism problems in the past, but good news: we keep logs and can usually figure out what happened! :-).

Altenatively you could remove permissions from everyone else on your home directory there, but this prevents you from being able to share files in specific folders that you do want to share with other students.


We are going to create a file to let Nigel (and others in the users group) send Brian messages which go in a file in his home directory.

As Brian, create a file message-brian.c in your home directory and add the following lines:

#include <stdio.h>
#include <stdlib.h>

const char *filename ="/home/brian/messages.txt";

int main(int argc, char **argv) {
  if (argc != 2) {
    puts("Usage: message-brian MESSAGE");
    return 1;
  FILE *file = fopen(filename, "a");
  if (file == NULL) {
    puts("Error opening file");
    return 2;
  int r = fputs(argv[1], file);
  if (r == EOF) {
    puts("Error writing message");
    return 2;
  r = fputc('\n', file);
  if (r == EOF) {
    puts("Error writing newline");
    return 2;
  return 0;

Compile it with cc -Wall message-brian.c -o message-brian (you should not get any warnings) and check with ls -l, you will see a line like

-rwxr-xr-x    1 brian     brian         19984 Oct 28 13:26 message-brian

These are the default permissions for a newly created executable file; note that gcc has set the three +x bits for you. Still as Brian, run chmod u+s message-brian and check the file again: you should now see -rwsr-xr-x for the file permissions. The s is the setuid bit.

As Nigel (su nigel), go into Brian's home directory and run ./message-brian "Hi from Nigel!". The quotes are needed here because the program accepts only a single argument.

Now run ls -l and notice that a messages.txt has appeared with owner and group brian. Check the contents with cat messages.txt. Although Nigel cannot create and edit files in Brian's home directory himself (he can't edit messages.txt for example, although he can read it), the program message-brian ran as Brian, which let it create the file. Nigel can send another message like this (./message-brian "Hi again!"), which gets appended to the file: try this out.

This shows how setuid programs can be used to allow other users to selectively perform specific tasks under a different user account.

Warning: writing your own setuid programs is extremely dangerous if you don't know the basics of secure coding and hacking C programs, because a bug in such a program could let someone take over your user account. The absolute minimum you should know is the contents of our security units up to and including 4th year.

A general task for a security analyst might be finding all files with the setuid bit set on a system. You can try this yourself, but return to a vagrant shell first so that you're allowed to use sudo:

sudo find / -perm /4000

You might get some errors relating to /proc files, which you can ignore: these are subprocesses that find uses to look at individual files.

Apart from message-brian, you'll find a few files by default: sudo, mount, umount and su. The first one you already know; look up what the next two do and think about why they are setuid. Specifically, what kinds of (un)mounting are non-root users allowed to do according to the manual pages?

Look up the passwd program in the manual pages. Why might that program need to be setuid?


Make sure your terminal is running as brian and try a sudo ls. You will see a general message, you will be asked for your password, and then you will get the error brian is not in the sudoers file. This incident will be reported. (This means that an entry has been logged in /var/log/messages.)

So, brian can currently not use sudo. Switch back to vagrant and run the command sudo cat /etc/sudoers. Everything is commented out except root ALL=(ALL) ALL and the last line #includedir /etc/sudoers.d (this is not a comment!) which contains a single file vagrant with the line vagrant ALL=(ALL) NOPASSWD: ALL which is why vagrant can use sudo in the first place.

However, note the commented lines such as

# %sudo ALL=(ALL) ALL

If uncommented, the first one would let everyone in group wheel run commands using sudo (this is the default on some other linux distributions), whereas the second one would allow everyone in the group sudo to do this, but would prompt for their own password beforehand.

Let's allow people in the users group to reboot the machine. Open a root shell with sudo su as vagrant; this is so we don't get locked out if we break sudo.

Edit the sudoers file with visudo as root, and add the following line:

%users ALL=(ALL) /sbin/reboot

and save the sudoers file.

Warning:: Never edit /etc/sudoers directly and always use visudo instead. If you make a mistake and add a syntax error to the file then sudo will refuse to work. If your root account doesn't have a password (some people don't like that as a security precaution) then you'll have to spend the next half-hour figuring out how to break into your own computer and wrestle back control. There is almost always a command to check a config file before replacing the current one: the same advice also applies to the ssh config files. If you break them you might have to travel to wherever the server is with a keyboard and a monitor.

You can now switch back to brian (check the prompt to make sure you are Brian) and do sudo reboot. After asking for Brian's password, the virtual machine will now reboot, which you notice because you get kicked out of your ssh connection. Another vagrant ssh after a few seconds will get you back in again.

After rebooting, your /vagrant shared folder might not work. In this case, log out and do vagrant halt then vagrant up and vagrant ssh again on the host machine.

When vagrant boots your VM, it automatically sets up the shared folder, but this doesn't always work if you reboot the VM yourself.