The su (aka substitute user) command is used by Linux users to transition from the current user to another user or super user (root account) without changing the current working directory or user environment.
A standard user with sudo access has extra privileges while using the su command. For instance, if a user with sudo access uses the su command to try to switch to the root account, they are required to supply their own password.
Switching to the root account with sudo permission
However, users who don’t have access to sudo try to switch to the root account using the su command. They are required to provide a root account password.
Switching to the root account without sudo permission
What do you think? Which one is way more secure to prevent unnecessary access to root or any other account? No doubt, the second option is secure and recommended unless you own the system, trust the user, or are using your own personal desktop.
As a system administrator, you have access to revoke su permission for a user or group of users from the sudoers file. The sudoers file is used to control and manage a user’s sudo privileges.
By default, the location for the sudoers file is at /etc/sudoers, but it is recommended to edit the file using the sudo visudo command.
Let’s stop this talk and get to the real meat by learning how to disable su for sudo users and groups of users in Linux.
Revoke su Access for a Sudo User
First, you should always take a backup of the sudo configuration file before making any changes. Execute the following command to take a backup of the sudoers file.
$ sudo cp /etc/sudoers /etc/sudoers.bak
Then open your visudo configuration file by issuing the following command.
# Execute below command if you logged as super user
$ visudo
# Execute below command if you are a normal user with sudo privileges
$ sudo visudo
Below is the output of the above command.
sudo configuration file
In the next step, you need to create the following alias under the comment section “Cmnd alias specification“.
Cmnd_Alias DISABLE_SU = /bin/su
Below is the output of the above command.
Creating alias
Then add the following line by replacing “trendoceans” with the actual user at the end of the sudoers file.
trendoceans ALL=ALL, !DISABLE_SU
Below is the output of the above command.
Disabling sudo user from su access
Save/close the file and restart your terminal session.
To verify the changes are made properly, open a new terminal session and execute the sudo su command. The system should return an error with the message “Sorry, user trendoceans is not allowed to execute '/usr/bin/su' as root on linux.“, as shown below.
$ sudo su
Below is the output of the above command.
Privilege error
Revoke su Access for a Group of Sudo Users
By using the same DISABLE_SU variable, you can prevent a group of sudo users from accessing the su command. For example, in the sudoers file, you will find an admin group.
Let’s see, how you can prevent the user added to the admin group from accessing the su command. For that, replace the below line.
%admin ALL=(ALL) ALL
to this.
%admin ALL=(ALL) ALL, !DISABLE_SU
In the sudoers file and then, save and close the file.
Lastly, add the user to the admin group by using the following command.
$ sudo usermod -aG admin trendoceans
Do not forget to replace “trendoceans” with the actual user. Now if the users try to access su from the sudo command, they will get the same error with the message “Sorry, user trendoceans is not allowed to execute '/usr/bin/su' as root on linux.“.
That’s all for now. If you found this article useful and want to know more about different ways to restrict user access in Linux systems, then let us know in the comment section.
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