User management is a critical part of maintaining a secure system. Ineffective user and privilege management often leads to a system being compromised. Therefore, it is important that you understand how to protect your server through simple and effective user account management techniques.
Where is root?
Ubuntu developers decided to disable the administrative root account by default in all Ubuntu installations. This does not mean that the root account has been deleted, or that it may not be accessed. Instead, it has been given a password hash that matches no possible value, and so may not log in directly by itself.
Instead, the sudo utility (“superuser do”) is used to carry out system administrative duties. sudo allows an authorised user to temporarily elevate their privileges using their own password instead of having to know the password belonging to the root account. This provides accountability for all user actions, and gives the administrator control over which actions a user can perform with said privileges.
Enabling the root account
If for some reason you wish to enable the root account, you will need to give it a password:
sudo passwd
sudo will prompt you for your password, and then ask you to supply a new password for root as shown below:
[sudo] password for username: (enter your own password)
Enter new UNIX password: (enter a new password for root)
Retype new UNIX password: (repeat new password for root)
passwd: password updated successfully
Disabling the root account password
To disable the root account password, use the following passwd syntax:
sudo passwd -l root
You can learn more about sudo by reading the sudo man page: man sudo
By default, the initial user created by the Ubuntu installer is a member of the group sudo which is added to the file /etc/sudoers as an authorised sudo user. To give any other account full root access through sudo, add them to the sudo group.
Adding and deleting users
Managing local users and groups differs very little from most other GNU/Linux operating systems. Ubuntu and other Debian-based distributions encourage the use of the adduser package for account management.
Add a user
To add a user account, use the following syntax, and follow the prompts to give the account a password and identifiable characteristics, such as a full name, phone number, etc:
sudo adduser username
Delete a user
To delete a user account and its primary group, use the following syntax:
sudo deluser username
Deleting an account does not remove their respective home folder. You must decide whether or not to delete the folder manually, or to keep it according to your desired retention policies.
Remember, any user added later with the same UID/GID as the previous owner will now have access to this folder if you have not taken the necessary precautions.
You may want to change the UID/GID values to something more appropriate, such as the root account, and perhaps even relocate the folder to avoid future conflicts:
sudo chown -R root:root /home/username/
sudo mkdir /home/archived_users/
sudo mv /home/username /home/archived_users/
Lock or unlock a password
To temporarily lock a user password, use the following syntax:
sudo passwd -l username
Similarly, to unlock a user password:
sudo passwd -u username
Add or delete a group
To add or delete a personalised group, use the following syntax, respectively:
sudo addgroup groupname
sudo delgroup groupname
Add a user to a group
To add a user to a group, use the following syntax:
sudo adduser username groupname
User profile security
When a new user is created, the adduser utility creates a brand new home directory named /home/username. The default profile is modelled after the contents found in the directory of /etc/skel, which includes all profile basics.
If your server will be home to multiple users, you should pay close attention to the user home directory permissions to ensure confidentiality. By default, user home directories in Ubuntu are created with world read/execute permissions. This means that all users can browse and access the contents of other users home directories, which may not be suitable for your environment.
To verify your current user home directory permissions, use the following syntax:
ls -ld /home/username
The following output shows that the directory /home/username has world-readable permissions:
drwxr-xr-x 2 username username 4096 2007-10-02 20:03 username
You can remove the world readable-permissions using the following command:
sudo chmod 0750 /home/username
Note:
Some people use the recursive option (-R) indiscriminately, which modifies all child folders and files. However, this is not necessary and may have undesirable/unintended consequences. Modifying only the parent directory is enough to prevent unauthorised access to anything below the parent.
A more efficient approach would be to modify the adduser global default permissions when creating user home folders. To do this, edit the /etc/adduser.conf file and modify the DIR_MODE variable to something appropriate, so that all new home directories will receive the correct permissions.
DIR_MODE=0750
After correcting the directory permissions using any of the previously mentioned techniques, verify the results as follows:
ls -ld /home/username
The output below shows that world-readable permissions have been removed:
drwxr-x--- 2 username username 4096 2007-10-02 20:03 username
Password policy
A strong password policy is one of the most important aspects of your security posture. Many successful security breaches involve simple brute force and dictionary attacks against weak passwords.
If you intend to offer any form of remote access involving your local password system, make sure you address minimum password complexity requirements, maximum password lifetimes, and frequent audits of your authentication systems.
Minimum password length
By default, Ubuntu requires a minimum password length of 6 characters, as well as some basic entropy checks. These values are controlled in the file /etc/pam.d/common-password, which is outlined below.
password [success=1 default=ignore] pam_unix.so obscure sha512
To adjust the minimum length to 8 characters, change the appropriate variable to min=8. The modification is outlined below:
password [success=1 default=ignore] pam_unix.so obscure sha512 minlen=8
Note:
Basic password entropy checks and minimum length rules do not apply to the administrator usingsudo-level commands to setup a new user.
Password expiration
When creating user accounts, you should make it a policy to have a minimum and maximum password age, forcing users to change their passwords when they expire.
To view the current status of a user account:
sudo chage -l username
The output below shows interesting facts about the user account, namely that there are no policies applied:
Last password change : Jan 20, 2015
Password expires : never
Password inactive : never
Account expires : never
Minimum number of days between password change : 0
Maximum number of days between password change : 99999
Number of days of warning before password expires : 7
To set any of these values, use the chage (“change age”) command, and follow the interactive prompts:
sudo chage username
The following is also an example of how you can manually change the explicit expiration date (-E) to 01/31/2015, minimum password age (-m) of 5 days, maximum password age (-M) of 90 days, inactivity period (-I) of 30 days after password expiration, and a warning time period (-W) of 14 days before password expiration:
sudo chage -E 01/31/2015 -m 5 -M 90 -I 30 -W 14 username
To verify changes, use the same syntax as mentioned previously:
sudo chage -l username
The output below shows the new policies that have been established for the account:
Last password change : Jan 20, 2015
Password expires : Apr 19, 2015
Password inactive : May 19, 2015
Account expires : Jan 31, 2015
Minimum number of days between password change : 5
Maximum number of days between password change : 90
Number of days of warning before password expires : 14
Other security considerations
Many applications use alternate authentication mechanisms that can be easily overlooked by even experienced system administrators. Therefore, it is important to understand and control how users authenticate and gain access to services and applications on your server.
SSH access by disabled passwords
Disabling or locking a user password will not prevent a user from logging into your server remotely if they have previously set up SSH public key authentication. They will still be able to gain shell access to the server, without the need for any password. Remember to check the user’s home directory for files that will allow for this type of authenticated SSH access, e.g. /home/username/.ssh/authorized_keys.
Remove or rename the directory .ssh/ in the user’s home folder to prevent further SSH authentication access.
Be sure to check for any established SSH connections by the disabled account, as it is possible they may have existing inbound or outbound connections – then pkill any that are found.
who | grep username (to get the pts/# terminal)
sudo pkill -f pts/#
Restrict SSH access to only user accounts that should have it. For example, you may create a group called sshlogin and add the group name as the value associated with the AllowGroups variable located in the file /etc/ssh/sshd_config:
AllowGroups sshlogin
Then add your permitted SSH users to the group sshlogin, and restart the SSH service.
sudo adduser username sshlogin
sudo systemctl restart sshd.service
External user database authentication
Most enterprise networks require centralised authentication and access controls for all system resources. If you have configured your server to authenticate users against external databases, be sure to disable the user accounts both externally and locally. This way you ensure that local fallback authentication is not possible.