User Management

User management is a critical part of maintaining a secure system. Ineffective user and privilege management often lead many systems into being compromised. Therefore, it is important that you understand how you can protect your server through simple and effective user account management techniques.

Where is root?

Ubuntu developers made a conscientious decision 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. It merely has been given a password which matches no possible encrypted value, therefore may not log in directly by itself.

Instead, users are encouraged to make use of a tool by the name of sudo to carry out system administrative duties. Sudo allows an authorized user to temporarily elevate their privileges using their own password instead of having to know the password belonging to the root account. This simple yet effective methodology provides accountability for all user actions, and gives the administrator granular control over which actions a user can perform with said privileges.

  • If for some reason you wish to enable the root account, simply 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
  • To disable the root account, use the following passwd syntax:

    sudo passwd -l root
  • You should read more on Sudo by checking out it's man page:

    man sudo

By default, the initial user created by the Ubuntu installer is a member of the group "admin" which is added to the file /etc/sudoers as an authorized sudo user. If you wish to give any other account full root access through sudo, simply add them to the admin group.

Adding and Deleting Users

The process for managing local users and groups is straight forward and 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.

  • 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 nomusuari
  • To delete a user account and its primary group, use the following syntax:

    sudo deluser nomusuari

    Deleting an account does not remove their respective home folder. It is up to you whether or not you wish to delete the folder manually or keep it according to your desired retention policies.

    Remember, any user added later on 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 these 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/nomusuari/
    sudo mkdir /home/usuaris_arxivats/
    sudo mv /home/username /home/usuaris_arxivats/
  • To temporarily lock or unlock a user account, use the following syntax, respectively:

    sudo passwd -l nomusari
    sudo passwd -u nomusari
  • To add or delete a personalized group, use the following syntax, respectively:

    sudo addgroup nomgrup
    sudo delgroup nomgrup
  • To add a user to a group, use the following syntax:

    sudo adduser nomusuari nomgrup

Seguretat del perfil d'usuari

When a new user is created, the adduser utility creates a brand new home directory named /home/username, respectively. The default profile is modeled 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. This may not be suitable for your environment.

  • To verify your current users 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 syntax:

    sudo chmod 0750 /home/nomusuari

    Some people tend to use the recursive option (-R) indiscriminately which modifies all child folders and files, but this is not necessary, and may yield other undesirable results. The parent directory alone is sufficient for preventing unauthorized access to anything below the parent.

    A much more efficient approach to the matter would be to modify the adduser global default permissions when creating user home folders. Simply edit the file /etc/adduser.conf and modify the DIR_MODE variable to something appropriate, so that all new home directories will receive the correct permissions.

  • After correcting the directory permissions using any of the previously mentioned techniques, verify the results using the following syntax:

    ls -ld /home/username

    The results below show 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 adequately 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 4 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   required nullok obscure min=4 max=8 md5

If you would like to adjust the minimum length to 6 characters, change the appropriate variable to min=6. The modification is outlined below.

password   required nullok obscure min=6 max=8 md5

The max=8 variable does not represent the maximum length of a password. It only means that complexity requirements will not be checked on passwords over 8 characters. You may want to look at the libpam-cracklib package for additional password entropy assistance.

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 easily view the current status of a user account, use the following syntax:

    sudo chage -l nomusuari

    The output below shows interesting facts about the user account, namely that there are no policies applied:

    Last password change                                    : Jan 20, 2008
    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, simply use the following syntax, and follow the interactive prompts:

    sudo chage nomusuari

    The following is also an example of how you can manually change the explicit expiration date (-E) to 01/31/2008, minimum password age (-m) of 5 days, maximum password age (-M) of 90 days, inactivity period (-I) of 5 days after password expiration, and a warning time period (-W) of 14 days before password expiration.

    sudo chage -E 01/31/2008 -m 5 -M 90 -I 30 -W 14 nomusuari
  • To verify changes, use the same syntax as mentioned previously:

    sudo chage -l nomusuari

    The output below shows the new policies that have been established for the account:

    Last password change                                    : Jan 20, 2008
    Password expires                                        : Apr 19, 2008
    Password inactive                                       : May 19, 2008
    Account expires                                         : Jan 31, 2008
    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

Altres consideracions de seguretat

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 Users

Simply disabling/locking a user account will not prevent a user from logging into your server remotely if they have previously set up RSA 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 users 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 capabilities.

Be sure to check for any established SSH connections by the disabled user, as it is possible they may have existing inbound or outbound connections. Kill any that are found.

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 nomusuari sshlogin
sudo /etc/init.d/ssh restart

External User Database Authentication

Most enterprise networks require centralized 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.