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While Ubuntu comes secure and ready to use, many people decide to offer a wide range of services on their computer, such as running a FTP server or Apache. The purpose of this page is to advise these users on possible settings that could be changed to have an even stricter environment. Be warned that these recommendations are not always a good idea, as they can cause usability trade-offs that the Ubuntu Security Team has traditionally not agreed with.

Shared Memory

By default, /dev/shm is mounted read/write, with permission to execute programs. In recent years, many security mailing lists have noted many exploits where /dev/shm is used in an attack against a running service, such as httpd. Most of these exploits, however, rely on an insecure web application rather than a vulnerability in Apache or Ubuntu. There are a few reasons for it to be mounted read/write in specific configurations, such as real-time configuration of a Synaptics touchpad for laptops, but for servers and desktop installations there is no benefit to mounting /dev/shm read/write. To change this setting, edit the /etc/fstab file to include the following line:

tmpfs     /dev/shm     tmpfs     defaults,ro     0     0

This will mount /dev/shm in read-only mode. If you have a good reason to keep it writable, put this line in /etc/fstab instead:

tmpfs     /dev/shm     tmpfs     defaults,noexec,nosuid     0     0

This will mount /dev/shm writable, but without permission to execute programs and without permission to change the UID of running programs.

The changes will take effect the next time you reboot, unless you remount /dev/shm with the command sudo mount -o remount /dev/shm.

Note: MANY programs will not work if you make /dev/shm read-only (e.g. Google Chrome). Note also that on Ubuntu 8.10, if you make /dev/shm read-only, then reboot, the mount command will show that /dev/shm has not been mounted as read-only and the output will look like this:

tmpfs on /dev/shm type tmpfs (rw,nosuid,nodev)

So /dev/shm must be remounted manually with the following command, in order for changes to take effect:

sudo mount -o remount /dev/shm

SSH Settings

While the SSH daemon is secure enough for most people, some may wish to further enhance their security by changing certain sshd settings. Some settings which could be changed to enhance security are given here. All changes, unless otherwise stated, are made in the /etc/ssh/sshd_config file. Lines with a pound sign (#) are commented and not read. To edit this file from a terminal:

sudoedit /etc/ssh/sshd_config

For a Gnome editor, press Alt+F2 and use:

gksudo gedit /etc/ssh/sshd_config

For a KDE editor, press Alt+F2 and use:

kdesu kate /etc/ssh/sshd_config

Please remember, after making any changes, sshd must be restarted, which can be done from the terminal with this command:

service ssh restart

On older versions, e.g. Ubuntu 8.10, the above command might not work, so you may need to run this command instead:

sudo /etc/init.d/ssh restart

SSH Root Login

By default, the SSH daemon ships with remote root logins enabled. Normally Ubuntu does not allow direct access to the root user, so this setting is unimportant. If you have set a password on the root account, this setting can be a potential security risk, and should be disabled. To disable root login, edit the /etc/ssh/sshd_config file and replace the following line:

PermitRootLogin yes

with this line:

PermitRootLogin no

Sometimes it is necessary to allow root logins when doing automated tasks such as backups. To disallow normal logins but allow forced commands, you can use:

PermitRootLogin forced-commands-only

See man (5) sshd_config for details.

SSH Login Grace Time

The login grace time is a period of time where a user may be connected and not begin the authentication process. By default, sshd will allow a connected user to wait for 120 seconds (2 minutes) before starting to authenticate. This could be used to conduct a Denial of Service (DoS) or a brute force attack against a running SSH daemon. One solution is lowering this to 20 seconds. This should be used in combination with iptables rules to reduce the number of SSH connection attempts, otherwise a DoS can be launched well within the login timeout. To change this, replace this line:

LoginGraceTime 120

with this line:

LoginGraceTime 20

SSH Welcome Banner

The SSH daemon will allow a message to be displayed to users attempting to log in to the SSH server. To enable login messages, remove the pound sign from this line:

#Banner /etc/issue.net

so it looks like this:

Banner /etc/issue.net

Now, edit /etc/issue.net and place a warning to unauthorized users. The following is taken from the Advanced OpenSSH page and is modified from a U.S. Department of Defense warning banner.

***************************************************************************
                            NOTICE TO USERS


This computer system is the private property of its owner, whether
individual, corporate or government.  It is for authorized use only.
Users (authorized or unauthorized) have no explicit or implicit
expectation of privacy.

Any or all uses of this system and all files on this system may be
intercepted, monitored, recorded, copied, audited, inspected, and
disclosed to your employer, to authorized site, government, and law
enforcement personnel, as well as authorized officials of government
agencies, both domestic and foreign.

By using this system, the user consents to such interception, monitoring,
recording, copying, auditing, inspection, and disclosure at the
discretion of such personnel or officials.  Unauthorized or improper use
of this system may result in civil and criminal penalties and
administrative or disciplinary action, as appropriate. By continuing to
use this system you indicate your awareness of and consent to these terms
and conditions of use. LOG OFF IMMEDIATELY if you do not agree to the
conditions stated in this warning.

****************************************************************************

Once this is in place, restart sshd and all users will see this warning before they get the login prompt. This will obviously not dissuade automated SSH attacks, and will potentially worsen DoS effects, but it may tip off a human attacker that the system is being looked after closely, and that they should move on to some other system on the network.

SSH Allowed Users

By default, SSH will permit every user with an account to attempt to log in. PAM (the authentication system under SSH) already does not allow attackers to guess user accounts. However, if you have a set of users you want to never allow SSH access to, you can use the AllowUsers directive. To do this, add a line like this in your sshd configuration file:

AllowUsers jsmith tallen

Or to limit access to a specific user from a specific ip address, use something like:

AllowUsers jsmith@192.168.0.2 tallen

The AllowUsers directive is the list of all users that are allowed to log in through SSH. If you have a large number of users, or you intend to have a changing list of users, you can also use the AllowGroups directive and create a group specifically for users allowed to log in through SSH. You can add a group for this purpose with this command:

sudo addgroup sshlogin

Using the example name of 'sshlogin', you would then add this line to your sshd configuration file:

AllowGroups sshlogin

After you restart sshd, only users in the AllowUsers list (or users who are members of the 'sshlogin' group if you chose that method instead) will be allowed to log in through SSH. See man (5) sshd_config for details.

Disable Password Authentication

One may also tighten security by forcing public key authentication and disabling password authentication entirely. While this greatly enhances security and completely defeats brute-force attacks, it will lock you out of the machine if you lose your private key. Use with caution.

On the client, generate a key pair with a strong passphrase using:

$ ssh-keygen -t rsa -b 4096
Generating public/private rsa key pair.
Enter file in which to save the key (/home/<username>/.ssh/id_rsa):
Enter passphrase (empty for no passphrase):
Enter same passphrase again:
Your identification has been saved in /home/<username>/.ssh/id_rsa.
Your public key has been saved in /home/<username>/.ssh/id_rsa.pub.
The key fingerprint is:
xx:xx:xx:xx:xx:xx:xx:xx:xx:xx:xx:xx:xx:xx:xx:xx <username>@<client hostname>

Next, copy the above key to the machine you'd like to login to (this puts the key in /home/<username>/.ssh/authorized_keys on the server):

$ ssh-copy-id -i /home/<username>/.ssh/id_rsa.pub <username>@<server hostname>

Verify you can login to the server with your new key:

$ ssh <username>@<server hostname>
Enter passphrase for key '/home/<username>/.ssh/id_rsa':
<username>@<server hostname:~$

Once you have verified you can login to the server with your new key pair, update /etc/ssh/sshd_config on the server to have:

# Change to no to disable tunnelled clear text passwords
#PasswordAuthentication yes
PasswordAuthentication no

See man (5) sshd_config and man (1) ssh-keygen for more details.

Use SFTP instead of FTP

Suppose you want to give some users the ability to upload and download files from their home directories (or ~/public_html), but not a full shell account. In Ubuntu 8.10 (Intrepid) and later, this is simple to achieve with OpenSSH's SFTP component.

In /etc/ssh/sshd_config, change

Subsystem sftp /usr/lib/openssh/sftp-server

to

Subsystem sftp internal-sftp

Add a stanza like the following to the *bottom* of the same file. It must be at the bottom, because Match groups are only terminated by the end of the file (or another Match stanza).

Match User alice
  ChrootDirectory %h/public_html
  X11Forwarding no
  AllowAgentForwarding no
  AllowTcpForwarding no
  ForceCommand internal-sftp

Now alice can use SFTP to read from and write to her ~/public_html/ directory, but can't use OpenSSH to get a shell, nor even to read files outside of ~/public_html.

If you have many such users, you can change User alice to `Group sftponly, and add users to the sftponly group using usermod(8)`.

Chrooting SFTP was considerably more tedious in Ubuntu 8.04 (Hardy) and earlier, where ChrootDirectory was not available. For FTP on such systems, consider vsftpd: it is more security-conscious than other FTP implementations.

"su" program available to non-admin users

This is not necessarily a problem alone, but if there are accounts with weak passwords on the system a malicious non-admin user (or malicious software they are using) might use su to gain access to such accounts. To deny non-admin users access to su, type this in a terminal:

sudo dpkg-statoverride --update --add root admin 4750 /bin/su

Disable IPv6

IPv6 is enabled on Ubuntu by default. Most firewalls only apply to IPv4, and completely ignore IPv6. If you don't use IPv6 at all, you can prevent it loading at boot time by changing alias net-pf-10 ipv6 to alias net-pf-10 off in /etc/modprobe.d/aliases, and scheduling a reboot.


CategorySecurity

StricterDefaults (last edited 2013-10-18 13:06:29 by nick-humphrey)