.. SPDX-FileCopyrightText: 2023 Dominik Wombacher <dominik@wombacher.cc>
..
.. SPDX-License-Identifier: CC-BY-SA-4.0

FreeBSD 13 Base System OpenSSH Server Hardening
###############################################

:date: 2022-03-17
:modified: 2022-03-18
:tags: FreeBSD, OpenSSH, Hardening, SSH
:description: Howto harden the built-in OpenSSH Server on FreeBSD 13
:category: Unix
:slug: freebsd_13_base_system_openssh_server_hardening
:author: Dominik Wombacher
:lang: en
:transid: freebsd_13_base_system_openssh_server_hardening 
:status: published

Default sshd configs tend to focus more on compatibility instead security. Therefore hardening 
should be one of the first things after setup a new system. 

I'm using the OpenSSH Daemon which comes with the FreeBSD Base System. 
When you want to use the :code:`openssh-portable` port instead, skip *Step 3*, the rest should be identical.

Step 1) Delete existing host keys, generate new rsa and ed25519 key:

.. code-block:: bash

  rm /etc/ssh/ssh_host_*
  ssh-keygen -q -t rsa -b 4096 -f ssh_host_rsa_key -N ""
  ssh-keygen -q -t ed25519 -f ssh_host_ed25519_key -N ""

Step 2) Create new Diffie-Hellman groups and avoid small moduli

.. code-block:: bash

  ssh-keygen -G moduli-3072.candidates -b 3072
  ssh-keygen -T moduli-3072 -f moduli-3072.candidates
  mv moduli-3072 /etc/ssh/moduli
  rm moduli-3072.candidates

Step 3) Disable DSA and ECDSA host keys, only use RSA and ED25519

.. code-block:: bash

  sysrc sshd_dsa_enable="NO"
  sysrc sshd_ecdsa_enable="NO"
  sysrc sshd_ed25519_enable="YES"
  sysrc sshd_rsa_enable="YES"

Step 4) Optimize :code:`/etc/ssh/sshd_config`, improve security, restrict allowed key exchange, cipher and MAC algorithms

.. code-block::

  # Hardening
  HostKey /etc/ssh/ssh_host_rsa_key
  HostKey /etc/ssh/ssh_host_ed25519_key
  KexAlgorithms curve25519-sha256,curve25519-sha256@libssh.org,diffie-hellman-group16-sha512,diffie-hellman-group18-sha512,diffie-hellman-group-exchange-sha256
  Ciphers chacha20-poly1305@openssh.com,aes256-gcm@openssh.com,aes128-gcm@openssh.com,aes256-ctr,aes192-ctr,aes128-ctr
  MACs hmac-sha2-256-etm@openssh.com,hmac-sha2-512-etm@openssh.com,umac-128-etm@openssh.com
  HostKeyAlgorithms ssh-ed25519,ssh-ed25519-cert-v01@openssh.com

  # Security
  PermitRootLogin no
  AuthenticationMethods publickey
  ChallengeResponseAuthentication no
  UsePAM no 
  VersionAddendum none
  X11Forwarding no
  AuthorizedKeysFile .ssh/authorized_keys
  Subsystem sftp /usr/libexec/sftp-server

Run :code:`service sshd restart` to apply the new settings, to verify the results of your hardening, 
you can use the CLI Tool `ssh-audit <https://github.com/jtesta/ssh-audit>`_ 
which is also available as `Online Version <https://www.ssh-audit.com>`_.

Following a Custom Policy for ssh-audit based on the above recommendations:

.. code-block::

  name = "Custom Policy - FreeBSD 13 Base System OpenSSH Daemon (2022/03/17)"
  version = 1
  dh_modulus_size_diffie-hellman-group-exchange-sha256 = 2048
  host keys = ssh-ed25519
  key exchanges = curve25519-sha256, curve25519-sha256@libssh.org, diffie-hellman-group16-sha512, diffie-hellman-group18-sha512, diffie-hellman-group-exchange-sha256
  ciphers = chacha20-poly1305@openssh.com, aes256-gcm@openssh.com, aes128-gcm@openssh.com, aes256-ctr, aes192-ctr, aes128-ctr
  macs = hmac-sha2-256-etm@openssh.com, hmac-sha2-512-etm@openssh.com, umac-128-etm@openssh.com

Copy the policy into a file and run :code:`ssh-audit -P=<policy.txt> <servername>`. 
The result should be similar to following example without warnings or errors:

.. code-block::

  # general
  (gen) banner: SSH-2.0-OpenSSH_8.8
  (gen) software: OpenSSH 8.8
  (gen) compatibility: OpenSSH 7.4+, Dropbear SSH 2018.76+
  (gen) compression: enabled (zlib@openssh.com)
  
  # key exchange algorithms
  (kex) curve25519-sha256                     -- [info] available since OpenSSH 7.4, Dropbear SSH 2018.76
  (kex) curve25519-sha256@libssh.org          -- [info] available since OpenSSH 6.5, Dropbear SSH 2013.62
  (kex) diffie-hellman-group16-sha512         -- [info] available since OpenSSH 7.3, Dropbear SSH 2016.73
  (kex) diffie-hellman-group18-sha512         -- [info] available since OpenSSH 7.3
  (kex) diffie-hellman-group-exchange-sha256 (2048-bit) -- [info] available since OpenSSH 4.4
  
  # host-key algorithms
  (key) ssh-ed25519                           -- [info] available since OpenSSH 6.5
  
  # encryption algorithms (ciphers)
  (enc) chacha20-poly1305@openssh.com         -- [info] available since OpenSSH 6.5
                                              `- [info] default cipher since OpenSSH 6.9.
  (enc) aes256-gcm@openssh.com                -- [info] available since OpenSSH 6.2
  (enc) aes128-gcm@openssh.com                -- [info] available since OpenSSH 6.2
  (enc) aes256-ctr                            -- [info] available since OpenSSH 3.7, Dropbear SSH 0.52
  (enc) aes192-ctr                            -- [info] available since OpenSSH 3.7
  (enc) aes128-ctr                            -- [info] available since OpenSSH 3.7, Dropbear SSH 0.52
  
  # message authentication code algorithms
  (mac) hmac-sha2-256-etm@openssh.com         -- [info] available since OpenSSH 6.2
  (mac) hmac-sha2-512-etm@openssh.com         -- [info] available since OpenSSH 6.2
  (mac) umac-128-etm@openssh.com              -- [info] available since OpenSSH 6.2
  
  # fingerprints
  (fin) ssh-ed25519: SHA256:uN9Oton+VmLL793KirVFB+ilD3Bndra4I/3yFntgX8k
  
  # algorithm recommendations (for OpenSSH 8.8)
  (rec) +diffie-hellman-group14-sha256        -- kex algorithm to append 
  (rec) +rsa-sha2-256                         -- key algorithm to append 
  (rec) +rsa-sha2-512                         -- key algorithm to append 

Sources:

- https://ozgurkazancci.com/ssh-server-security-audit-hardening-freebsd
  (Archive: `[1] <https://web.archive.org/web/20220311055553/https://ozgurkazancci.com/ssh-server-security-audit-hardening-freebsd/>`__,
  `[2] <https://archive.today/2022.03.17-211815/https://ozgurkazancci.com/ssh-server-security-audit-hardening-freebsd/>`__)

- https://gist.github.com/koobs/e01cf8869484a095605404cd0051eb11
  (Archive: `[1] <https://web.archive.org/web/20220313140130/https://gist.github.com/koobs/e01cf8869484a095605404cd0051eb11>`__,
  `[2] <https://archive.today/2021.09.16-082343/https://gist.github.com/koobs/e01cf8869484a095605404cd0051eb11>`__)

- https://www.ssh-audit.com/hardening_guides.html
  (Archive: `[1] <https://web.archive.org/web/20220113100935/https://www.ssh-audit.com/hardening_guides.html>`__,
  `[2] <https://archive.today/2022.03.17-211934/https://www.ssh-audit.com/hardening_guides.html>`__)