ssh-hardening (Ansible Role)

Description

This role provides secure ssh-client and ssh-server configurations. It is intended to be compliant with the DevSec SSH Baseline.

Warning: This role disables root-login on the target server! Please make sure you have another user with su or sudo permissions that can login into the server.

Requirements

• Ansible > 2.2.1

Role Variables

Name	Default Value	Description
network_ipv6_enable	false	true if IPv6 is needed
ssh_client_cbc_required	false	true if CBC for ciphers is required. This is usually only necessary, if older M2M mechanism need to communicate with SSH, that don't have any of the configured secure ciphers enabled. CBC is a weak alternative. Anything weaker should be avoided and is thus not available.
ssh_server_cbc_required	false	true if CBC for ciphers is required. This is usually only necessary, if older M2M mechanism need to communicate with SSH, that don't have any of the configured secure ciphers enabled. CBC is a weak alternative. Anything weaker should be avoided and is thus not available.
ssh_client_weak_hmac	false	true if weaker HMAC mechanisms are required. This is usually only necessary, if older M2M mechanism need to communicate with SSH, that don't have any of the configured secure HMACs enabled.
ssh_server_weak_hmac	false	true if weaker HMAC mechanisms are required. This is usually only necessary, if older M2M mechanism need to communicate with SSH, that don't have any of the configured secure HMACs enabled.
ssh_client_weak_kex	false	true if weaker Key-Exchange (KEX) mechanisms are required. This is usually only necessary, if older M2M mechanism need to communicate with SSH, that don't have any of the configured secure KEXs enabled.
ssh_server_weak_kex	false	true if weaker Key-Exchange (KEX) mechanisms are required. This is usually only necessary, if older M2M mechanism need to communicate with SSH, that don't have any of the configured secure KEXs enabled.
ssh_server_ports	['22']	ports on which ssh-server should listen
ssh_client_port	'22'	port to which ssh-client should connect
ssh_listen_to	['0.0.0.0']	one or more ip addresses, to which ssh-server should listen to. Default is all adresseses, but should be configured to specific addresses for security reasons!
ssh_host_key_files	[]	Host keys for sshd. If empty ['/etc/ssh/ssh_host_rsa_key', '/etc/ssh/ssh_host_ecdsa_key', '/etc/ssh/ssh_host_ed25519_key'] will be used, as far as supported by the installed sshd version
ssh_client_alive_interval	600	specifies an interval for sending keepalive messages
ssh_client_alive_count	3	defines how often keep-alive messages are sent
ssh_permit_tunnel	false	true if SSH Port Tunneling is required
ssh_remote_hosts	[]	one or more hosts and their custom options for the ssh-client. Default is empty. See examples in defaults/main.yml.
ssh_allow_root_with_key	false	false to disable root login altogether. Set to true to allow root to login via key-based mechanism.
ssh_allow_tcp_forwarding	false	false to disable TCP Forwarding. Set to true to allow TCP Forwarding.
ssh_allow_agent_forwarding	false	false to disable Agent Forwarding. Set to true to allow Agent Forwarding.
ssh_use_pam	false	false to disable pam authentication.
ssh_deny_users	''	if specified, login is disallowed for user names that match one of the patterns.
ssh_allow_users	''	if specified, login is allowed only for user names that match one of the patterns.
ssh_deny_groups	''	if specified, login is disallowed for users whose primary group or supplementary group list matches one of the patterns.
ssh_allow_groups	''	if specified, login is allowed only for users whose primary group or supplementary group list matches one of the patterns.
ssh_authorized_keys_file	''	change default file that contains the public keys that can be used for user authentication.
ssh_print_motd	false	false to disable printing of the MOTD
ssh_print_last_log	false	false to disable display of last login information
sftp_enabled	false	true to enable sftp configuration
sftp_chroot_dir	/home/%u	change default sftp chroot location
ssh_client_roaming	false	enable experimental client roaming
sshd_moduli_minimum	2048	remove Diffie-Hellman parameters smaller than the defined size to mitigate logjam
ssh_challengeresponseauthentication	false	Specifies whether challenge-response authentication is allowed (e.g. via PAM)
ssh_client_password_login	false	true to allow password-based authentication with the ssh client
ssh_server_password_login	false	true to allow password-based authentication with the ssh server
ssh_banner	false	true to print a banner on login
ssh_client_hardening	true	false to stop harden the client
ssh_client_port	'22'	Specifies the port number to connect on the remote host.
ssh_compression	false	Specifies whether compression is enabled after the user has authenticated successfully.
ssh_max_auth_retries	2	Specifies the maximum number of authentication attempts permitted per connection.
ssh_print_debian_banner	false	true to print debian specific banner
ssh_server_enabled	true	false to disable the opensshd server
ssh_server_hardening	true	false to stop harden the server
ssh_server_match_group	''	Introduces a conditional block. If all of the criteria on the Match line are satisfied, the keywords on the following lines override those set in the global section of the config file, until either another Match line or the end of the file.
ssh_server_match_user	''	Introduces a conditional block. If all of the criteria on the Match line are satisfied, the keywords on the following lines override those set in the global section of the config file, until either another Match line or the end of the file.
ssh_server_permit_environment_vars	false	true to specify that ~/.ssh/environment and environment= options in ~/.ssh/authorized_keys are processed by sshd
ssh_use_dns	false	Specifies whether sshd should look up the remote host name, and to check that the resolved host name for the remote IP address maps back to the very same IP address.
ssh_server_revoked_keys	[]	a list of revoked public keys that the ssh server will always reject, useful to revoke known weak or compromised keys.

Example Playbook

- hosts: localhost
  roles:
    - dev-sec.ssh-hardening

Local Testing

The preferred way of locally testing the role is to use Docker. You will have to install Docker on your system. See Get started for a Docker package suitable to for your system.

You can also use vagrant and Virtualbox or VMWare to run tests locally. You will have to install Virtualbox and Vagrant on your system. See Vagrant Downloads for a vagrant package suitable for your system. For all our tests we use test-kitchen. If you are not familiar with test-kitchen please have a look at their guide.

Next install test-kitchen:

# Install dependencies
gem install bundler
bundle install

Testing with Docker

# fast test on one machine
bundle exec kitchen test default-ubuntu-1204

# test on all machines
bundle exec kitchen test

# for development
bundle exec kitchen create default-ubuntu-1204
bundle exec kitchen converge default-ubuntu-1204

Testing with Virtualbox

# fast test on one machine
KITCHEN_YAML=".kitchen.vagrant.yml" bundle exec kitchen test default-ubuntu-1204

# test on all machines
KITCHEN_YAML=".kitchen.vagrant.yml" bundle exec kitchen test

# for development
KITCHEN_YAML=".kitchen.vagrant.yml" bundle exec kitchen create default-ubuntu-1204
KITCHEN_YAML=".kitchen.vagrant.yml" bundle exec kitchen converge default-ubuntu-1204

For more information see test-kitchen

FAQ / Pitfalls

I can't log into my account. I have registered the client key, but it still doesn't let me it.

If you have exhausted all typical issues (firewall, network, key missing, wrong key, account disabled etc.), it may be that your account is locked. The quickest way to find out is to look at the password hash for your user:

sudo grep myuser /etc/shadow

If the hash includes an !, your account is locked:

myuser:!:16280:7:60:7:::

The proper way to solve this is to unlock the account (passwd -u myuser). If the user doesn't have a password, you should can unlock it via:

usermod -p "*" myuser

Alternatively, if you intend to use PAM, you enabled it via ssh_use_pam: true. PAM will allow locked users to get in with keys.

Why doesn't my application connect via SSH anymore?

Always look into log files first and if possible look at the negotation between client and server that is completed when connecting.

We have seen some issues in applications (based on python and ruby) that are due to their use of an outdated crypto set. This collides with this hardening module, which reduced the list of ciphers, message authentication codes (MACs) and key exchange (KEX) algorithms to a more secure selection.

If you find this isn't enough, feel free to activate the attributes cbc_requires for ciphers, weak_hmac for MACs and weak_kexfor KEX in the variables ssh_client or ssh_server based on where you want to support them.

After using the role Ansibles template/copy/file module does not work anymore!

This role deactivates SFTP. Ansible uses by default SFTP to transfer files to the remote hosts. You have to set scp_if_ssh = True in your ansible.cfg. This way Ansible uses SCP to copy files.

Cannot restart sshd-service due to lack of privileges

If you get the following error when running handler "restart sshd"

Unable to restart service ssh: Failed to restart ssh.service: Access denied

or

failure 1 running systemctl show for 'ssh': Failed to connect to bus: No such file or directory

either run the playbook as root (without become: yes at the playbook level), or add become: yes to the handler.

This is a bug with Ansible: see here and here for more information.

Contributing

See contributor guideline.

License and Author

• Author:: Sebastian Gumprich [email protected]
• Author:: Christoph Hartmann [email protected]

Licensed under the Apache License, Version 2.0 (the "License"); you may not use this file except in compliance with the License. You may obtain a copy of the License at

http://www.apache.org/licenses/LICENSE-2.0

Unless required by applicable law or agreed to in writing, software distributed under the License is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the License for the specific language governing permissions and limitations under the License.