sshd Policy

This policy is an application of the recommendations given by ssh-audit¹.

Usage

Insert the following into your Butane configuration.

variant: fcos
version: 1.4.0
ignition:
  config:
    merge:
    - source: https://docs.doubleu.codes/kb/coreos/sshd-policy/sshd-policy.ign.gz
      compression: gzip
      verification:
        hash: sha512-fe9d38ffb06fcaa83ed7778612d3fc12415a9e54c577c908713620cabc58438f9a741eceac72803c7b3d480650548d3f7e539b5365d050f4b3735b215336dd96

Ensure the sha512 sum reflects the one below, which is loaded directly from the sum file.

fe9d38ffb06fcaa83ed7778612d3fc12415a9e54c577c908713620cabc58438f9a741eceac72803c7b3d480650548d3f7e539b5365d050f4b3735b215336dd96

Policy Butane Format

# yaml-language-server: $schema=https://raw.githubusercontent.com/Relativ-IT/Butane-Schemas/Release/Butane-Schema.json
variant: fcos
version: 1.4.0
storage:
  files:
  - path: /etc/ssh/sshd_config.d/10_host-keys.conf
    # Use only RSA and Ed25519 host keys.
    # Explicitly exclude ECDSA as it can be vulnerable if weak RNG or low
    # entropy was used to generate keys.
    mode: 0644
    contents:
      inline: |
        HostKey /etc/ssh/ssh_host_rsa_key
        HostKey /etc/ssh/ssh_host_ed25519_key
  - path: /etc/ssh/sshd_config.d/10_crypto-policy.conf
    # The crypto-policy is defined like this instead of using a 'real'
    # crypto-policy because Fedora CoreOS does not ship with the
    # 'update-crypto-policies' tools, and including it depends on Python.
    # The default policy is loaded by '/etc/ssh/sshd_config.d/50-redhat.conf'
    # from '/etc/crypto-policies/back-ends/opensshserver.config'.
    # Loading the these policies earlier than that ensures that the duplicate
    # options defined in those policies are ignored.
    mode: 0644
    contents:
      inline: |
        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
        KexAlgorithms curve25519-sha256,curve25519-sha256@libssh.org,diffie-hellman-group16-sha512,diffie-hellman-group18-sha512,diffie-hellman-group-exchange-sha256
        HostKeyAlgorithms ssh-ed25519,ssh-ed25519-cert-v01@openssh.com,rsa-sha2-256,rsa-sha2-512
        PubkeyAcceptedKeyTypes ssh-ed25519,ssh-ed25519-cert-v01@openssh.com,rsa-sha2-256,rsa-sha2-512
systemd:
  units:
  - name: sshd-moduli.service
    # Mitigation for the LOGJAM attack. Moduli with lengths less than 2048 are
    # removed. Default moduli lengths are 2047, 3071, 4095, 6143, 7678, and
    # 8191. Instead of regenerating (which takes a long time), truncate to the
    # next smallest uneffected (3071).
    enabled: true
    contents: |
      [Unit]
      After=coreos-ignition-firstboot-complete.service
      Before=sshd.service

      [Service]
      Type=oneshot
      ExecStart=/usr/bin/bash -c "/usr/bin/awk '$5 >= 3071' /etc/ssh/moduli > /etc/ssh/moduli.safe"
      ExecStartPost=/usr/bin/mv -f /etc/ssh/moduli.safe /etc/ssh/moduli

      [Install]
      RequiredBy=multi-user.target
  - name: sshd-disable-default-host-keys.service
    # As of Fedora CoreOS 35, HostKeys are commented out by default, but just in
    # case that changes in the future, ensure they are disabled in 'sshd_config'
    # so that the only HostKey options are loaded from the file defined above at
    # '/etc/ssh/sshd_config/10_host-keys.conf'.
    enabled: true
    contents: |
      [Unit]
      After=coreos-ignition-firstboot-complete.service
      Before=sshd.service

      [Service]
      Type=oneshot
      ExecStart=/usr/bin/sed -i 's/^HostKey\(.*\)/#HostKey\1/g' /etc/ssh/sshd_config

      [Install]
      RequiredBy=multi-user.target
  - name: sshd-keygen@ecdsa.service
    # Disable ECDSA host keys from generating
    # tl;dr It's spooky and possibly glows in the dark
    #
    # ECDSA has known vulnerabilities as explained above, but also has magic
    # seed numbers that have never been explained, leading many to believe that
    # it's backdoored by the NSA. Not much you can do if a nation state is in
    # your threat model, but why make it easy?
    mask: true

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1. https://github.com/jtesta/ssh-audit ↩

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Last update: April 2, 2022
Created: April 2, 2022