Audit OpenSSH

Audit OpenSSH

Synopsis #

  1. Install ssh-audit with pkg_add(1) .
  2. Run ssh-audit against the target to inventory supported algorithms.
  3. Constrain key exchange, host-key, and MAC algorithms in sshd_config(5) .
  4. Validate configuration with sshd(8) and restart via rcctl(8) .
  5. Re-run ssh-audit to confirm the intended policy.
  6. Optionally remove small Diffie–Hellman moduli per moduli(5) .
  7. Optionally rotate host keys with ssh-keygen(1) .

Overview #

This chapter describes how to assess and harden an OpenSSH server on OpenBSD using the ssh-audit utility. The workflow is to install and run ssh-audit, constrain algorithms in sshd_config(5) , validate the daemon with sshd(8) , restart via rcctl(8) , and optionally curate Diffie–Hellman moduli per moduli(5) and rotate host keys with ssh-keygen(1) .

A key exchange algorithm (KEX) establishes shared secrets. A host-key algorithm authenticates the server identity. A message authentication code (MAC) authenticates message integrity. Restricting these sets reduces attack surface while maintaining required client compatibility.

Install and Run ssh-audit #

Install the package with pkg_add(1) and perform a local audit.

# pkg_add ssh-audit
$ ssh-audit localhost

To audit a remote host, specify the target and optional port. IPv6 literals require brackets.

$ ssh-audit example.org
$ ssh-audit -p 2222 example.org
$ ssh-audit -p 22 '[2001:db8::1]'

The utility reports the server banner and the offered KEX, host keys, ciphers, and MACs, highlighting weak or deprecated choices.

Harden Algorithm Policy in sshd_config #

Edit /etc/ssh/sshd_config to permit a concise, modern subset of algorithms. The commands below append conservative settings suitable for a wide range of clients. Review and adjust to match the deployment’s compatibility requirements. See sshd_config(5) for directive semantics.

Key exchange. Prefer Curve25519 and strong finite-field Diffie–Hellman groups. The hybrid sntrup761x25519-sha512@openssh.com may be included where supported by both server and clients.

# sh -c 'printf "\n# Restrict key exchange\nKexAlgorithms \
curve25519-sha256,curve25519-sha256@libssh.org,\
diffie-hellman-group16-sha512,diffie-hellman-group18-sha512,\
diffie-hellman-group-exchange-sha256\n" >> /etc/ssh/sshd_config'

MACs. Prefer encrypt-then-MAC variants and avoid SHA-1.

# sh -c 'printf "# Restrict MACs\nMACs \
umac-128-etm@openssh.com,hmac-sha2-256-etm@openssh.com,\
hmac-sha2-512-etm@openssh.com\n" >> /etc/ssh/sshd_config'

Host keys. Prefer Ed25519 and RSA with SHA-2 signatures.

# sh -c 'printf "# Restrict HostKey algorithms\nHostKeyAlgorithms \
ssh-ed25519,rsa-sha2-256,rsa-sha2-512\n" >> /etc/ssh/sshd_config'

When reducing HostKeyAlgorithms, ensure that HostKey file paths in /etc/ssh/sshd_config reference only the retained key types. For example, omit ssh_host_ecdsa_key when ECDSA is disabled. See sshd_config(5) .

Validate and Apply the Configuration #

Test the daemon configuration and restart the service. Use sshd(8) with -t to validate settings and rcctl(8) to restart.

# sshd -t
# rcctl restart sshd

Re-run the audit to confirm the intended algorithm set is offered.

$ ssh-audit localhost

Optional: Remove Small Diffie–Hellman Moduli #

Group-exchange parameters reside in /etc/ssh/moduli. Filter out entries with size below 3071 bits to target at least approximately 128-bit strength. Refer to moduli(5) for field details.

# awk '$5 >= 3071' /etc/ssh/moduli > /etc/ssh/moduli.safe
# mv -f /etc/ssh/moduli.safe /etc/ssh/moduli
# rcctl restart sshd

Optional: Rotate Host Keys #

When deprecating algorithms or strengthening parameters, rotate host keys. Back up existing keys, remove unwanted types, and generate Ed25519 and RSA (3072-bit) keys. Use ssh-keygen(1) .

# cd /etc/ssh
# install -d -m 0700 ./backup-keys && mv ssh_host_* ./backup-keys/
# ssh-keygen -t ed25519 -N '' -f /etc/ssh/ssh_host_ed25519_key
# ssh-keygen -t rsa -b 3072 -o -a 100 -N '' -f /etc/ssh/ssh_host_rsa_key

Ensure that HostKey directives reference the new files:

HostKey /etc/ssh/ssh_host_ed25519_key
HostKey /etc/ssh/ssh_host_rsa_key

Validate and restart:

# sshd -t
# rcctl restart sshd

Client Compatibility Considerations #

Tightening policies can break older clients that lack support for Curve25519 or RSA SHA-2 signatures. Where legacy access is unavoidable, conditional match blocks in sshd_config can scope exceptions, or a separate bastion with a relaxed policy can be provided. See sshd_config(5) and ssh(1) .

Verification #

Use ssh-audit to confirm removal of weak items such as SHA-1 MACs, small DH groups, or disabled host-key types. Monitor authentication and negotiation logs via the system’s syslog(3) facilities after deployment.

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