SSH Hardening Auditor

A single-file, dependency-free Python script that audits an sshd_config (or a live server's effective config via sshd -T) against CIS Benchmark / NIST-aligned SSH hardening recommendations — catches weak ciphers/KEX/MACs, password auth left enabled, root login, empty passwords, and more.

By Errant Solutions (errant.solutions), makers of Rustinion (device provisioning & monitoring), /Dev/Out Digital (security awareness training), OOPS! (hands-on security labs), and SSHerpa (Android SSH client).

Why

Every "harden your SSH server" checklist online is a blog post you have to manually diff against your config by eye. This is a real, runnable, testable tool instead — one Python file, stdlib only, works on any box with Python 3.8+.

Install

pipx install git+https://git.canutethegreat.com/CanuteTheGreat/sshd-hardening-auditor.git        # latest release branch; puts `sshd-hardening-auditor` on PATH
pipx install git+https://git.canutethegreat.com/CanuteTheGreat/sshd-hardening-auditor.git@v1.2.2  # a pinned release
brew install canutethegreat/errant-tools/sshd-hardening-auditor   # after: brew tap canutethegreat/errant-tools https://git.canutethegreat.com/CanuteTheGreat/homebrew-errant-tools
scoop install errant-tools/sshd-hardening-auditor                 # after: scoop bucket add errant-tools https://git.canutethegreat.com/CanuteTheGreat/scoop-errant-tools

Or download the single file (stdlib-only Python 3.8+) and run it directly: curl -fsSLO https://errant.solutions/downloads/sshd_audit.py && python3 sshd_audit.py --help. Every release has a changelog, the wheel, the sdist and a SHA256SUMS file. Not on PyPI yet, so plain pipx install sshd-hardening-auditor doesn't work until it's published there.

Usage

sshd-hardening-auditor /etc/ssh/sshd_config      # a config file (the default path)
sudo sshd-hardening-auditor --live               # the effective config from `sshd -T`
sshd-hardening-auditor --remote host.example.com # what a running server offers on the wire (port 22)
sshd-hardening-auditor --remote 10.0.0.5:2222 --json
sshd-hardening-auditor --fail-on medium sshd_config   # CI gate: exit 1 on MEDIUM or worse
sshd-hardening-auditor --html report.html sshd_config

--remote connects, exchanges SSH identification strings and reads the server's KEXINIT (the algorithm lists it offers). It never logs in or sends credentials, so it works on any server you're allowed to connect to, including appliances where you can't run sshd -T.

What it checks

• Authentication: PermitRootLogin, PasswordAuthentication, PermitEmptyPasswords, HostbasedAuthentication, Protocol (HIGH); KbdInteractiveAuthentication (ChallengeResponseAuthentication), StrictModes, X11Forwarding, MaxAuthTries, PermitUserEnvironment, IgnoreRhosts (MEDIUM); GSSAPIAuthentication, UsePAM, AllowUsers/AllowGroups/DenyUsers/DenyGroups (LOW)
• CIS session limits and logging: LogLevel (INFO/VERBOSE), MaxSessions, MaxStartups (10:30:60 or stricter), Banner, ClientAliveInterval, ClientAliveCountMax, LoginGraceTime, AllowTcpForwarding (LOW)
• Algorithms (configured lists, sshd -T output, or what --remote sees offered):
  • key exchange: SHA-1 and 1024-bit DH groups (HIGH), NIST ecdh-sha2-nistp* (LOW), and whether a hybrid post-quantum KEX (sntrup761x25519, mlkem768x25519) is offered (LOW)
  • host keys: ssh-dss (HIGH), SHA-1 ssh-rsa (MEDIUM), ecdsa-sha2-nistp* (LOW)
  • ciphers: every CBC mode, arcfour, 3DES, blowfish, cast128, none (HIGH)
  • MACs: MD5 and truncated SHA-1 (HIGH), hmac-sha1, umac-64 (MEDIUM), non-ETM (encrypt-and-MAC) variants (LOW)
• Terrapin (CVE-2023-48795, --remote): chacha20-poly1305 or CBC+EtM offered without strict key exchange (MEDIUM); pre-authentication zlib compression (LOW)
• Files (CIS 5.2.1-5.2.3): sshd_config and host private keys owned by root and mode 0600; RSA host keys under 2048 bits (HIGH) or under 3072 (LOW), DSA host keys (HIGH). --no-file-checks skips these.

The config is read the way sshd itself reads it: Include drop-ins (e.g. /etc/ssh/sshd_config.d/*.conf) are expanded in place, the first value of a directive wins, settings inside Match blocks don't override the global ones, and an unset directive is judged on OpenSSH's built-in default (shown as <value> (default)). Times use sshd's format (90, 1m30s, 2h), and LoginGraceTime 0 / ClientAliveCountMax 0 (which switch the limit off) fail their rules. One version difference: OpenSSH before 10.1 keeps the last of several MaxStartups lines; --live always shows what your sshd really uses. An algorithm list written with +/-/^ is relative to OpenSSH's defaults, so only the named algorithms are judged; use --live or --remote to judge the full effective list. The test suite cross-checks file mode against the real sshd -T (including 200 random configs in tests/test_accuracy.py), and --remote against a real running sshd, on every CI run.

Exit codes (same convention as every Errant Solutions tool):

Code	Meaning
0	no failed check at or above --fail-on (default high)
1	at least one
2	the config couldn't be read, sshd -T couldn't run, or the remote server couldn't be reached

Each finding includes the current value, the expected value, and a one-line rationale — so it doubles as a mini security-training doc for whoever reads the output.

CI integration example

# .forgejo/workflows/ssh-audit.yml (or any CI)
- name: Audit sshd_config
  run: pipx run --spec git+https://git.canutethegreat.com/CanuteTheGreat/sshd-hardening-auditor.git sshd-hardening-auditor --no-file-checks deploy/sshd_config

License

MIT — see LICENSE. Free to use, modify, and redistribute. If it saves you time, pay what you think it's worth here — or check out Rustinion for full fleet-wide config compliance monitoring.

Roadmap ideas (not yet built)

• Compare against a specific CIS Benchmark level (1 vs 2) profile flag
• --fix mode that writes a hardened config with an explained diff

Tests

tests/ runs every option and check end-to-end (including --live and --remote against a real OpenSSH server) on Python 3.8 and current Python in Forgejo CI: sh ci/run-tests.sh 3.13 reproduces it locally (needs docker).