sshield

SSH server hardening auditor in portable C11 with zero dependencies. Grades sshd_config and authorized_keys files the way certwatch grades TLS: one letter per file, one line per problem, CI-friendly exit codes.

It knows what the file doesn't say, too: OpenSSH defaults to PasswordAuthentication yes, and a config that disables passwords but leaves keyboard-interactive PAM at its default is still accepting them.

Demo

$ sshield examples/sshd_config examples/authorized_keys

sshd_config  examples/sshd_config  GRADE F
  WARN  line 3: PermitRootLogin prohibit-password - prefer 'no' unless root keys are required
  CRIT  line 4: PasswordAuthentication yes - passwords can be brute-forced; use keys only
  WARN  line 5: X11Forwarding yes - exposes the client X server; disable unless required
  WARN  line 6: MaxAuthTries 6 - allows many guesses per connection; 3-4 is plenty
  CRIT  line 7: Ciphers include a CBC-mode cipher - plaintext-recovery attacks
  INFO  ClientAliveInterval not set - idle sessions never time out

authorized_keys  examples/authorized_keys  GRADE F
  CRIT  line 2: 1024-bit RSA key - too small to be safe; replace with ed25519
  WARN  line 3: 2048-bit RSA key - below current 3072-bit guidance; rotate to ed25519
  INFO  line 4: ssh-ed25519 key without from=/command=/restrict options - usable from anywhere
  WARN  line 6: duplicate of the key on line 4

3 critical, 5 warning(s), 5 info

What it checks

sshd_config (global section; Match blocks are skipped)

• Root login, password / empty-password / keyboard-interactive authentication
• Weak Ciphers, MACs, and KexAlgorithms entries (CBC, RC4, 3DES, MD5, SHA-1, group1/group14 key exchange)
• MaxAuthTries, LoginGraceTime, ClientAliveInterval, X11Forwarding, PermitUserEnvironment, legacy protocol 1
• Dangerous defaults for directives the file never sets

authorized_keys (the base64 blob is actually decoded - RFC 4253 wire format)

• DSA keys (rejected by modern OpenSSH), RSA below 2048/3072 bits
• ECDSA NIST-curve advisories, with ed25519 as the recommendation
• Keys without from= / command= / restrict options
• Duplicate keys, unparseable entries sshd would silently ignore
• Group/world-writable file permissions

File type is detected from content, so you can just point it at both:

sshield /etc/ssh/sshd_config ~/.ssh/authorized_keys

Build

Requires CMake 3.16+ and any C11 compiler. No libraries beyond libc.

cmake -B build -DCMAKE_BUILD_TYPE=Release
cmake --build build
./build/sshield --version

Usage

usage: sshield [--json] <sshd_config | authorized_keys> ...

Exit codes

Code	Meaning
0	clean, or informational findings only
1	warnings
2	critical findings, or a file could not be read

Drop it in a cron job or CI step and alert on non-zero:

sshield --json /etc/ssh/sshd_config | jq '.files[0].grade'

Design notes

• Built with -Wall -Wextra -Werror, C11, no VLAs, no sprintf.
• Checks are table-driven: adding a directive check is one struct entry.
• The key parser implements base64 and the SSH public-key wire format directly (~100 lines) rather than depending on OpenSSL.
• Tests run with CTest against real ssh-keygen output embedded as fixtures.

Development

ctest --test-dir build --output-on-failure

License

MIT