highaccess-controlverified ✓rollback-safe

Configure SSH to use FIPS-approved ciphers (Ubuntu)

ssh-ciphers-fips-ubuntu · UBUNTU ≥ 22 · 1 impl

Description

The SSH daemon must be configured to use only FIPS 140-3 approved ciphers for session encryption. Ubuntu's approved cipher set differs from RHEL's (it excludes aes192-ctr), so this rule carries the Ubuntu-specific allowlist and remediation value.

Rationale

FIPS-approved ciphers have been validated for cryptographic strength. Restricting the SSH daemon to the Ubuntu-approved set ensures encrypted sessions cannot be downgraded to a weaker algorithm.

Check → Remediate

Checkcommand

command -v sshd >/dev/null 2>&1 || exit 0
ciphers=$(sshd -T 2>/dev/null | awk '/^ciphers /{print $2}')
if [ -z "$ciphers" ]; then
  echo "FAIL: could not read effective ciphers (sshd -T needs root)"
  exit 1
fi
if echo "$ciphers" | tr ',' '\n' | grep -qvE '^(aes128-ctr|aes256-ctr|aes128-gcm@openssh\.com|aes256-gcm@openssh\.com)$'; then
  echo "FAIL: non-FIPS cipher in effective config: $ciphers"
  exit 1
fi
echo "OK: $ciphers"

expected_exit:: 0

Remediateconfig_set_dropin

dir:: /etc/ssh/sshd_config.d
file:: 00-kensa-fips-ciphers.conf
key:: Ciphers
value:: aes256-gcm@openssh.com,aes128-gcm@openssh.com,aes256-ctr,aes128-ctr
restart:: sshd

Framework references

STIG

V-270667 / UBTU-24-100820V-260531 / UBTU-22-255050

NIST 800-53

SC-8SC-13

Live verification

ubuntu22:checkubuntu24:checkubuntu26:check

#ssh#fips#ciphers#cryptography#stig