mediumaccess-controlverified ✓rollback-safe

Configure SSH to use FIPS-validated key exchange algorithms (Ubuntu)

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

Description

The SSH server must be configured to use only FIPS 140-3 validated key exchange algorithms. Ubuntu's approved set is broader than RHEL's (it adds the diffie-hellman-group16-sha512 and -group14-sha256 algorithms), so this rule carries the Ubuntu-specific allowlist and remediation value.

Rationale

FIPS-validated key exchange algorithms protect the establishment of the session key against cryptographic attack. Restricting the SSH server to the approved set prevents negotiation of a weaker exchange.

Check → Remediate

Checkcommand

command -v sshd >/dev/null 2>&1 || exit 0
kex=$(sshd -T 2>/dev/null | awk '/^kexalgorithms /{print $2}')
if [ -z "$kex" ]; then
  echo "FAIL: could not read effective key exchange algorithms (sshd -T needs root)"
  exit 1
fi
if echo "$kex" | tr ',' '\n' | grep -qvE '^(ecdh-sha2-nistp256|ecdh-sha2-nistp384|ecdh-sha2-nistp521|diffie-hellman-group-exchange-sha256|diffie-hellman-group16-sha512|diffie-hellman-group14-sha256)$'; then
  echo "FAIL: non-FIPS key exchange algorithm in effective config: $kex"
  exit 1
fi
echo "OK: $kex"

expected_exit:: 0

Remediateconfig_set_dropin

dir:: /etc/ssh/sshd_config.d
file:: 00-kensa-fips-kex.conf
key:: KexAlgorithms
value:: ecdh-sha2-nistp521,ecdh-sha2-nistp384,ecdh-sha2-nistp256,diffie-hellman-group-exchange-sha256,diffie-hellman-group16-sha512,diffie-hellman-group14-sha256
restart:: sshd

Framework references

STIG

V-270669 / UBTU-24-100840V-260533 / UBTU-22-255060

NIST 800-53

SC-8SC-13

Live verification

ubuntu22:checkubuntu24:checkubuntu26:check

#ssh#fips#kex#cryptography#stig