os_sshd_fips_compliant Limit SSHD to FIPS Compliant Connections

Title

Limit SSHD to FIPS Compliant Connections

Control

fips_sshd_config=("Ciphers [email protected]" "HostbasedAcceptedAlgorithms ecdsa-sha2-nistp256,[email protected]" "HostKeyAlgorithms [email protected],[email protected],ecdsa-sha2-nistp256,[email protected]" "KexAlgorithms ecdh-sha2-nistp256" "MACs [email protected],hmac-sha2-256" "PubkeyAcceptedAlgorithms ecdsa-sha2-nistp256,[email protected],[email protected]" "CASignatureAlgorithms ecdsa-sha2-nistp256,[email protected]")
total=0
for config in $fips_sshd_config; do
total=$(expr $(/usr/sbin/sshd -G | /usr/bin/grep -i -c "$config") + $total)
done

echo $total

Discussion

If SSHD is enabled then it _MUST_ be configured to limit the Ciphers, HostbasedAcceptedAlgorithms, HostKeyAlgorithms, KexAlgorithms, MACs, PubkeyAcceptedAlgorithms, CASignatureAlgorithms to algorithms that are FIPS 140 validated.

FIPS 140-3 is the current standard for validating that mechanisms used to access cryptographic modules utilize authentication that meet federal requirements.

Operating systems utilizing encryption _MUST_ use FIPS validated mechanisms for authenticating to cryptographic modules.

NOTE: For more information on FIPS compliance with the version of SSHD included in the macOS, the manual page apple_ssh_and_fips has additional information.

NOTE: On macOS 15.2 and higher the SSH configuration can be reset to the macOS default by running /usr/libexec/reset-ssh-configuration.

Related controls

ac-17.2, ia-7, sc-13, sc-8.1

Fix

[source,bash]
----
if [ -f /etc/ssh/crypto.conf ] && /usr/bin/grep -q "Include /etc/ssh/crypto.conf" /etc/ssh/sshd_config.d/100-macos.conf 2>/dev/null; then
/bin/ln -fs /etc/ssh/crypto/fips.conf /etc/ssh/crypto.conf
fi

include_dir=$(/usr/bin/awk '/^Include/ {print $2}' /etc/ssh/sshd_config | /usr/bin/tr -d '*')

if [[ -z $include_dir ]]; then
/usr/bin/sed -i.bk "1s/.*/Include \/etc\/ssh\/sshd_config.d\/\*/" /etc/ssh/sshd_config
fi

fips_sshd_config=("Ciphers [email protected]" "HostbasedAcceptedAlgorithms ecdsa-sha2-nistp256,[email protected]" "HostKeyAlgorithms [email protected],[email protected],ecdsa-sha2-nistp256,[email protected]" "KexAlgorithms ecdh-sha2-nistp256" "MACs [email protected],hmac-sha2-256" "PubkeyAcceptedAlgorithms ecdsa-sha2-nistp256,[email protected],[email protected]" "CASignatureAlgorithms ecdsa-sha2-nistp256,[email protected]")
sshd_config=$(/usr/sbin/sshd -G)
for config in $fips_sshd_config; do
if ! echo $sshd_config | /usr/bin/grep -q -i "$config" 2>/dev/null; then
/usr/bin/grep -qxF "$config" "${include_dir}01-mscp-sshd.conf" 2>/dev/null || echo "$config" >> "${include_dir}01-mscp-sshd.conf"
fi
done

for file in $(ls ${include_dir}); do
if [[ "$file" == "100-macos.conf" ]]; then
continue
fi
if [[ "$file" == "01-mscp-sshd.conf" ]]; then
break
fi
/bin/mv ${include_dir}${file} ${include_dir}20-${file}
done
----