highaccess-controlunverified

Ensure the SSH client uses only FIPS-approved ciphers

ssh-client-ciphers-fips · RHEL ≥ 8 · 1 impl

Description

The SSH client Ciphers in /etc/crypto-policies/back-ends/openssh.config must be exactly the DoD/FIPS 140-3 approved set — the AES-GCM and AES-CTR algorithms — with no weaker cipher (e.g. any *-cbc) present.

Rationale

CBC-mode and other non-approved ciphers are vulnerable to known attacks against SSH confidentiality. Restricting the client to FIPS-approved AES-GCM/CTR ciphers ensures session data stays protected.

Check → Remediate

Checkcommand

cfg=/etc/crypto-policies/back-ends/openssh.config
approved="aes128-ctr,aes128-gcm@openssh.com,aes256-ctr,aes256-gcm@openssh.com"
line=$(grep -iE '^[[:space:]]*Ciphers[[:space:]]' "$cfg" 2>/dev/null | awk '{print $2}' | tail -1)
if [ -z "$line" ]; then
  echo "FAIL: no Ciphers line in $cfg"
  exit 1
fi
got=$(printf '%s' "$line" | tr ',' '\n' | sort | paste -sd, -)
if [ "$got" = "$approved" ]; then
  echo "OK: SSH client Ciphers are exactly the FIPS-approved set"
  exit 0
fi
echo "FAIL: SSH client Ciphers are '$line' (expected only the FIPS-approved AES-GCM/CTR set)"
exit 1

expected_exit:: 0

Remediatemanual

note:: The client Ciphers are derived from the system crypto policy. Set the FIPS policy and reinstall crypto-policies so the openssh back-end is regenerated: update-crypto-policies --set FIPS dnf -y reinstall crypto-policies (See rule crypto-policy-fips for the system-wide policy.)

Framework references

STIG

V-281010 / RHEL-10-300030V-270177 / RHEL-09-255064V-272483 / RHEL-08-010297

NIST 800-53

AC-17SC-13

#ssh#client#ciphers#fips#crypto-policy#stig