SSH Server CBC Mode Ciphers Enabled

Tenable plugin 70658 fires when an SSH server *offers* CBC-mode ciphers such as aes128-cbc, aes256-cbc, or 3des-cbc. Tenable maps it to CVE-2008-5161, an old SSH protocol weakness where CBC-mode traffic can leak small amounts of plaintext under narrow conditions. Important nuance: the plugin explicitly says it checks cipher *configuration*, not whether the underlying SSH implementation/version is itself vulnerable.

The vendor LOW is directionally right, and I'd actually lean slightly lower in operational urgency. Real exploitation needs an *on-path* attacker, repeated session disruption, and favorable plaintext positioning; impact is limited to partial confidentiality loss, not code execution or auth bypass. For a 10,000-host estate, this belongs in SSH hardening and exposure reduction, not in the same queue as remotely exploitable edge RCEs.

Why this verdict

• Requires MITM/on-path position: this is not an unauthenticated edge-to-root bug; the attacker must already control the path between client and server.
• Negotiation has to fall back to CBC: if modern ciphers like CTR, GCM, or [email protected] are negotiated instead, the attack chain stops.
• Impact is narrow: the weakness leaks limited plaintext under constrained conditions; it does not directly provide RCE, privilege escalation, or auth bypass.

Why not higher?

A higher rating would require a realistic path from Internet reachability to material compromise. That is missing here. The exploit chain assumes post-initial-access network control, repeated session interference, and useful plaintext recovery, which is too much friction for a HIGH or CRITICAL call.

Why not lower?

I would not mark it IGNORE because CBC on admin protocols is still unnecessary legacy cryptography and occasionally appears on exposed network gear or brittle appliances. The detection is usually accurate, and disabling CBC is a sensible hardening step even if the practical exploitability is poor.

1. Disable CBC ciphers on SSH servers — Remove *-cbc and 3des-cbc from the server cipher list and standardize on [email protected], aes*[email protected], or aes*-ctr. For a LOW finding there is no SLA; handle it in normal hardening/backlog hygiene, starting with Internet-exposed and admin-plane systems.
2. Reduce SSH exposure — Move SSH behind VPN, bastions, ACLs, or management-plane segmentation so an attacker cannot easily obtain the traffic path in the first place. For a LOW finding there is no SLA; fold this into the next remote-admin exposure review.
3. Kill legacy client exceptions — CBC often survives because of a few old automation clients, appliances, or embedded stacks. Identify those dependencies, isolate them, and remove broad CBC allowances during normal crypto-baseline maintenance; for LOW, this is backlog hygiene rather than emergency work.
4. Prefer key auth and MFA on jump paths — Even if a session were manipulated, environments using key-based auth and MFA reduce the value of recovered plaintext compared with password-heavy workflows. For LOW, implement during routine SSH access-hardening cycles.

Run this on the target Linux/Unix SSH server or on a mounted image of its config, not from an auditor workstation. Invoke it as sudo bash check_ssh_cbc.sh /etc/ssh/sshd_config; it needs read access to the SSH daemon config and may need sudo so sshd -T can evaluate the effective settings cleanly.

#!/usr/bin/env bash
# check_ssh_cbc.sh
# Exit codes:
#   0 = PATCHED (no CBC ciphers enabled)
#   1 = VULNERABLE (one or more CBC ciphers enabled)
#   2 = UNKNOWN (could not determine)

set -u

CFG="${1:-/etc/ssh/sshd_config}"

if ! command -v sshd >/dev/null 2>&1; then
  echo "UNKNOWN: sshd binary not found on this host"
  exit 2
fi

if [ ! -r "$CFG" ]; then
  echo "UNKNOWN: cannot read config file: $CFG"
  exit 2
fi

# Try to get effective config first.
CIPHERS="$(sshd -T -f "$CFG" 2>/dev/null | awk '/^ciphers / {print $2; exit}')"

# Fallback: parse configured line if sshd -T did not return a value.
if [ -z "$CIPHERS" ]; then
  CIPHERS="$(grep -Ei '^[[:space:]]*Ciphers[[:space:]]+' "$CFG" | tail -n 1 | sed -E 's/^[[:space:]]*Ciphers[[:space:]]+//I')"
fi

if [ -z "$CIPHERS" ]; then
  echo "UNKNOWN: unable to determine configured SSH ciphers"
  exit 2
fi

BAD=()
IFS=',' read -r -a ARR <<< "$CIPHERS"
for c in "${ARR[@]}"; do
  c="${c// /}"
  if [[ "$c" == *-cbc ]] || [[ "$c" == "3des-cbc" ]]; then
    BAD+=("$c")
  fi
done

if [ ${#BAD[@]} -gt 0 ]; then
  echo "VULNERABLE: CBC ciphers enabled -> ${BAD[*]}"
  exit 1
else
  echo "PATCHED: no CBC ciphers enabled -> $CIPHERS"
  exit 0
fi

Sources

1. Tenable Nessus Plugin 70658
2. NVD CVE-2008-5161
3. CERT VU#958563 SSH CBC vulnerability
4. OpenSSH Security page
5. OpenSSH Legacy Options
6. Mozilla OpenSSH Guidelines
7. CISA Known Exploited Vulnerabilities Catalog
8. FIRST EPSS documentation