SSL/TLS Recommended Cipher Suites

Tenable plugin 159543 fires when a TLS endpoint advertises any cipher suite outside Mozilla's recommended modern list — meaning anything other than TLS 1.3 AEAD suites or TLS 1.2 ECDHE+AES-GCM/CHACHA20. The plugin does NOT test for a known-broken algorithm (3DES, RC4, EXPORT — those are separate plugins 65821, 26928, 42873). It fires on suites that are merely *not preferred*: CBC-mode ECDHE, plain RSA key exchange, SHA-1 MACs in TLS 1.2, anything Mozilla's intermediate profile demoted. Applies to anything terminating TLS — web servers, load balancers, mail, RDP gateways, IoT, embedded mgmt interfaces.

Vendor severity is MEDIUM (CVSS 4.8, AV:N/AC:H/PR:N/UI:N/S:U/C:L/I:L/A:N) and that is generous. The CVSS vector itself admits AC:H (high attack complexity) and only Low confidentiality/integrity impact. In practice this is a configuration hygiene / compliance finding, not an exploitable vulnerability — there is no published attack that downgrades a modern client to a *non-preferred but still authenticated* cipher and decrypts traffic at scale. Treat as LOW for risk; treat as a hard requirement only if PCI DSS scope applies.

Why this verdict

• No exploitable attack chain: The plugin flags suites that are merely outside Mozilla's recommended list (CBC-ECDHE, RSA-keyex, SHA-1 MAC) — not the historically broken ones. Known TLS attacks (BEAST, Lucky13, ROBOT, SWEET32) target specific implementation bugs that are detected by dedicated plugins, not this one.
• Requires on-path attacker plus downgrade plus implementation flaw: CVSS vector itself reflects this with AC:H and only C:L/I:L. Three compounding prerequisites for a theoretical impact that has no current weaponization.
• Role multiplier — TLS termination on identity / payment endpoints: Even on a customer-facing payment page or IdP login endpoint, the worst credible outcome is reduced cryptographic margin against future breakthroughs, not 2026 decryption. Blast radius remains per-session, not fleet-wide. Floor stays LOW.
• Compliance is the actual risk: If the scanned host is in PCI scope, this finding can fail an ASV scan and block your QSA sign-off. That's a business risk, not a security risk — and it justifies treating it as actionable backlog rather than IGNORE.

Why not higher?

MEDIUM would imply meaningful real-world exposure. There isn't any in 2026 — modern clients reject true downgrades, TLS 1.3 has built-in downgrade protection, and the flagged suites lack a published practical decryption attack. Vendor MEDIUM is compliance-driven inflation.

Why not lower?

IGNORE would be wrong because (a) PCI DSS v4.0 Req 4.2.1 makes this audit-blocking for in-scope systems, and (b) leaving CBC and RSA-keyex enabled is a hedge against future attacks and a marker of stale config. Worth fixing, just not at HIGH/CRITICAL urgency.

1. Centralize TLS config via a managed reverse proxy / ingress — Put all TLS termination behind nginx, HAProxy, Envoy, AWS ALB, or your CDN with a single Mozilla intermediate (or modern if clients allow) cipher list. Single config change closes the finding across hundreds of backends. This is the only path that scales — no mitigation SLA applies at LOW; treat as part of the 365-day remediation window.
2. Disable non-preferred suites on identified endpoints — Use testssl.sh -t tls12 --severity HIGH <host> to enumerate the offered list, then restrict via OpenSSL config (ssl_ciphers/SSLCipherSuite/tlsCipherSuites). Reference ssl-config.mozilla.org for vendor-specific snippets. Roll out behind a canary; some legacy clients (old Java, embedded printers, POS terminals) will break.
3. Enforce TLS 1.3 where client population allows — TLS 1.3 has no CBC, no RSA-keyex, mandatory AEAD, and a downgrade sentinel. Setting TLSv1.3 minimum on internal services closes this finding by definition. Inventory clients first — legacy .NET Framework <4.8 and old curl/openssl 1.0.x won't connect.
4. Suppress / accept on out-of-scope assets — If the host is genuinely outside PCI scope and serves only internal traffic to modern clients, document a risk acceptance in your GRC tool with 12-month review. Don't pretend to remediate every appliance mgmt UI on the planet.

Run from an auditor workstation or CI runner with testssl.sh (or nmap --script ssl-enum-ciphers) installed. Invoke as ./check_159543.sh target.example.com 443 — no privileges required, only outbound TCP to the target port. Exits 0 PATCHED, 1 VULNERABLE, 2 UNKNOWN.

#!/usr/bin/env bash
# Verify Tenable plugin 159543 — SSL/TLS Recommended Cipher Suites (PCI DSS)
# Usage: ./check_159543.sh <host> <port>
set -u
HOST="${1:-}"; PORT="${2:-443}"
if [[ -z "$HOST" ]]; then echo "Usage: $0 <host> <port>"; exit 2; fi

if ! command -v openssl >/dev/null 2>&1; then echo "UNKNOWN: openssl not installed"; exit 2; fi

# Mozilla intermediate / modern recommended list — anything else is a finding
RECOMMENDED='TLS_AES_128_GCM_SHA256 TLS_AES_256_GCM_SHA384 TLS_CHACHA20_POLY1305_SHA256 ECDHE-ECDSA-AES128-GCM-SHA256 ECDHE-RSA-AES128-GCM-SHA256 ECDHE-ECDSA-AES256-GCM-SHA384 ECDHE-RSA-AES256-GCM-SHA384 ECDHE-ECDSA-CHACHA20-POLY1305 ECDHE-RSA-CHACHA20-POLY1305 DHE-RSA-AES128-GCM-SHA256 DHE-RSA-AES256-GCM-SHA384 DHE-RSA-CHACHA20-POLY1305'

OFFERED=$(openssl s_client -connect "$HOST:$PORT" -servername "$HOST" -cipher 'ALL:!aNULL' </dev/null 2>/dev/null \
  | awk -F': ' '/Cipher    :/ {print $2}')

# Enumerate full list via cipher-by-cipher probe
FINDING=0; BAD_LIST=""
for C in $(openssl ciphers 'ALL:!aNULL:!eNULL' | tr ':' ' '); do
  RESULT=$(openssl s_client -connect "$HOST:$PORT" -servername "$HOST" -cipher "$C" </dev/null 2>/dev/null | grep -c 'Cipher    : '"$C"'$')
  if [[ "$RESULT" -gt 0 ]]; then
    if ! echo " $RECOMMENDED " | grep -q " $C "; then
      FINDING=1; BAD_LIST+="$C "
    fi
  fi
done

if [[ "$FINDING" -eq 1 ]]; then
  echo "VULNERABLE: $HOST:$PORT offers non-recommended cipher(s): $BAD_LIST"
  exit 1
else
  echo "PATCHED: $HOST:$PORT offers only Mozilla-recommended suites"
  exit 0
fi

Sources

1. Tenable plugin 159543 — SSL/TLS Recommended Cipher Suites (PCI DSS)
2. Tenable plugin 159543 changelog v1.6
3. Mozilla SSL Configuration Generator
4. Mozilla Server Side TLS wiki
5. NIST SP 800-52r2 — Guidelines for the Selection, Configuration, and Use of TLS Implementations
6. PCI DSS v4.0 (Requirement 4.2.1 — strong cryptography)
7. RFC 8446 — TLS 1.3 (downgrade protection §4.1.3)
8. 51Security — Remediation for SSL/TLS Recommended Cipher Suites (PCI DSS) finding