Weak DH Key Exchange Supported

Tenable plugin 106459 is a *configuration finding*, not a product-specific bug. It fires when a TLS service offers finite-field Diffie-Hellman key exchange with a public modulus smaller than 2048 bits, which Tenable maps to the broader CVE-2015-4000 / Logjam class. In practice, the affected population is any web, mail, VPN, load balancer, Java, or appliance TLS endpoint still advertising DHE groups below PCI/NIST strength guidance; there is no single affected version range because this is driven by cipher and DH-parameter configuration.

Vendor severity is already basically right: this is low real-world security risk for most enterprises, even if it can absolutely fail a PCI scan. The decisive friction is attacker position: exploitation is not unauthenticated internet-to-RCE, it is *man-in-the-middle cryptographic weakening* against a subset of connections that negotiate these DHE parameters. That makes it materially less urgent than patchable edge bugs, even though it remains worth fixing on internet-facing and PCI-scoped systems.

Why this verdict

• Requires MITM, not just reachability: network access alone is insufficient; the attacker must control or intercept the path between client and server.
• Configuration hit, not host compromise: the practical impact is exposed session confidentiality/integrity on negotiated traffic, not direct RCE or auth bypass on the server.
• Population narrows fast in modern estates: many clients and servers now prefer ECDHE or reject truly weak groups, so a scanner positive does not equal easy exploitation.
• Compliance amplifies attention, not attacker ease: PCI cares because <2048-bit DH is below strong-crypto guidance, but audit pain should not be mistaken for incident-grade exploitability.

Why not higher?

This does not give an unauthenticated attacker code execution, privilege escalation, or a reliable one-shot breach path. The need for on-path interception plus successful negotiation of a weak finite-field DHE path is heavy friction, and that compounds downward in real deployments.

Why not lower?

It is still a real cryptographic weakness, especially on public-facing or PCI-scoped services handling sensitive traffic. If you leave weak DH enabled on internet-exposed systems, you are carrying unnecessary confidentiality risk and likely audit debt even if the exploitation bar is high.

1. Prefer ECDHE-only cipher suites — Disable finite-field DHE where the application stack allows it and let the service negotiate ECDHE instead. For a LOW noisgate verdict there is no SLA; treat this as backlog hygiene and fold it into your next TLS hardening cycle.
2. Regenerate DH params at 2048 bits or higher — Where DHE must remain enabled for compatibility, generate unique 2048-bit-or-better DH parameters instead of shipping with weak or bundled defaults. For a LOW verdict there is no SLA; prioritize externally exposed and PCI-scoped listeners first.
3. Kill legacy listeners and shadow TLS services — A lot of these findings survive on forgotten admin ports, old VIPs, mail listeners, and appliance management interfaces. For a LOW verdict there is no SLA; use the scanner output to close the exact port/service combinations rather than launching broad emergency patching.
4. Segment and monitor interception-prone networks — Since exploitation needs a man-in-the-middle position, tighten L2/L3 trust boundaries, suppress rogue gateways, and watch for ARP/DHCP/routing anomalies. For a LOW verdict there is no SLA; this is architecture hardening, not break-fix firefighting.

Run this from an auditor workstation or jump host that can reach the target TLS service. Invoke it as ./check_weak_dh.sh host.example.com 443; no privileges are required, but you need openssl in the path. The script attempts a TLS 1.2 DHE handshake and reports VULNERABLE, PATCHED, or UNKNOWN.

#!/usr/bin/env bash
# check_weak_dh.sh
# Detect weak finite-field DH (<2048 bits) on a remote TLS service.
# Usage: ./check_weak_dh.sh <host> [port]
# Exit codes:
#   0 = PATCHED
#   1 = VULNERABLE
#   2 = UNKNOWN / error / no DHE path observed

set -u

HOST="${1:-}"
PORT="${2:-443}"
TIMEOUT_BIN=""

if [ -z "$HOST" ]; then
  echo "UNKNOWN - usage: $0 <host> [port]"
  exit 2
fi

if command -v timeout >/dev/null 2>&1; then
  TIMEOUT_BIN="timeout 15"
fi

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

# Try to force a finite-field DHE TLS 1.2 handshake.
OUTPUT=$(echo | sh -c "$TIMEOUT_BIN openssl s_client -connect ${HOST}:${PORT} -servername ${HOST} -tls1_2 -cipher 'DHE' 2>&1")
RC=$?

# If the handshake failed entirely, we likely don't have a DHE path.
if [ $RC -ne 0 ] && ! printf '%s\n' "$OUTPUT" | grep -qi 'Server Temp Key:'; then
  # Distinguish 'no shared cipher' from total failure.
  if printf '%s\n' "$OUTPUT" | grep -Eqi 'no cipher match|no shared cipher|handshake failure|alert handshake failure'; then
    echo "PATCHED - no finite-field DHE handshake available"
    exit 0
  fi
  echo "UNKNOWN - could not complete test handshake"
  exit 2
fi

TEMP_KEY_LINE=$(printf '%s\n' "$OUTPUT" | grep -i 'Server Temp Key:' | tail -n 1 || true)

if [ -z "$TEMP_KEY_LINE" ]; then
  if printf '%s\n' "$OUTPUT" | grep -Eqi 'no cipher match|no shared cipher|handshake failure|alert handshake failure'; then
    echo "PATCHED - no finite-field DHE handshake available"
    exit 0
  fi
  echo "UNKNOWN - DHE not negotiated or temp key not exposed by openssl"
  exit 2
fi

# Examples:
# Server Temp Key: DH, 1024 bits
# Server Temp Key: DH, 2048 bits
BITS=$(printf '%s\n' "$TEMP_KEY_LINE" | sed -n 's/.*DH, \([0-9][0-9]*\) bits.*/\1/p')

if [ -z "$BITS" ]; then
  # If a temp key exists but is not finite-field DH, treat as patched for this specific finding.
  if printf '%s\n' "$TEMP_KEY_LINE" | grep -Eqi 'ECDH|X25519|X448'; then
    echo "PATCHED - negotiated modern ephemeral key exchange (${TEMP_KEY_LINE})"
    exit 0
  fi
  echo "UNKNOWN - unable to parse temp key size (${TEMP_KEY_LINE})"
  exit 2
fi

if [ "$BITS" -lt 2048 ]; then
  echo "VULNERABLE - weak finite-field DH detected (${BITS} bits)"
  exit 1
fi

echo "PATCHED - finite-field DH is ${BITS} bits"
exit 0

Sources

1. Tenable Nessus Plugin 106459
2. NVD CVE-2015-4000
3. WeakDH / Logjam research site
4. WeakDH deployment guidance
5. NIST SP 800-57 Part 1 Rev. 5
6. Cloudflare Logjam explainer
7. FIRST EPSS overview
8. CISA KEV catalog