Firewall UDP Packet Source Port 53 Ruleset Bypass

Plugin 11580 is an old network check for firewalls that trust UDP source port 53 too broadly. Tenable ties it primarily to Kerio Personal Firewall 2.1.4 (CVE-2003-1491), while also cross-referencing similar behavior in older Symantec Firewall/VPN Appliance and Gateway Security firmware (CVE-2004-1473). The flaw does not execute code by itself; it lets an external sender slip UDP packets past a filtering rule that should have blocked them.

Tenable's HIGH label is too generous for enterprise triage in 2026. The real-world attack chain is narrow: the attacker must find a genuinely exposed firewall with this exact source-port trust mistake, then there must be a useful UDP service behind it, and usually a second bug or dangerous service is needed for meaningful impact. Add the age of the affected products and the scanner's false-positive history on stateless firewalls, and this drops to backlog-grade risk unless you verify a real internet-facing exposure.

Why this verdict

• Downgrade #1: this is a filter bypass, not an RCE. The flaw only helps packets cross a policy boundary; it does not itself run code, dump creds, or hand over admin access.
• Downgrade #2: it requires a useful UDP target behind the firewall. No reachable SNMP/TFTP/IKE/other UDP service means the attack dies after the packet trick.
• Downgrade #3: exposed population is narrow and old. The core product mapping is Kerio Personal Firewall 2.1.4 from 2003, and comparable Symantec appliance cases are also legacy.
• Downgrade #4: modern controls should already reduce payoff. Stateful firewalls, management-plane isolation, allowlists, and segmentation all cut the reachable population hard.
• Downgrade #5: scanner reliability is imperfect. Broadcom's ESXi guidance shows this exact finding can fire on stateless DNS-permitting behavior that is not the same thing as a practical perimeter bypass.

Why not higher?

There is no fetched KEV signal, no current exploitation evidence, and no modern mass-exploitation pattern attached to this plugin. More importantly, the attacker still needs a second-stage opportunity after reaching a UDP service; the finding is rarely the compromise itself.

Why not lower?

If this is real on an internet-facing edge and it exposes administrative UDP services that were supposed to be blocked, it can absolutely become the first domino in a larger appliance compromise. That keeps it above IGNORE: confirmed source-port trust on a perimeter control is sloppy and should be fixed, just not treated like a crisis.

1. Stop trusting inbound udp src port 53 — Rewrite rules so DNS responses are allowed by state/conntrack or by explicit resolver IP allowlists plus expected destination ephemeral ranges, not by trusting the source port alone. For a LOW verdict there is no SLA (treat as backlog hygiene), so fold this into normal firewall policy cleanup unless validation proves real internet exposure to a sensitive UDP service.
2. Restrict DNS to known resolvers — If the platform cannot do stateful DNS handling, allow inbound DNS-response traffic only from your approved recursive resolvers or upstream DNS providers. This shrinks the attack population dramatically and is the cleanest interim control for stateless products; again, no formal mitigation deadline applies for LOW, but it belongs in the next routine network change window.
3. Audit reachable UDP services behind the edge — Inventory what would actually answer if this bypass were real: 161/udp, 69/udp, 500/udp, custom agents, appliance management listeners, or legacy daemons. If you find a sensitive service that becomes reachable from the internet with source-port-53 probes, pull that specific case forward even though the plugin itself remains LOW.
4. Validate from outside before opening a ticket storm — Run a controlled external test with and without -g 53 against a UDP port that should be blocked. This matters because the plugin is prone to false positives and product-by-design flags; spending ten minutes validating can save weeks of bad remediation work across a large estate.

Run this from an external auditor workstation or a test host on the untrusted side of the firewall, not from the target itself. Invoke it as sudo ./check_udp53_bypass.sh 203.0.113.10 161 where the second argument is a UDP port that should be blocked from the internet; it needs nmap installed and root/admin rights for raw-packet scans.

#!/usr/bin/env bash
# check_udp53_bypass.sh
# Test for UDP source-port-53 firewall ruleset bypass.
# Usage: sudo ./check_udp53_bypass.sh <target_ip_or_name> <udp_port>
# Exit codes:
#   0 = PATCHED (no bypass observed)
#   1 = VULNERABLE (baseline filtered, source-port-53 less filtered)
#   2 = UNKNOWN (inconclusive / prerequisites missing)

set -euo pipefail

if [[ $# -ne 2 ]]; then
  echo "UNKNOWN: usage: sudo $0 <target_ip_or_name> <udp_port>"
  exit 2
fi

TARGET="$1"
PORT="$2"

if ! command -v nmap >/dev/null 2>&1; then
  echo "UNKNOWN: nmap is required"
  exit 2
fi

if [[ "$PORT" =~ ^[0-9]+$ ]] && (( PORT >= 1 && PORT <= 65535 )); then
  :
else
  echo "UNKNOWN: invalid UDP port '$PORT'"
  exit 2
fi

scan_port_state() {
  local extra_args="$1"
  local out state
  out=$(nmap -n -Pn -sU -p "$PORT" --max-retries 1 --host-timeout 30s $extra_args "$TARGET" 2>/dev/null || true)
  state=$(awk -v p="$PORT/udp" '$1==p {print $2}' <<< "$out" | head -n1)
  if [[ -z "$state" ]]; then
    echo "unknown"
  else
    echo "$state"
  fi
}

BASE_STATE=$(scan_port_state "")
SRC53_STATE=$(scan_port_state "-g 53")

echo "Target: $TARGET"
echo "Test UDP port: $PORT"
echo "Baseline state: $BASE_STATE"
echo "Source-port-53 state: $SRC53_STATE"

# Interpretation guidance:
# - If baseline is filtered/open|filtered but source-port-53 becomes open or closed,
#   the firewall likely treated source port 53 specially and the port became reachable.
# - If both are identical, no bypass was observed.
# - If both are open/open|filtered, choose a different UDP port that is EXPECTED to be blocked.

if [[ "$BASE_STATE" == "filtered" || "$BASE_STATE" == "open|filtered" || "$BASE_STATE" == "unknown" ]]; then
  if [[ "$SRC53_STATE" == "open" || "$SRC53_STATE" == "closed" || "$SRC53_STATE" == "unfiltered" ]]; then
    echo "VULNERABLE"
    exit 1
  fi
fi

if [[ "$BASE_STATE" == "$SRC53_STATE" ]]; then
  if [[ "$BASE_STATE" == "open" || "$BASE_STATE" == "closed" ]]; then
    echo "UNKNOWN"
    echo "Note: chosen UDP port may already be reachable without source-port manipulation; pick a port that should be blocked."
    exit 2
  fi
  echo "PATCHED"
  exit 0
fi

echo "UNKNOWN"
echo "Note: results were inconclusive; validate with a protocol-aware UDP service test and packet capture."
exit 2

Sources

1. Tenable Plugin 11580
2. NVD CVE-2003-1491
3. Full Disclosure: UDP bypassing in Kerio Firewall 2.1.4
4. Nmap Firewall/IDS Evasion and Spoofing
5. Microsoft Learn: Network Ports Used by DNS
6. CERT/CC VU#329230
7. Broadcom KB 341588
8. NVD CVE-2004-1473