hunt-tls-network skill

HUNT-TLS-NETWORK — TLS/SSL & DNS Security

Reality Check (Read First)

Most findings in this class are Info/Low and routinely rejected as "best-practice" / "missing-hardening" by triage. This skill exists to stop you wasting a submission. Two questions before you report anything here:

1. Is there a real victim and a real action? "Missing HSTS" is not a vulnerability — demonstrated session-cookie capture from a victim you MitM'd is. "Missing CAA" is never a vulnerability you can demonstrate.
2. Does the program accept it? Many programs explicitly list missing SPF/DMARC, missing security headers, weak ciphers without exploit, and CAA as out of scope. Read scope first; quote the in-scope line in your report.

What actually pays in this class (in order):

• Dangling-CNAME / dangling-A subdomain takeover — you control content on target.com subdomain. Real impact, real bounty. (Owned in depth by hunt-subdomain; covered here for the TLS/DNS recon angle.)
• Spoofable DMARC, proven by delivered-to-inbox email — not "p=none exists" but an actual mail from ceo@target.com landing in a real inbox with a passing/none DMARC verdict in the headers.
• DNS AXFR returning internal hosts — full internal hostname/IP map. Concrete recon value, often Medium.
• mTLS / client-cert bypass on an internal service — reaching authenticated-only functionality without the cert. Real auth bypass = High.
• Exploited TLS weakness with a working decrypt/MitM PoC — almost never achievable remotely in 2024-2026 against a patched stack; see Phase 1 caveats.

What does NOT pay (do not report standalone): missing CAA, missing HSTS with no MitM PoC, missing security headers alone, weak-cipher support without an exploit, self-signed cert on a non-prod host, TLS 1.0/1.1 enabled without a downgrade victim.

Phase 1 — TLS/SSL Audit

# Quick TLS test with testssl.sh
brew install testssl
testssl.sh --fast $TARGET 2>/dev/null | grep -E "CRITICAL|HIGH|MEDIUM|OK|NOT" | head -30

# Or use sslyze (Python)
pip3 install sslyze
python3 -m sslyze $TARGET --json_out /tmp/sslyze_$TARGET.json 2>/dev/null
cat /tmp/sslyze_$TARGET.json | python3 -m json.tool | grep -i "vulnerability\|insecure\|error" | head -20

# Check certificate expiry and chain
echo | openssl s_client -connect $TARGET:443 -servername $TARGET 2>/dev/null | \
  openssl x509 -noout -dates -subject -issuer 2>/dev/null

# Check for weak ciphers manually (a successful handshake = the cipher is OFFERED, not exploitable)
openssl s_client -connect $TARGET:443 -cipher RC4-SHA 2>/dev/null | grep -i "cipher\|handshake"
openssl s_client -connect $TARGET:443 -cipher DES-CBC3-SHA 2>/dev/null | grep -i "cipher\|handshake"

# Protocol downgrade surface — TLS 1.0/1.1 still negotiable?
openssl s_client -connect $TARGET:443 -tls1   2>/dev/null | grep -E "Protocol|Cipher"
openssl s_client -connect $TARGET:443 -tls1_1 2>/dev/null | grep -E "Protocol|Cipher"

Accuracy / triage notes — do not over-claim TLS bugs:

• Offered ≠ exploitable. testssl/sslyze flagging RC4, 3DES, or TLS 1.0 means the server negotiates it. That is a hardening finding, not a demonstrated decrypt. Without a PoC it is Info/Low and frequently OOS.
• SWEET32 (CVE-2016-2183) — 3DES birthday attack. Requires a long-lived TLS session, an on-path attacker, and ~hundreds of GB / hours of same-key traffic. Realistically un-demonstrable in a bug bounty; report only the support of 3DES, expect Low/Info.
• POODLE (CVE-2014-3566) — SSLv3 CBC padding oracle. Needs SSLv3 actually enabled; almost no modern stack offers it. Confirm with testssl.sh --poodle (or nmap --script ssl-poodle) — modern OpenSSL 3.x dropped the -ssl3 flag. If SSLv3 won't negotiate, there is no POODLE.
• FREAK (CVE-2015-0204) and DROWN (CVE-2016-0800) — require export-grade RSA / a shared SSLv2 endpoint respectively. Both are pre-conditions you must prove present, not assume. DROWN needs SSLv2 reachable on some host sharing the cert/key — scan for SSLv2 with testssl.sh --drown (or nmap --script sslv2-drown) across the cert's SAN list before claiming it; modern OpenSSL has no -ssl2 flag.
• Heartbleed (CVE-2014-0160) — if you genuinely find an unpatched OpenSSL 1.0.1 leaking memory, that is High/Critical with a real PoC (dump containing keys/cookies). Verify with testssl.sh --heartbleed and capture leaked bytes; this is the rare TLS bug worth a full report.

Phase 2 — HSTS Check

# Check HSTS header on main domain and all subdomains
curl -sI "https://$TARGET/" | grep -i "strict-transport-security"
# Expected: Strict-Transport-Security: max-age=31536000; includeSubDomains; preload

# Check critical subdomains (login, api, auth)
for sub in login auth api account pay www; do
  HSTS=$(curl -sI "https://$sub.$TARGET/" 2>/dev/null | grep -i "strict-transport-security")
  if [ -z "$HSTS" ]; then
    echo "[!] MISSING HSTS: https://$sub.$TARGET/"
  else
    echo "[OK] $sub.$TARGET: $HSTS"
  fi
done

# Check HTTP (non-HTTPS) redirect
curl -sI "http://$TARGET/" | grep -i "location"
# Should redirect to HTTPS immediately

# HSTS preload check
curl -s "https://hstspreload.org/api/v2/status?domain=$TARGET" | python3 -m json.tool 2>/dev/null

Phase 3 — DNS Zone Transfer (AXFR)

# Find nameservers
dig NS $TARGET +short

# Attempt zone transfer on each nameserver
for NS in $(dig NS $TARGET +short); do
  echo "=== Trying AXFR from $NS ==="
  dig AXFR $TARGET @$NS 2>/dev/null | grep -v "^;" | head -30
done

# Zone transfer via alternative tools
host -t AXFR $TARGET $(dig NS $TARGET +short | head -1) 2>/dev/null | head -30
nmap -sn --script dns-zone-transfer $TARGET 2>/dev/null | head -30

# If AXFR succeeds → full internal hostname map
# Look for: internal IPs, staging servers, admin hostnames, CI/CD servers

Phase 4 — Email Security (SPF/DKIM/DMARC)

# Check SPF record
dig TXT $TARGET +short | grep "v=spf1"
# Missing SPF → potential email spoofing

# Check DMARC
dig TXT _dmarc.$TARGET +short
# Missing DMARC → attacker can send as @target.com with no enforcement

# Check DKIM selectors (common: default, google, mail, k1)
for selector in default google mail k1 selector1 selector2 s1 s2 dkim; do
  RESULT=$(dig TXT $selector._domainkey.$TARGET +short 2>/dev/null)
  [ -n "$RESULT" ] && echo "DKIM selector found: $selector → $RESULT"
done

# --- Spoofability evaluation (heuristic only; PROOF is the swaks test below) ---
SPF=$(dig +short TXT $TARGET | tr -d '"' | grep -i "v=spf1")
DMARC=$(dig +short TXT _dmarc.$TARGET | tr -d '"' | grep -i "v=DMARC1")

# SPF "+all" / "all" with no qualifier = pass-everything = spoofable from any IP
echo "$SPF" | grep -Eq '[+ ]all($|[^-~?])' && echo "[CRITICAL] SPF passes all senders (+all)"
echo "$SPF" | grep -q "~all" && echo "[INFO] SPF softfail (~all) — may still deliver to inbox"
[ -z "$SPF" ] && echo "[INFO] No SPF record"

# Correct DMARC-absence check: test the variable for emptiness, do NOT pipe dig|wc -c
if [ -z "$DMARC" ]; then
  echo "[INFO] No DMARC record (no published policy)"
else
  POL

Why the original dig ... | wc -c | grep '^1$' check was broken: empty dig +short output is a zero-length string; piped through wc -c it usually yields 0, and the surrounding newline handling is shell-dependent, so the ^1$ match misfires both ways. Always capture into a variable and test [ -z "$VAR" ].

Spoofability is a RECEIVER decision, not a record-reading exercise

Do not report "missing DMARC = email spoofing" from dig output alone. DMARC p=none (or absent) means the sending domain published no enforcement — but the receiving mail provider (Gmail, M365, the program's own MX) may still junk or reject your spoof based on SPF, its own heuristics, or ARC. The only proof that survives triage is a message you delivered to a real inbox.

# PROOF: send a spoofed mail and confirm INBOX delivery (use a tester account you own)
# Use an account on the receiver the program actually uses (check their MX: dig MX $TARGET)
swaks --to your-tester@gmail.com \
      --from "CEO <ceo@$TARGET>" \
      --header "Subject: [TEST] DMARC spoof PoC for $TARGET" \
      --body "Authorized bug-bounty test. Spoofed from-domain: $TARGET" \
      --server <an-smtp-relay-you-control-or-localhost>

Confirmation gate — a spoof PoC is only valid if you can show:

1. The message landed in Inbox (not Spam/Junk), screenshot the folder.
2. The raw headers: Authentication-Results: showing dmarc=none|fail AND the mail was still delivered (not bounced). A bounce or a Spam-folder landing is NOT a finding — note it and move on.
3. The visible From: shows @$TARGET to the recipient (header-from spoof, the one that matters for phishing), not just an envelope-from trick.

Severity is Medium at best, and only if delivered-to-inbox. Many programs mark email-auth findings OOS outright — check scope first.

Phase 5 — Security Headers Audit

# Check all security headers
HEADERS=$(curl -sI "https://$TARGET/")

# Check each critical header
for HEADER in "Strict-Transport-Security" "Content-Security-Policy" "X-Frame-Options" \
              "X-Content-Type-Options" "Referrer-Policy" "Permissions-Policy"; do
  RESULT=$(echo "$HEADERS" | grep -i "$HEADER")
  if [ -z "$RESULT" ]; then
    echo "[MISSING] $HEADER"
  else
    echo "[OK] $HEADER: $RESULT"
  fi
done

# Automated security headers check
curl -s "https://securityheaders.com/?q=https://$TARGET&followRedirects=on" | \
  grep -oP "grade-\K[A-F+]" | head -3

Phase 6 — Certificate Transparency (Subdomain Discovery)

# crt.sh — certificate transparency logs
curl -s "https://crt.sh/?q=%25.$TARGET&output=json" | \
  python3 -m json.tool 2>/dev/null | grep "name_value" | \
  grep -oP '"name_value": "\K[^"]+' | \
  sed 's/\*\.//g' | sort -u > recon/$TARGET/ct-subdomains.txt

echo "[+] CT subdomains found: $(wc -l < recon/$TARGET/ct-subdomains.txt)"

# Compare with existing subdomain list
comm -23 <(sort recon/$TARGET/ct-subdomains.txt) \
         <(sort recon/$TARGET/subdomains.txt 2>/dev/null) | head -20
# New entries = recently issued certs = new services to investigate

Phase 6.5 — Dangling Records → Subdomain Takeover (the finding that actually pays)

This is the highest-impact item in the whole skill. A CNAME/A record pointing at a deprovisioned third-party resource (S3 bucket, Azure CDN/App Service, GitHub Pages, Heroku, Fastly, etc.) lets you claim that resource and serve content from *.target.com. Full depth lives in hunt-subdomain; here is the TLS/DNS-recon entry point.

# For each subdomain from CT logs, resolve the CNAME chain and check for a live origin
while read sub; do
  CNAME=$(dig +short CNAME "$sub" | head -1)
  [ -z "$CNAME" ] && continue
  CODE=$(curl -s -o /dev/null -w "%{http_code}" --max-time 8 "https://$sub/" 2>/dev/null)
  echo "$sub -> $CNAME  [http $CODE]"
done < recon/$TARGET/ct-subdomains.txt | tee recon/$TARGET/cname-map.txt

# Flag CNAMEs that point at known takeoverable providers, then confirm the
# fingerprint string in the body (e.g. "NoSuchBucket", "There isn't a GitHub Pages site here",
# "Fastly error: unknown domain", "The specified bucket does not exist", Azure "Web App - Unavailable").

Validation gate — a takeover claim requires you to actually claim it:

1. Confirm the dangling target is unregistered/claimable (the S3 bucket name is free, the Heroku app does not exist, etc.) — the provider error fingerprint alone is necessary but NOT sufficient.
2. Register the resource yourself and serve a unique canary file, e.g. https://$sub/.txt returning a string only you know. Screenshot it served over the victim subdomain with valid TLS.
3. Tear it down immediately after PoC; never leave attacker-controlled content live on the target's domain.

Impact: cookie scope theft (cookies set for .target.com), OAuth redirecturi/CORS-trust abuse, phishing on a trusted origin. Typically High** (Critical if it sits at an OAuth/SSO redirect or shares session cookies).

Phase 7 — CAA Records (recon signal only — NOT a reportable finding)

# CAA records DECLARE which CAs the domain owner permits to issue certs.
dig CAA $TARGET +short
dig CAA "*.$TARGET" +short

Do NOT report "missing CAA" as a vulnerability. This is the most common false positive in this class. Correct framing:

• A missing CAA record does not let any attacker obtain a certificate. It only means the owner has not opted into restricting which CAs may issue. With or without CAA, an attacker still needs to pass Domain Control Validation (HTTP-01 / DNS-01 / email) — which requires already controlling the domain, DNS, or web root.
• The "fraudulent issuance" scenario requires CA compromise or social-engineering a CA into mis-issuing. That is out of scope for essentially every bug-bounty program and is not something you can demonstrate. CAA enforcement is a CA-side control, not an attacker-facing surface.
• CAA is Info-tier hardening at most, and routinely closed as Won't-Fix / OOS. Mention it in a recon notes appendix if at all; never file it standalone.

Where CAA recon IS useful (no finding, just intel): the issue/issuewild values tell you which CA the org uses (e.g. letsencrypt.org, digicert.com, amazon.com). That hints at automation (ACME) and at where a real takeover (Phase 6.5 dangling records) could let you mint a valid cert via DCV because you'd control the host.

Phase 8 — mTLS Bypass Attempts

# Check if endpoint requires client certificate
curl -sk "https://$TARGET/internal/" 2>&1 | grep -i "ssl\|certificate\|403\|401"

# Try without client cert (should fail)
curl -sk --cert "" "https://$TARGET/internal/api" | head -5

# Try common bypass paths (some apps skip mTLS on health checks)
for path in /health /ping /status /metrics /api/health; do
  STATUS=$(curl -sk -o /dev/null -w "%{http_code}" "https://$TARGET$path")
  echo "$path: $STATUS"
done

# Header-injection bypass — nginx/HAProxy/Envoy commonly terminate mTLS at the edge and
# forward the verdict as a request header the backend trusts. If the edge does NOT strip
# client-supplied copies of that header, you spoof a verified client. Try the real header
# names used by each proxy:
for combo in \
  "X-SSL-Client-Verify: SUCCESS|X-SSL-Client-S-DN: CN=admin" \
  "ssl-client-verify: SUCCESS|ssl-client-subject-dn: CN=admin" \
  "X-Client-Verify: SUCCESS|X-Client-DN: CN=admin,O=target" \
  "X-Forwarded-Client-Cert: By=spiffe://x;Hash=0;Subject=\"CN=admin\""; do
  H1="${combo%%|*}"; H2="${combo##*|}"
  echo "== $H1 / $H2 =="
  curl -sk "https://$TARGET/internal/api" -H "$H1" -H "$H2" -o /dev/null -w "%{http_code}\n"
done

mTLS-bypass validation — this is a real auth bypass, so prove access, not just a status code:

• A 200 could be a generic page. Confirm you reached authenticated-only functionality: show data/an action that is impossible without the client cert (e.g. an admin-only object, an internal-only response body).
• Distinguish server policy from server state: a 403 flipping to 200 with the spoofed header is meaningful; a 200 for both with and without the header means the path was never protected (no finding).
• For the bypass-path angle (/health, /metrics): only a finding if that endpoint exposes sensitive data (internal IPs, build secrets, Prometheus metrics revealing internal topology) — an empty 200 OK health probe is not.
• Capture the request/response pair in Burp Repeater so the spoofed header → privileged response is unambiguous.

Chain Table

Severities below are calibrated to what triage actually accepts. They are deliberately conservative; do not inflate them in a report.

Tools

# testssl.sh — comprehensive TLS audit
brew install testssl
testssl.sh $TARGET

# sslyze — Python TLS scanner
pip3 install sslyze

# MXToolbox for email security
curl -s "https://mxtoolbox.com/api/v1/Lookup/spf?argument=$TARGET" 2>/dev/null

# dmarc-inspector
curl -s "https://dmarcian.com/dmarc-inspector/?domain=$TARGET" 2>/dev/null

Validation

Each finding ships only with the proof listed — never the dig/header output alone.

• Subdomain takeover: you registered the dangling resource and served a unique canary over https://sub.target.com/ with valid TLS. Screenshot + canary string. (Tear down after.)
• mTLS bypass: spoofed client-verify header returns privileged data/action that the cert-required path otherwise denies. Burp request/response pair.
• AXFR: zone transfer returns internal hostnames/IPs from an authoritative NS. Full transcript.
• DMARC spoof: swaks-sent mail with From: @target.com delivered to a real Inbox (not Spam), raw Authentication-Results headers attached. A bounce or Spam landing = no finding.
• HSTS missing: only reportable with a working downgrade PoC capturing a victim cookie over plaintext — otherwise it is best-practice noise.

Severity (conservative — matches the Chain Table):

• Subdomain takeover (claimed): High (Critical at OAuth/SSO redirect or shared session cookie)
• mTLS bypass to authed functionality: High
• AXFR returning internal hosts: Medium
• DMARC spoof delivered-to-inbox: Medium (often OOS — verify scope)
• HSTS missing on auth (with downgrade PoC): Low–Medium; without PoC: Info
• Weak cipher support without decrypt PoC: Info–Low
• Missing security headers / missing CAA only: Info (usually do not file)

Pre-submission scope gate: before filing ANY item here, confirm the program does not list it as out of scope (email-auth, missing-headers, weak-TLS-without-exploit, and CAA are commonly OOS). Quote the in-scope line in your report.