Rigorous, defensible email-spoofability verdict and SPF supply-chain risk analysis computed from published DNS alone. Deepens the record-level SPF/DMARC/DKIM/BIMI/MTA-STS/DNSSEC fetch recipes in the offensive-osint arsenal (§16.14) with the reasoning that section doesn't do: a priority-ordered composite verdict for whether an attacker can actually land header-From-spoofed mail in an inbox, and by which vector (exact-domain vs subdomain) — grounded in the single most-misunderstood distinction in email security: the envelope MAIL FROM that SPF authenticates vs the visible header From: that only DMARC governs. Explains precisely why SPF -all/~all alone is NOT spoof-proof without DMARC enforcement, and why SPF +all bypasses DMARC even under p=reject pct=100. Covers RFC 7208 §4.6.4's 10-DNS-lookup / 2-void-lookup PermError fail-open condition with a runnable stdlib-only lookup-counter script, plus the SPF-include-takeover supply-chain vector (an attacker re-registering a dead include inherits SPF-pass authority ov
Install with Codex or Claude Copy this prompt, paste it into Codex, Claude, or another assistant, and let it review the skill page and install it for you.
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Rigorous, defensible email-spoofability verdict and SPF supply-chain risk analysis computed from published DNS alone. Deepens the record-level SPF/DMARC/DKIM/BIMI/MTA-STS/DNSSEC fetch recipes in the offensive-osint arsenal (§16.14) with the reasoning that section doesn't do: a priority-ordered composite verdict for whether an attacker can actually land header-From-spoofed mail in an inbox, and by which vector (exact-domain vs subdomain) — grounded in the single most-misunderstood distinction in email security: the envelope MAIL FROM that SPF authenticates vs the visible header From: that only DMARC governs. Explains precisely why SPF -all/~all alone is NOT spoof-proof without DMARC enforcement, and why SPF +all bypasses DMARC even under p=reject pct=100. Covers RFC 7208 §4.6.4's 10-DNS-lookup / 2-void-lookup PermError fail-open condition with a runnable stdlib-only lookup-counter script, plus the SPF-include-takeover supply-chain vector (an attacker re-registering a dead include inherits SPF-pass authority over the victim domain) with strict transient-vs-NXDOMAIN discrimination discipline so a temporary SERVFAIL is never mistaken for a takeover lead. Fully passive: DNS TXT reads only, no mail sent, no RCPT TO probe, no API keys. Use when auditing a domain's real spoofing resistance (not just its published records), explaining to a client why 'we have SPF -all' does not mean they're covered, investigating an SPF PermError or an unusually long include chain, evaluating a dead SPF include as a takeover lead, or writing a defensible spoofability finding for a deliverable.
version
1
sources
asm_reference_impl, rfc_7208, public_research
triggers
["email spoofability","email spoofing verdict","is this domain spoofable","spoof feasibility","header from spoofing","envelope from vs header from","BEC feasibility","business email compromise feasibility","SPF DMARC verdict","DMARC enforcement","DMARC alignment","SPF supply chain","SPF PermError","SPF lookup limit","10 DNS lookup limit","SPF void lookup","SPF include takeover","dead SPF include","SPF +all","DMARC p=none","DMARC p=reject","DMARC subdomain policy","duplicate DMARC record","email domain security","email authentication audit","phishing feasibility domain","spoof proof domain"]
Companion skills: offensive-osint §16.14 (raw record-fetch recipes — dig/PowerShell one-liners for SPF/DMARC/DKIM/BIMI/MTA-STS/TLS-RPT/DNSSEC/CAA, the MX→IdP inference table, the DMARC reporting-vendor table). osint-methodology (confidence levels, output format, severity rubric this skill inherits). Fetch the records with §16.14 first; bring the raw TXT text here for the verdict. This skill does not re-list what a record is — it reasons about what a domain's combination of records actually lets an attacker do.
0. When to Use / When NOT
Use this skill when:
Asked to audit spoof feasibility, produce an email-spoofability verdict, or explain "is domain X spoofable."
You already have raw SPF/DMARC TXT text (via offensive-osint §16.14 or your own dig) and need the verdict, not just the record dump.
Investigating an SPF PermError, a long or unusual include chain, or a dead include: target.
Writing a client-facing finding that has to survive the pushback "we have SPF -all, why is this flagged?"
Reasoning about DMARC subdomain policy inheritance, pct= partial enforcement, or duplicate-record handling.
Do NOT use this skill when:
You haven't fetched the raw records yet. Run offensive-osint §16.14's dig/PowerShell recipes first, then bring the text here.
You want to actually send a spoofed test message or run an SMTP RCPT TO liveness check. That is active engagement work requiring explicit authorization and is out of scope for this passive-DNS skill — see §14.
You're auditing TLS/cert posture rather than email auth — that's offensive-osint §16.15 / TLS deep audit territory.
You need AXFR / zone-transfer analysis. dns_deep-class modules run that check alongside the email-auth sweep, but it's a distinct DNS finding (open zone transfer, unrelated to spoofability) — see the note in §9.
1. Scope & Authorization Posture
Same posture as the companion skills: assets you own or have written authorization to assess. Everything in this skill is passive — reading published, public DNS TXT records that any resolver on the internet can already see — so it carries essentially zero detectability risk on its own.
But the verdict this skill produces is the input to a decision someone downstream might act on (an authorized phishing-simulation send, a client remediation ticket, a bug bounty report). Flag the boundary explicitly: a passive verdict of "spoofable" is a strong, defensible claim about DNS-published policy — it is not itself proof that a spoofed message was delivered. See §14.
A mechanism in the SPF chain could not be resolved (timeout / SERVFAIL / no nameservers) — the verdict for that mechanism is inconclusive. See §8's transient-vs-dead discipline; never silently upgrade this to FIRM.
FIRM
Record(s) present, parsed under RFC 7208 (SPF) / RFC 7489 (DMARC) grammar, verdict computed purely from directly observed TXT text. This is the ceiling for everything this skill produces on its own.
CONFIRMED
The verdict was cross-checked against actual mail-flow behavior (an authorized send-and-verify test). Outside this skill's passive scope — only ever assigned after an active test someone else ran.
Default posture: everything this skill outputs is at most FIRM. Never claim CONFIRMED spoofability from DNS reading alone, no matter how conclusive the record combination looks.
3. Output Format
Same finding schema as the companion skills, with the fields this domain actually populates in practice:
Finding:
module: email_spoof # or dns_deep / email-domain-security, depending on your pipeline
asset_key: domain:<target>
category: DNS_MISCONFIG # the real implementation groups ALL email-auth findings under
# this one category — these are DNS record misconfigurations,
# not mail-server misconfigurations. Don't invent a separate
# EMAIL_SECURITY category unless your own schema needs one.
severity: <high|medium|low|info> # see §7 / §8 for exact per-condition mapping
confidence: <tentative|firm|confirmed> # see §2 — almost always FIRM
title: "Email spoofable — <vector>" # or the specific SPF supply-chain title
description: <cite the exact reasons — see the "reasons" convention below>
evidence:
spf: <raw SPF TXT, or "(none)">
dmarc: <raw DMARC TXT, or "(none)">
vector: <e.g. "header-From spoof (exact domain, SPF +all bypasses DMARC)">
effective_subdomain_policy: <sp= value or the inherited p= value>
reasons: [<ordered list of full-sentence reasons — see below>]
dedup_key: "emailspoof:<target>" # or "spfsupply:<target>:<kind>", "spfmulti:<target>",
# "dmarcmulti:<target>" — stable across re-scans so the
# same finding doesn't duplicate on the next audit
remediation: <the specific DMARC/SPF ratchet needed to close the exact vector found>
The reasons convention: the verdict logic (§7) doesn't just return a label — it builds an ordered list of full, client-report-ready sentences explaining why the domain is spoofable by that vector. Quote them directly in a deliverable rather than re-writing the explanation from scratch; they're already written in the right register (e.g. "SPF terminates in +all (or bare all), returning an SPF pass for any sender IP. An attacker aligns the envelope sender to the target domain, so DMARC passes on the SPF leg and the spoofed message is delivered even if DMARC is set to reject.").
4. Source Hygiene & Citations
For every record pulled: FQDN queried + record type + UTC timestamp + resolver used + raw text verbatim.
Query against public resolvers (1.1.1.1 / 8.8.8.8 / 9.9.9.9), not local/system DNS — an operator's local resolver may be split-horizon or stale, masking what the internet actually sees. This matters more here than almost anywhere else in the arsenal: the whole verdict depends on seeing exactly what a real mail receiver's resolver sees.
Capture the raw TXT text verbatim, not a paraphrase — SPF/DMARC grammar is unforgiving (a missing ;, a pct= typo, a duplicate record) and the raw text is the only thing that lets someone else verify your parse.
If you walked an SPF include chain, log every hop (host → TXT-or-NXDOMAIN-or-transient) — the walk itself is evidence, not just the final lookup count.
5. Do NOT
Do NOT treat SPF -all/~all as spoof-proof by itself. This is the single misconception this skill exists to correct — see §6. SPF governs the invisible envelope return-path; the visible header From: is governed only by DMARC.
Do NOT assert CONFIRMED spoofability from DNS reading alone. Passive analysis caps at FIRM (§2). CONFIRMED requires an authorized send-and-verify test — out of this skill's scope (§14).
Do NOT send actual spoofed test mail, or run an SMTP RCPT TO/MAIL FROM probe, as "confirmation." That's active engagement work requiring explicit authorization; this skill is DNS-reads-only.
Do NOT flag a transient DNS failure (SERVFAIL / timeout / no-nameservers) on an SPF include as "dead" or a takeover lead. Only an explicit NXDOMAIN qualifies — see §8's transient-vs-dead discipline. A dead-include false-positive is the kind of finding that gets a report laughed out of a client review.
Do NOT count a macro-expanded SPF target (%{d}, %{i}, %{s}, …) as a resolvable takeover candidate. The mechanism still costs a DNS lookup toward the RFC 7208 budget, but you cannot passively resolve the literal macro string — doing so will NXDOMAIN and produce a false takeover claim.
Do NOT ignore a redirect= modifier that sits after a terminal all mechanism — per RFC 7208 §6.1 it is never reached (SPF evaluation stops at all), so don't count it toward the lookup budget or walk it.
Do NOT assert an SPF-include-takeover finding from NXDOMAIN alone. NXDOMAIN is necessary but not sufficient — you still have to check whether the registrable domain is actually available for an attacker to register (WHOIS/RDAP). NXDOMAIN + confirmed-available = takeover lead; NXDOMAIN + still-registered-to-someone = just a dead reference.
6. The Mental Model — Envelope vs Header (read this first)
This is the distinction operators get wrong more than any other single thing in email security, and it's the reasoning gap §16.14's record catalog doesn't close on its own.
Envelope (MAIL FROM / SMTP Return-Path)
Header (From:)
Who authenticates it
SPF — an IP allow-list check against the envelope-sender domain's own SPF record
Nothing directly. Only DMARC governs it, indirectly, via alignment
Does the recipient ever see it
No — buried in the SMTP transaction / bounce headers; essentially invisible in every common mail client UI
Yes — this is the line the user reads, trusts, and hits "Reply" on
What actually makes it spoof-proof
N/A on its own
DMARC p=reject at pct=100, with SPF-alignment or DKIM-alignment holding for legitimate mail
The operator's misconception
"We have SPF -all, we're covered"
(the axis they're actually not covering)
Two independent attack paths land a spoofed header From: <ceo@target.com>
in an inbox. Neither one requires cracking any cryptography.
── Path A — exact-domain header spoof (works whenever DMARC isn't fully enforced) ──
MAIL FROM: attacker@evil-throwaway.tld <- SPF checks THIS domain's record —
target.com's SPF is NEVER CONSULTED,
because the envelope sender isn't
target.com. Its `-all` never even runs.
From: ceo@target.com <- the user sees THIS
DMARC then asks: does the header-From domain (target.com) ALIGN with the domain
that just passed SPF (evil-throwaway.tld)? No. SPF-alignment fails, DKIM-alignment
fails (attacker can't sign as target.com), so DMARC falls through to its published
policy for target.com:
p=reject, pct=100, sp= enforced -> blocked
p=none / p=quarantine / absent / pct<100 / weak sp= -> delivered (or spam-foldered,
which many users still open)
target.com's SPF record — `-all` or otherwise — is IRRELEVANT to this path. This is
why "we have SPF -all" is not an answer to a DMARC-enforcement gap.
── Path B — SPF +all bypass (works even under DMARC p=reject, pct=100) ──
MAIL FROM: anything@target.com <- attacker aligns the envelope domain
to the TARGET this time
From: ceo@target.com
target.com's SPF record ends in `+all` (or bare `all`, which means the same thing):
an explicit PASS for any sending IP whatsoever. SPF passes. SPF-alignment holds
(envelope domain == header-From domain). DMARC passes on the SPF leg. Delivered —
even with p=reject at pct=100, because DMARC has nothing left to reject.
The rule of thumb to give a client:SPF -all alone stops nothing you can see in your inbox. DMARC p=reject at pct=100 stops Path A. Neither one alone stops Path B — an +all SPF record actively defeats even a fully-enforced DMARC policy.
DKIM's role — an explicit scope note: the composite verdict in §7 reasons about SPF and DMARC only, mirroring the real engine this skill is grounded in. DKIM alignment is tracked as a separate posture control (§9's scorecard), not folded into the priority chain. This isn't an oversight: an attacker who cannot produce a valid DKIM signature for the target domain (they don't have the private key) cannot achieve DKIM-alignment regardless of what the target's DKIM record says, so SPF+DMARC reasoning is sufficient to establish the attacker's available spoofing vectors. DKIM matters for whether the defender's own legitimate mail achieves alignment — a separate, defensive-posture question, not an extra bar the attacker has to clear.
7. The Composite Spoofability Verdict — Priority-Ordered Decision Tree
This is the exact order a rigorous engine evaluates in — priorities are not arbitrary; earlier conditions short-circuit later ones (a duplicate-DMARC domain is spoofable regardless of what either individual record says; an +all SPF domain is spoofable regardless of DMARC policy).
Before any branch runs, two context reasons are always evaluated and prepended to the eventual verdict's reasoning (informational — they explain why, they don't by themselves decide whether):
No SPF record present → "the envelope return-path is unauthenticated."
SPF present but its terminal qualifier is +all / bare all / absent entirely → "the envelope accepts any sender."
Then, in strict priority order:
Pri
Condition
Human label
Severity
Vector (exact wording)
1
More than one DMARC record published at _dmarc.<domain> (RFC 7489 §6.6.3: receivers must ignore the entire set)
DMARC present but p=none, or p= missing/malformed (unparseable)
SPOOFABLE
HIGH
header-From spoof (exact domain)
5
p=quarantine
SPOOFABLE
MEDIUM
header-From spoof (exact domain, quarantined)
6
p=reject but pct < 100
SPOOFABLE
MEDIUM
header-From spoof (pct=N, ~(100−N)% not rejected)
7
p=reject, pct=100, but the effective subdomain policy is not quarantine/reject
SPOOFABLE
MEDIUM
header-From spoof (subdomain)
—
p=reject, pct=100, effective subdomain policy also enforced
NOT SPOOFABLE
INFO
not spoofable (DMARC reject enforced)
"Effective subdomain policy" = the explicit sp= tag if present, otherwise it inherits the apex p= value (RFC 7489 §6.3). This is a common source of false findings both directions:
False "protected": don't assume an absentsp= is a gap — it inherits p=reject and subdomains ARE covered.
True gap: an explicitsp=none (or sp=quarantine) alongside p=reject overrides the inheritance — apex is enforced, subdomains are not. This is priority 7, and it's easy to miss on a quick read because the apex line looks fully hardened.
Severity ceiling: the real engine this skill is grounded in caps every one of these verdicts at HIGH — never CRITICAL, even for priorities 1–4 (the worst case). Preserve that ceiling; don't invent a CRITICAL tier for the composite verdict. If your own engagement's severity rubric wants to escalate a HIGH email-spoof finding to CRITICAL contextually (e.g. it's a domain with a finance/CEO-fraud history), do that as a documented escalation rule, not as this module's native output.
Worked micro-example (priority 2 firing over what looks like a hardened domain):
SPF: v=spf1 include:_spf.google.com ~all <- wait, this record ends -all/~all, no +all here
DMARC: v=DMARC1; p=reject; pct=100; sp=reject <- looks fully enforced at first glance
If instead the SPF record actually reads v=spf1 include:_spf.google.com +all (a copy-paste error appending +all after a legitimate include list is a real, observed misconfiguration — someone meant to add a permissive fallback and instead nullified the whole record), priority 2 fires and the domain is spoofable despite a textbook-perfect DMARC record. This is exactly the kind of finding that a records-only read (§16.14) would miss if the auditor's eye jumps straight to the DMARC line and calls it done.
8. SPF Supply-Chain Analysis (RFC 7208 §4.6.4)
The composite verdict in §7 answers "is the domain spoofable." This section answers a different question: can the SPF record itself be turned into a weapon, or can it silently stop working?
8.1 The 10-lookup / 2-void-lookup budget
RFC 7208 §4.6.4 caps SPF evaluation at:
10 DNS-lookup-costing mechanisms (counted across the entire recursive chain, not per-record)
2 void lookups (a mechanism that resolves to nothing)
Exceed either cap → the evaluating receiver returns PermError. Conformant receivers treat PermError as no SPF at all — this is a fail-open condition: the domain silently loses envelope-sender protection, and nobody publishing the record gets an error telling them so. It just quietly stops working the day someone's 11th SaaS include: gets added.
Mechanisms that cost one lookup each (count every one, across every hop of the recursive walk):
Mechanism
Costs a lookup?
Notes
include:
Yes (1)
Also recurses — walk its own SPF record and count its mechanisms too
a, a:host, a/24
Yes (1)
mx, mx:host
Yes (1)
Each MX lookup for the target implicitly costs additional A/AAAA lookups too, but the RFC counts the mx mechanism itself as 1 for this cap
ptr, ptr:host
Yes (1)
Also deprecated — see §8.4
exists:
Yes (1)
redirect= (modifier)
Yes (1)
Also recurses — unless the record contains a terminal all mechanism, in which case RFC 7208 §6.1 means processing stops at all and the redirect is never reached — don't count it or walk it in that case
ip4:, ip6:
No (0)
Static, no DNS query needed
all
No (0)
Terminal, no query
v=spf1 tag itself
No (0)
8.2 What counts as a "void" lookup
A void lookup is any lookup-costing mechanism that resolves to nothing usable — this is a distinct cap from the 10-lookup total, and it's the one people forget: you can be well under 10 total lookups and still PermError on void lookups alone.
NXDOMAIN (the queried name doesn't exist) → void. Also flag as a dead-include lead (§8.3).
NOERROR with no TXT / no SPF-shaped TXT (the name is registered, it just doesn't have an SPF record there) → void, but this is not a dead-include / takeover candidate — the domain is registered and owned by someone; there's simply no SPF policy published at that name.
8.3 Dead-include takeover — and the discipline that keeps it from being an FP machine
If an include: target NXDOMAINs, its registrable domain may be available for an attacker to register. If they do, and publish their own permissive SPF record there, they inherit SPF-pass authority for the victim domain's SPF chain — an attacker sending mail whose envelope aligns to the victim domain can now pass SPF via a supply-chain link the victim forgot to remove (a decommissioned vendor, a renamed SaaS product, a typo in the include target that happened to look plausible).
The discipline, stated precisely — this is the part that separates a real finding from an embarrassing FP:
DNS outcome for the include target
Classification
Action
NXDOMAIN
Dead — takeover lead
Verify registration status via WHOIS/RDAP before calling it a finding. NXDOMAIN alone is necessary but not sufficient.
NOERROR, no TXT (NoAnswer)
Void, not dead
The domain is registered and presumably owned by the original vendor. It's a stale reference costing a void lookup — a hygiene finding, not a takeover lead.
Timeout / SERVFAIL / no nameservers reachable
Transient — inconclusive
Skip judging entirely. Do not count it as void, do not count it as dead, do not report a takeover lead. Re-query later if it matters. Crying takeover on a temporary resolver hiccup is the single most common way this class of finding loses credibility with a client.
Contains a macro (%{d}, %{i}, %{s}, …)
N/A — can't be resolved literally
The mechanism still costs a lookup toward the budget (§8.1), but do not attempt to resolve the literal macro string — it will spuriously NXDOMAIN and produce a false takeover claim. Skip judgment on that specific host.
8.4 The deprecated ptr mechanism
RFC 7208 §5.5 explicitly deprecates ptr: it's slow, DNS-load-heavy, and its authorization basis (reverse DNS) is attacker-influenceable in ways ip4/ip6/include are not. Low-severity finding on its own; the fix is a straight swap to explicit ip4/ip6/include mechanisms.
8.5 Severity mapping for supply-chain issues
Issue
Severity
Trigger
Lookup-count overflow (PermError)
MEDIUM
Recursive lookup-costing mechanism count > 10
Void-lookup overflow (PermError)
MEDIUM
Void lookups > 2
Dead include (NXDOMAIN)
MEDIUM (base)
Every confirmed-NXDOMAIN include, regardless of whether it's later confirmed registerable
Deprecated ptr mechanism
LOW
Any ptr/ptr: mechanism present
These are the base engine's severities — they intentionally score the posture gap, not the exploited outcome. A dead include is MEDIUM the moment it's detected (you don't yet know if it's registerable). Once you separately confirm via WHOIS/RDAP that the registrable domain is actually available, that's an earned escalation for the deliverable narrative — write it up as HIGH/CRITICAL in the client-facing framing with the registration-availability evidence attached, while keeping the underlying engine's MEDIUM as the honest "what we know from DNS alone" baseline. Don't silently inflate the base severity; document the escalation and why.
9. Recipes — Fetching the Records
The raw fetch commands for SPF / DMARC / DKIM / BIMI / MTA-STS / TLS-RPT / DNSSEC / CAA, plus the MX→IdP inference table and the DMARC reporting-vendor table, live in offensive-osint §16.14 — use those verbatim, bash and PowerShell both included there.
What this skill adds on top (not duplicated in §16.14):
Duplicate-record detection (feeds §7 priority 1 / the SPF-multiple check):
D="target.example"
echo "SPF records:"; dig +short TXT "$D" | grep -ic 'v=spf1'
echo "DMARC records:"; dig +short TXT "_dmarc.$D" | grep -ic 'v=DMARC1'
# either count > 1 -> the entire record set for that mechanism is ignored (§7 pri 1 / §8.1)
Subdomain-policy inheritance check (feeds §7 priority 7 — is sp= explicit or inherited?):
dig +short TXT "_dmarc.$D" | grep -oE 'sp=[a-z]+'
# no match -> sp= is ABSENT -> inherits p= (protected if p=reject)
# match -> sp= is EXPLICIT -> use its value, do NOT assume inheritance
SPF mechanism inventory for one record (manual pre-count before running the full recursive walker in §10):
AXFR note: the real-world module this skill's logic is grounded in runs an AXFR (zone-transfer) attempt against every authoritative NS alongside its email-auth sweep — that's a distinct finding (open zone transfer, HIGH/CONFIRMED if it succeeds) unrelated to spoofability. Out of scope here; see offensive-osint's DNS record catalog / §16.14 area for that check if you need it.
10. Runnable Helper — SPF Lookup Counter
stdlib-only Python (no pip install) — shells out to dig, the same tool §16.14's recipes already assume is available (Git Bash / WSL / macOS / Linux all ship it; on stock Windows without Git Bash, adapt fetch_txt() to shell out to nslookup -type=TXT instead — output parsing will differ). It reproduces the exact walker discipline from §8: a 4-state classification per hop (SPF text to recurse / empty-but-registered / NXDOMAIN-dead / transient-inconclusive), never collapsing the transient case into a false void-or-dead verdict.
#!/usr/bin/env python3
"""spf_lookup_count.py — RFC 7208 SS4.6.4 SPF DNS-lookup counter.
Recursively walks a domain's SPF include:/redirect= chain, counts every DNS-lookup-costing
mechanism (include / a / mx / ptr / exists / redirect), counts void lookups (NXDOMAIN or
NOERROR-with-no-SPF-TXT), and separately reports dead includes (NXDOMAIN -- SPF-include-
takeover LEADS, not confirmed findings -- verify registration status separately) vs
transient failures (SERVFAIL/timeout -- inconclusive, deliberately never counted as void
or dead). Flags >10 total lookups / >2 void lookups as the PermError fail-open condition.
stdlib only -- shells out to `dig` (ships with Git Bash / WSL / macOS / Linux; on stock
Windows without Git Bash, swap fetch_txt() for an `nslookup -type=TXT` call instead).
Usage: python3 spf_lookup_count.py target.example
"""
from __future__ import annotations
import re
import subprocess
import sys
MAX_LOOKUPS = 10
MAX_VOID = 2
MAX_DEPTH = 5
TRANSIENT = object() # timeout / SERVFAIL / dig missing -- inconclusive, never void or dead
def fetch_txt(name: str) -> list[str] | None | object:
"""Returns TXT strings for `name`; [] if NOERROR/no-TXT-answer; None if NXDOMAIN;
TRANSIENT if the query itself was inconclusive (timeout/SERVFAIL/tool missing)."""
try:
out = subprocess.run(
["dig", "+noall", "+answer", "+comments", "TXT", name],
capture_output=True, text=True, timeout=8,
)
except (subprocess.TimeoutExpired, FileNotFoundError):
return TRANSIENT
stdout = out.stdout
if re.search(r"status:\s*NXDOMAIN", stdout):
return None
if re.search(r"status:\s*SERVFAIL", stdout):
return TRANSIENT
txts = re.findall(r'"((?:[^"\\]|\\.)*)"', stdout)
if not txts and not re.search(r"status:\s*NOERROR", stdout):
# couldn't positively confirm a clean NOERROR-empty answer -- be conservative
return TRANSIENT
return [t.replace('\\"', '"') for t in txts]
def spf_mechanisms(txt: str) -> tuple[list[str], list[str], int, bool]:
"""Parse ONE SPF record's lookup-costing mechanisms (no recursion).
Returns (includes, redirects, direct_lookup_count, has_ptr)."""
includes: list[str] = []
redirects: list[str] = []
direct = 0
has_ptr = False
has_all = False
for raw in txt.split():
tok = raw.strip('"')
if not tok or tok.lower().startswith("v=spf1"):
continue
if tok[0] in "+-~?":
tok = tok[1:]
low = tok.lower()
if low == "all":
has_all = True
elif low.startswith("include:"):
includes.append(tok.split(":", 1)[1].rstrip("."))
elif low.startswith("redirect="):
redirects.append(tok.split("=", 1)[1].rstrip("."))
elif low == "a" or low.startswith(("a:", "a/")):
direct += 1
elif low == "mx" or low.startswith(("mx:", "mx/")):
direct += 1
elif low == "ptr" or low.startswith("ptr:"):
direct += 1
has_ptr = True
elif low.startswith("exists:"):
direct += 1
if has_all:
# RFC 7208 SS6.1: processing terminates at `all`; a trailing redirect= is
# never reached -- don't count it toward the budget or walk it.
redirects = []
return includes, redirects, direct, has_ptr
def walk(root_domain: str) -> dict:
root_txts = fetch_txt(root_domain)
root_txts = root_txts if isinstance(root_txts, list) else []
root_spf = next((t for t in root_txts if "v=spf1" in t.lower()), None)
if not root_spf:
return {"error": f"no SPF record found on {root_domain}"}
lookups = 0
void = 0
dead: list[str] = []
skipped_transient: list[str] = []
skipped_macro: list[str] = []
expanded: set[str] = set()
def expand(spf_text: str, depth: int) -> None:
nonlocal lookups, void
if depth > MAX_DEPTH:
return
includes, redirects, direct, _ = spf_mechanisms(spf_text)
lookups += direct
for host in includes + redirects:
lookups += 1
h = host.lower()
if not h or h in expanded:
continue
expanded.add(h)
if "%" in h:
skipped_macro.append(h)
continue # macro-expanded target -- cost counted, resolution skipped
sub = fetch_txt(h)
if sub is None: # NXDOMAIN -- dead / takeover LEAD
void += 1
dead.append(h)
elif sub is TRANSIENT: # inconclusive -- skip, don't judge
skipped_transient.append(h)
continue
elif sub == []: # registered, no TXT -- void, NOT dead
void += 1
else:
spf_sub = next((t for t in sub if "v=spf1" in t.lower()), None)
if spf_sub:
expand(spf_sub, depth + 1)
expand(root_spf, 0)
return {
"root_spf": root_spf,
"lookup_count": lookups,
"void_count": void,
"dead_includes": sorted(set(dead)),
"skipped_transient (re-check these, not judged)": sorted(set(skipped_transient)),
"skipped_macro (cost counted, not resolved)": sorted(set(skipped_macro)),
"perm_error": lookups > MAX_LOOKUPS or void > MAX_VOID,
}
if __name__ == "__main__":
if len(sys.argv) != 2:
sys.exit("usage: spf_lookup_count.py <domain>")
result = walk(sys.argv[1])
for k, v in result.items():
print(f"{k}: {v}")
if result.get("perm_error"):
print("\n>>> PermError condition: lookups >10 or void >2. Conformant receivers")
print(">>> treat this domain as having NO SPF at all (fail-open).")
for h in result.get("dead_includes", []):
print(f">>> Dead include '{h}' -- LEAD only. Verify registration status (WHOIS/RDAP)")
print(f">>> before calling it an SPF-include-takeover finding.")
Reading the output:perm_error: True is the fail-open condition (§8.1). Every entry under dead_includes is a lead, not a finding — cross-check registration status before writing it up (§8.3). Every entry under skipped_transient deserves a re-run later, not a report line now.
Your "reject spoofed mail" policy is silently void — the receiving mail server can't even tell which of your two conflicting policies to trust, so it honors neither.
SPF +all
HIGH
Your own DNS explicitly tells every mail server on the internet "trust any sender claiming to be us" — this defeats even a perfect DMARC policy.
No DMARC published
HIGH
Anyone can send email that displays your domain in the From: line, and nothing on the receiving end enforces a rejection.
DMARC p=none
HIGH
Same as above — you're only collecting reports about the spoofing, not stopping it.
DMARC p=quarantine
MEDIUM
Spoofed mail lands in spam/junk — meaningfully better, but a fraction of recipients still open and act on quarantined mail.
DMARC p=reject, pct<100
MEDIUM
You're rejecting spoofed mail, but only for a percentage of attempts — a persistent attacker who simply resends lands some fraction in the inbox.
DMARC p=reject, weak/absent-override sp=
MEDIUM
Your main domain is protected, but any made-up subdomain (invoices.yourcompany.com) is not — attackers pivot there instead.
SPF absent
MEDIUM (context)
Your outbound mail servers aren't authenticated to receivers — not itself the exploit primitive, but it removes one of the two authentication legs DMARC alignment can use.
SPF lookup-count / void-lookup PermError
MEDIUM (base) → escalates once confirmed exploitable
Your SPF record has grown too complex to evaluate — receivers silently stop checking it at all, on a date you don't control and won't be notified about.
Dead SPF include
MEDIUM (base) → HIGH/CRITICAL once registration-availability confirmed
A forgotten vendor reference in your SPF record could let an attacker who notices it re-register the old vendor's domain name and inherit the ability to send mail that authenticates as you.
Deprecated ptr mechanism
LOW
A legacy, slow, and weaker authentication method is still in use — hardening gap, not itself exploitable.
Header-From spoofing of this domain is not feasible via SPF/DMARC. Worth confirming DKIM alignment separately for completeness (§6).
12. Worked Examples
Example A — the classic false sense of security (the mandated trap, worked through):
SPF: v=spf1 include:_spf.google.com -all
DMARC: (no record)
Verdict:SPOOFABLE, HIGH — §7 priority 3 (no DMARC at all). The SPF -all is real and hardfails envelope spoofs of target.com's own return-path — but per §6 Path A, an attacker doesn't need to pass target.com's SPF at all. They send from an envelope domain they control, forge the header From: to target.com, and since there's no DMARC to check alignment, the mail is delivered with target.com visible to the recipient. -all protects a channel the recipient never looks at.
Example B — the +all bypass over a "hardened" DMARC:
Verdict:SPOOFABLE, HIGH — §7 priority 2, which wins over the DMARC branches entirely. The +all at the end (likely a copy-paste accident when someone added the mailgun.org include) makes SPF pass for literally any sending IP. An attacker aligns the envelope to target.com directly, SPF passes, DMARC's SPF-alignment leg is satisfied, and the reject policy never triggers.
Example C — dead include, correctly held as a lead, not a finding, until verified:
SPF: v=spf1 include:_spf.oldvendor-2019.com -all
dig TXT _spf.oldvendor-2019.com → NXDOMAIN. This is a lead: void lookup, dead include, §8.3 table row 1. Before writing "SPF-include takeover" in a report, run a WHOIS/RDAP check on oldvendor-2019.com. If it's genuinely unregistered → escalate to HIGH/CRITICAL takeover finding with the registration-availability evidence attached (§8.5). If it's still registered to someone (even a defunct-looking parked page) → it's a stale-reference hygiene finding at the base MEDIUM, not a takeover.
Example D — transient failure correctly NOT flagged:
Same record as Example C, but the dig query times out (upstream resolver had a bad moment). §8.3 row 3: skip judging this hop entirely. It is neither void nor dead. Re-run later. Reporting a takeover lead off a single timed-out query is exactly the FP class this skill's discipline exists to prevent.
Example E — lookup-count overflow with no single obviously "bad" record:
Seven include: mechanisms + a (1) + mx (1) + ptr (1) = 10 lookup-costing mechanisms at the top level — already sitting exactly on the RFC 7208 §4.6.4 ceiling (PermError triggers at >10, §8.1), and that's before a single recursion. Each include: then pulls its own record and several expand into multiples (_spf.google.com alone chains into _netblocks*.google.com), so the true resolved total blows well past 10. PermError — receivers treat this domain as having no SPF, and every legitimate -all intent is void. This is a realistic pattern: each individual include: was added for a good reason (a real SaaS migration) by someone who never counted the running total. (Counting rule: §8.1. Note the ceiling is >10, not ≥10 — a record that lands on exactly 10 is at the edge but not yet a PermError; this one overflows only once the includes recurse.)
13. Anti-Patterns & Common Misreadings
"SPF -all means we're covered." No — see §6. It protects the envelope, not the header the user reads.
"DMARC p=quarantine is basically the same as p=reject." No — quarantined mail still gets delivered to spam, and a non-trivial fraction of users still open and act on it. It's MEDIUM, not INFO.
"No sp= means subdomains are unprotected." No — absent sp=inherits the apex p=. The actual gap is an explicitsp=none/sp=quarantine sitting under a p=reject apex.
"Any SPF include that NXDOMAINs is a takeover finding." No — it's a lead. Verify the registrable domain is actually available before calling it a finding (§8.3).
"A dig timeout on an include means it's dead." No — transient failures are inconclusive by design; never treat them as void or dead (§8.3 row 3, §5).
"We're under 10 total include: mechanisms, so we're fine on lookups." No — count every lookup-costing mechanism (a/mx/ptr/exists/redirect too), across the entire recursive chain, not just the top-level include count. Also check the void-lookup cap (2) separately — you can PermError on void lookups alone while nowhere near 10 total.
"A redirect= after all still needs to be walked and counted." No — RFC 7208 §6.1 means it's dead code; SPF evaluation stops at all.
"DKIM passing means the domain isn't spoofable." Only relevant if the attacker can produce a valid DKIM signature for your domain, which they can't without your private key. DKIM protects your own legitimate mail's alignment; it isn't a bar the attacker has to clear (§6).
"This module's HIGH severity should really be CRITICAL — it's a full account-takeover-adjacent primitive." Preserve the engine's HIGH ceiling for the composite verdict (§7) — don't invent a CRITICAL tier here. Escalate in your own engagement's contextual rubric if warranted, and document that it's a deliberate escalation, not the base tool's native output.
14. Active-Verification Boundary
This skill's output is a passive DNS verdict: given the published SPF/DMARC text, is the domain spoofable, and by which vector. It is deliberately bounded there. What it explicitly does not do:
Send a spoofed test message to a mailbox you control to confirm delivery.
Run an SMTP RCPT TO / MAIL FROM liveness probe against the target's mail infrastructure.
Attempt to actually register a dead SPF-include domain to prove the takeover (that's a real acquisition with legal/financial implications, not a recon step).
If a finding from this skill needs to graduate from FIRM to CONFIRMED (§2), the natural next step is an authorized send-and-verify test: from an engagement-approved sending platform, send a message with the target domain in the header From: (and, for the exact-domain vectors in §7, an envelope domain you control) to a mailbox you control, and observe whether it lands in inbox/spam/rejected. That is active engagement work — it requires the same explicit authorization posture as any other active probe in the companion skills (osint-methodology §1), and it is out of scope for this skill to execute. Hand the passive verdict + the specific vector off to whoever owns that authorization decision.
15. Skill Self-Test
Drop these into a fresh session to verify the skill loads and reasons correctly.
"target.com has SPF -all and no DMARC record. Is it spoof-proof?" → No. §6 Path A + §7 priority 3 — no DMARC means the exact-domain header spoof succeeds regardless of SPF. [the mandated trap]
"SPF ends in +all, DMARC is p=reject at pct=100. Are we safe?" → No. §7 priority 2 — +all bypasses DMARC entirely, wins over every DMARC branch.
"We found two DMARC records at _dmarc.target.com — one says p=reject, one says p=none. Which one applies?" → Neither. §7 priority 1 / RFC 7489 §6.6.3 — the whole set is ignored by conformant receivers.
"DMARC is p=reject at pct=100, and sp=none is explicitly set. Are subdomains protected?" → No. §7 priority 7 — an explicit sp= overrides inheritance; the apex is enforced but subdomains are not.
"An SPF record has 12 include: mechanisms. What happens at evaluation time?" → PermError, fail-open, treated as no SPF at all. §8.1.
"An SPF include target NXDOMAINs. Is that automatically an SPF-include-takeover finding?" → No — it's a lead. Verify registrable-domain availability via WHOIS/RDAP first. §8.3.
"dig on an SPF include times out (SERVFAIL). Should I flag it as a dead include?" → No — transient failure, inconclusive, skip judging entirely. §8.3, §5.
"SPF record has redirect=_spf.vendor.com after a terminal -all. Does the redirect get evaluated?" → No — RFC 7208 §6.1, unreachable once all terminates; don't count or walk it. §8.1, §9.
"What raw DNS records do I need before I can run this skill's verdict?" → SPF TXT + _dmarc TXT at minimum; for the full posture also DKIM selectors / BIMI / MTA-STS / DNSSEC — fetch via offensive-osint §16.14. §0, §9.
"List the RFC 7208 §4.6.4 lookup-costing mechanisms." → include, a, mx, ptr, exists, redirect=. §8.1.
"Run the SPF lookup counter against a target and tell me if it PermErrors." → §10 helper script.
"A client pushes back: 'we have SPF -all, why is this HIGH?'" → §6 mental model — walk them through the envelope-vs-header distinction and the exact-domain Path A that -all doesn't touch.
"We want to actually send a spoofed test email to prove it." → Out of scope for this passive skill — that's an authorized active send-and-verify test. §14.
"An SPF record has a void-lookup count of 3, but the total lookup count is only 6 — well under 10. Still an issue?" → Yes — the void-lookup cap (2) is a separate PermError trigger from the total-lookup cap. §8.1, §8.2.
"DMARC is p=reject with pct=50. What fraction of spoofed mail actually gets through?" → ~50% falls through to the next-lower policy (quarantine) rather than being rejected. §7 priority 6.
"What severity should a dead SPF include get once I've confirmed the domain is actually available to register?" → Base engine scores MEDIUM on detection; escalate to HIGH/CRITICAL in the deliverable once registration-availability is independently confirmed — document the escalation. §8.5, §12 Example C.
16. Changelog
v1.0 (2026-08-06) — initial release. Grounded in the real composite-spoofability-verdict and SPF-supply-chain decision logic (priority-ordered verdict chain; RFC 7208 §4.6.4 lookup/void-lookup accounting; transient-vs-NXDOMAIN discrimination; macro-expansion and redirect-after-all handling). Companion to offensive-osint §16.14 (record-fetch recipes) — this skill adds the reasoning layer §16.14 does not cover: the envelope-vs-header mental model, the exact priority-ordered verdict conditions, and the SPF-include-takeover supply-chain vector with its FP-discipline. Includes a runnable stdlib-only SPF lookup-counter script (§10) and a 16-prompt self-test including the mandated -all-without-DMARC trap.
