anti
7a89fbb357
SMTP message-level technique tagger per Appendix A.6: open relay abuse (rcpt_count + foreign From), mass phishing (rcpt_count + body simhash), phishing-kit X-Mailer, IDN/punycode URL, sender masquerade composite (From/Return-Path/DKIM/SPF), malicious attachment (macro/.lnk/.iso/.img/ hash match), BEC subject+body composite, encoded payload in body. PII discipline (TTP_TAGGING.md §'Hard parts §6') is enforced at the lifter layer via _filter_evidence(): emitted TTPTag.evidence is restricted to the EmailEvidence-allowed allowlist (body_sha256, matched_headers — names only, rcpt_domain_set — domains only, attachment_sha256s, rcpt_count) plus PII-safe match discriminators (matched_kit, matched_trigger, matched_url_host, etc). Raw addresses, raw body bytes, full URLs, and decoded base64 previews NEVER appear in evidence — defense-in-depth over the YAML evidence_fields hint. Tests: tests/ttp/test_email_lifter.py per-rule positive + negative + PII allowlist guard + state modulation. tests/ttp/rule_precision/ test_email_rules.py xfail flipped to real precision (R0041-R0048 H-band ≥95%). Corpus rows updated to acknowledge that R0045 (masquerade) co-fires with R0041 / R0047 when the sender-masquerade signals are present alongside open-relay or BEC patterns — overlap is by design, not a precision bug.
403 lines
14 KiB
Python
403 lines
14 KiB
Python
"""Email lifter — SMTP message-level technique tagger (E.3.12).
|
|
|
|
Reads pre-parsed SMTP message payload (headers as a name-only list,
|
|
body sha + body text already truncated/scrubbed by the upstream worker,
|
|
attachment hashes + names) and emits Initial-Access / Phishing /
|
|
Resource-Development techniques per Appendix A.6.
|
|
|
|
PII discipline (TTP_TAGGING.md §"Hard parts §6") is enforced at the
|
|
lifter layer: emitted ``TTPTag.evidence`` only carries fields that
|
|
conform to :class:`~decnet.web.db.models.ttp.EmailEvidence`
|
|
(``body_sha256``, ``matched_headers`` — names not values,
|
|
``rcpt_domain_set`` — domains not addresses, ``attachment_sha256s``,
|
|
``rcpt_count``) plus a small set of match-discriminator strings
|
|
(``matched_kit``, ``matched_trigger``, ``matched_url``). Raw From /
|
|
Return-Path / RCPT addresses, raw body bytes, and decoded payload
|
|
previews NEVER appear in evidence.
|
|
"""
|
|
from __future__ import annotations
|
|
|
|
import base64
|
|
import binascii
|
|
import hashlib
|
|
import re
|
|
from collections.abc import Callable
|
|
from typing import Any, Final
|
|
|
|
from decnet.ttp.base import TaggerEvent, TolerantTagger
|
|
from decnet.ttp.impl._emit import emit_tags
|
|
from decnet.ttp.impl._rule_index import RuleIndex
|
|
from decnet.ttp.impl._state import is_active
|
|
from decnet.ttp.impl.rule_engine import CompiledRule
|
|
from decnet.ttp.store.base import RuleStore
|
|
from decnet.web.db.models.ttp import TTPTag
|
|
|
|
|
|
Predicate = Callable[
|
|
[dict[str, Any], dict[str, Any]],
|
|
"dict[str, Any] | None",
|
|
]
|
|
|
|
|
|
# ── Helpers ─────────────────────────────────────────────────────────
|
|
|
|
|
|
def _domain(addr_or_domain: str | None) -> str | None:
|
|
if not isinstance(addr_or_domain, str):
|
|
return None
|
|
if not addr_or_domain:
|
|
return None
|
|
if "@" in addr_or_domain:
|
|
return addr_or_domain.split("@", 1)[1].lower().strip()
|
|
return addr_or_domain.lower().strip()
|
|
|
|
|
|
def _safe_evidence(payload: dict[str, Any]) -> dict[str, Any]:
|
|
"""Build the EmailEvidence-conformant base evidence dict.
|
|
|
|
Only PII-safe keys: body sha (already a hash), header NAMES (not
|
|
values), recipient DOMAINS (not addresses), attachment hashes,
|
|
rcpt count. Raw addresses, raw body, raw header values explicitly
|
|
excluded.
|
|
"""
|
|
rcpt_domains_raw = payload.get("rcpt_domains") or []
|
|
rcpt_domains = [
|
|
d.lower() for d in rcpt_domains_raw if isinstance(d, str)
|
|
]
|
|
attachment_hashes = payload.get("attachment_sha256s") or []
|
|
if not isinstance(attachment_hashes, list):
|
|
attachment_hashes = []
|
|
body_sha = payload.get("body_sha256") or ""
|
|
if not isinstance(body_sha, str):
|
|
body_sha = ""
|
|
rcpt_count = payload.get("rcpt_count")
|
|
if not isinstance(rcpt_count, int):
|
|
rcpt_count = 0
|
|
return {
|
|
"body_sha256": body_sha,
|
|
"matched_headers": [],
|
|
"rcpt_domain_set": sorted(set(rcpt_domains)),
|
|
"attachment_sha256s": [
|
|
h for h in attachment_hashes if isinstance(h, str)
|
|
],
|
|
"rcpt_count": rcpt_count,
|
|
}
|
|
|
|
|
|
# ── Per-rule predicates ─────────────────────────────────────────────
|
|
|
|
|
|
def _p_open_relay(
|
|
spec: dict[str, Any], payload: dict[str, Any],
|
|
) -> dict[str, Any] | None:
|
|
threshold = int(spec.get("rcpt_threshold", 10))
|
|
rcpt_count = payload.get("rcpt_count")
|
|
if not isinstance(rcpt_count, int) or rcpt_count < threshold:
|
|
return None
|
|
if spec.get("require_foreign_from"):
|
|
from_domain = _domain(payload.get("from_domain") or payload.get("from"))
|
|
mail_from = _domain(
|
|
payload.get("mail_from_domain") or payload.get("mail_from"),
|
|
)
|
|
if not from_domain or not mail_from or from_domain == mail_from:
|
|
return None
|
|
return {"matched_headers": ["From", "Mail-From"]}
|
|
|
|
|
|
def _p_mass_phish(
|
|
spec: dict[str, Any], payload: dict[str, Any],
|
|
) -> dict[str, Any] | None:
|
|
threshold = int(spec.get("rcpt_threshold", 25))
|
|
rcpt_count = payload.get("rcpt_count")
|
|
if not isinstance(rcpt_count, int) or rcpt_count < threshold:
|
|
return None
|
|
# The "campaign" half: upstream must have observed body simhash
|
|
# recurring across recipients. Without that signal, high-RCPT alone
|
|
# is open-relay territory (R0041), not mass-phish. The simhash
|
|
# derivation lives in the SMTP worker (out of scope here).
|
|
if not isinstance(payload.get("body_simhash"), (str, int)):
|
|
return None
|
|
return {}
|
|
|
|
|
|
def _p_xmailer_kit(
|
|
spec: dict[str, Any], payload: dict[str, Any],
|
|
) -> dict[str, Any] | None:
|
|
x_mailer = payload.get("x_mailer")
|
|
if not isinstance(x_mailer, str) or not x_mailer:
|
|
return None
|
|
matched_kit = payload.get("matched_kit")
|
|
if isinstance(matched_kit, str) and matched_kit:
|
|
return {"matched_kit": matched_kit, "matched_headers": ["X-Mailer"]}
|
|
# Catalogue match flag — upstream marks it via xmailer_kit_match.
|
|
if payload.get("xmailer_kit_match") is True:
|
|
return {"matched_headers": ["X-Mailer"]}
|
|
return None
|
|
|
|
|
|
_PUNYCODE_PREFIX_DEFAULT: Final[str] = "xn--"
|
|
|
|
|
|
def _p_idn_url(
|
|
spec: dict[str, Any], payload: dict[str, Any],
|
|
) -> dict[str, Any] | None:
|
|
prefix = spec.get("punycode_prefix") or _PUNYCODE_PREFIX_DEFAULT
|
|
if not isinstance(prefix, str):
|
|
prefix = _PUNYCODE_PREFIX_DEFAULT
|
|
urls = payload.get("urls") or []
|
|
if not isinstance(urls, list):
|
|
return None
|
|
for url in urls:
|
|
if isinstance(url, str) and prefix in url:
|
|
# Carry only the punycode-bearing host portion as a match
|
|
# discriminator. NEVER carry the full URL (could contain
|
|
# credential-harvest path with PII).
|
|
host = _extract_host(url)
|
|
return {
|
|
"matched_url_host": host or "",
|
|
"matched_headers": ["body"],
|
|
}
|
|
return None
|
|
|
|
|
|
def _extract_host(url: str) -> str | None:
|
|
m = re.match(r"https?://([^/]+)", url)
|
|
if m:
|
|
return m.group(1).lower()
|
|
return None
|
|
|
|
|
|
def _p_sender_masquerade(
|
|
spec: dict[str, Any], payload: dict[str, Any],
|
|
) -> dict[str, Any] | None:
|
|
signals_raw = spec.get("signals", [])
|
|
if not isinstance(signals_raw, list):
|
|
return None
|
|
signals = {s for s in signals_raw if isinstance(s, str)}
|
|
matched: list[str] = []
|
|
if "from_returnpath_mismatch" in signals:
|
|
if (
|
|
_domain(payload.get("from_domain")) is not None
|
|
and _domain(payload.get("return_path_domain")) is not None
|
|
and _domain(payload.get("from_domain"))
|
|
!= _domain(payload.get("return_path_domain"))
|
|
):
|
|
matched.append("from_returnpath_mismatch")
|
|
if "from_mailfrom_mismatch" in signals:
|
|
if (
|
|
_domain(payload.get("from_domain")) is not None
|
|
and _domain(payload.get("mail_from_domain")) is not None
|
|
and _domain(payload.get("from_domain"))
|
|
!= _domain(payload.get("mail_from_domain"))
|
|
):
|
|
matched.append("from_mailfrom_mismatch")
|
|
if "dkim_fail" in signals and payload.get("dkim_signed") is False:
|
|
matched.append("dkim_fail")
|
|
if "spf_fail" in signals and payload.get("spf_pass") is False:
|
|
matched.append("spf_fail")
|
|
if not matched:
|
|
return None
|
|
headers: list[str] = []
|
|
if any("from_" in m for m in matched):
|
|
headers.extend(["From", "Return-Path"])
|
|
if "dkim_fail" in matched:
|
|
headers.append("DKIM-Signature")
|
|
if "spf_fail" in matched:
|
|
headers.append("Authentication-Results")
|
|
return {
|
|
"matched_signals": matched,
|
|
"matched_headers": sorted(set(headers)),
|
|
}
|
|
|
|
|
|
def _p_malicious_attachment(
|
|
spec: dict[str, Any], payload: dict[str, Any],
|
|
) -> dict[str, Any] | None:
|
|
triggers_raw = spec.get("triggers", [])
|
|
triggers = (
|
|
{t for t in triggers_raw if isinstance(t, str)}
|
|
if isinstance(triggers_raw, list)
|
|
else set()
|
|
)
|
|
if "office_macro" in triggers and payload.get("attachment_macros") is True:
|
|
return {"matched_trigger": "office_macro"}
|
|
if (
|
|
"protected_archive" in triggers
|
|
and payload.get("attachment_password_protected") is True
|
|
):
|
|
return {"matched_trigger": "protected_archive"}
|
|
if "html_smuggling" in triggers and payload.get("html_smuggling") is True:
|
|
return {"matched_trigger": "html_smuggling"}
|
|
if "mal_hash_match" in triggers and payload.get("mal_hash_match") is True:
|
|
return {"matched_trigger": "mal_hash_match"}
|
|
extensions = payload.get("attachment_extensions") or []
|
|
if isinstance(extensions, list):
|
|
ext_set = {
|
|
e.lower().lstrip(".") for e in extensions if isinstance(e, str)
|
|
}
|
|
for ext_trigger in ("lnk", "iso", "img"):
|
|
if ext_trigger in triggers and ext_trigger in ext_set:
|
|
return {"matched_trigger": ext_trigger}
|
|
return None
|
|
|
|
|
|
def _p_bec(
|
|
spec: dict[str, Any], payload: dict[str, Any],
|
|
) -> dict[str, Any] | None:
|
|
subject = payload.get("subject")
|
|
body_text = payload.get("body_text")
|
|
if not isinstance(subject, str) or not isinstance(body_text, str):
|
|
return None
|
|
subj_kws = spec.get("subject_keywords", [])
|
|
body_kws = spec.get("body_action_keywords", [])
|
|
if not isinstance(subj_kws, list) or not isinstance(body_kws, list):
|
|
return None
|
|
subj_l = subject.lower()
|
|
body_l = body_text.lower()
|
|
subj_hit = next(
|
|
(k for k in subj_kws if isinstance(k, str) and k.lower() in subj_l),
|
|
None,
|
|
)
|
|
body_hit = next(
|
|
(k for k in body_kws if isinstance(k, str) and k.lower() in body_l),
|
|
None,
|
|
)
|
|
if not subj_hit or not body_hit:
|
|
return None
|
|
return {
|
|
"matched_subject_kw": subj_hit,
|
|
"matched_body_kw": body_hit,
|
|
"matched_headers": ["Subject"],
|
|
}
|
|
|
|
|
|
_BASE64_RE = re.compile(r"[A-Za-z0-9+/]{32,}={0,2}")
|
|
|
|
|
|
def _p_encoded_payload(
|
|
spec: dict[str, Any], payload: dict[str, Any],
|
|
) -> dict[str, Any] | None:
|
|
min_bytes = int(spec.get("min_bytes", 4096))
|
|
body_text = payload.get("body_text")
|
|
if not isinstance(body_text, str) or not body_text:
|
|
return None
|
|
# Upstream may pre-compute the largest decoded base64 length.
|
|
body_b64_bytes = payload.get("body_base64_bytes")
|
|
if isinstance(body_b64_bytes, int) and body_b64_bytes >= min_bytes:
|
|
return {"encoded_byte_count": body_b64_bytes}
|
|
# Fallback: best-effort scan of the body text. Cap the work at the
|
|
# first match >= threshold to avoid quadratic behavior on a hostile
|
|
# body. Decoded bytes are NEVER returned — only the count.
|
|
for m in _BASE64_RE.finditer(body_text):
|
|
chunk = m.group(0)
|
|
try:
|
|
decoded = base64.b64decode(chunk, validate=True)
|
|
except (binascii.Error, ValueError):
|
|
continue
|
|
if len(decoded) >= min_bytes:
|
|
return {"encoded_byte_count": len(decoded)}
|
|
return None
|
|
|
|
|
|
_PREDICATES: Final[dict[str, Predicate]] = {
|
|
"lifter:email_open_relay": _p_open_relay,
|
|
"lifter:email_mass_phish": _p_mass_phish,
|
|
"lifter:email_xmailer_kit": _p_xmailer_kit,
|
|
"lifter:email_idn_url": _p_idn_url,
|
|
"lifter:email_sender_masquerade": _p_sender_masquerade,
|
|
"lifter:email_malicious_attachment": _p_malicious_attachment,
|
|
"lifter:email_bec": _p_bec,
|
|
"lifter:email_encoded_payload": _p_encoded_payload,
|
|
}
|
|
|
|
|
|
# Allowed keys in TTPTag.evidence for source_kind=email. Used both as
|
|
# the assembly contract here AND by tests/ttp/test_email_lifter.py to
|
|
# guard against a future predicate accidentally leaking PII.
|
|
_EMAIL_EVIDENCE_ALLOWED_KEYS: Final[frozenset[str]] = frozenset({
|
|
# EmailEvidence base
|
|
"body_sha256",
|
|
"matched_headers",
|
|
"rcpt_domain_set",
|
|
"attachment_sha256s",
|
|
"rcpt_count",
|
|
# PII-safe match discriminators
|
|
"matched_kit",
|
|
"matched_trigger",
|
|
"matched_url_host",
|
|
"matched_signals",
|
|
"matched_subject_kw",
|
|
"matched_body_kw",
|
|
"encoded_byte_count",
|
|
})
|
|
|
|
|
|
def _filter_evidence(evidence: dict[str, Any]) -> dict[str, Any]:
|
|
"""Drop any key not in the PII-safe allowlist.
|
|
|
|
Defense-in-depth: even if a predicate accidentally returns a raw
|
|
address or body field, this filter strips it before the tag is
|
|
constructed. Asserted by ``test_email_lifter.py``.
|
|
"""
|
|
return {
|
|
k: v for k, v in evidence.items()
|
|
if k in _EMAIL_EVIDENCE_ALLOWED_KEYS
|
|
}
|
|
|
|
|
|
class EmailLifter(TolerantTagger):
|
|
name = "email"
|
|
HANDLES = frozenset({"email"})
|
|
OWNED_PREFIX: Final[str] = "lifter:email_"
|
|
|
|
def __init__(self, store: RuleStore) -> None:
|
|
self._store = store
|
|
self._index = RuleIndex()
|
|
|
|
@classmethod
|
|
def _owns(cls, rule: CompiledRule) -> bool:
|
|
kind = rule.match_spec.get("kind", "")
|
|
return isinstance(kind, str) and kind.startswith(cls.OWNED_PREFIX)
|
|
|
|
async def watch_store(self) -> None:
|
|
await self._index.watch(self._store, predicate=self._owns)
|
|
|
|
async def _tag_impl(self, event: TaggerEvent) -> list[TTPTag]:
|
|
out: list[TTPTag] = []
|
|
base_evidence = _safe_evidence(event.payload)
|
|
for rule in self._index.values():
|
|
if event.source_kind not in rule.applies_to:
|
|
continue
|
|
if not is_active(rule.state):
|
|
continue
|
|
kind = rule.match_spec.get("kind", "")
|
|
handler = _PREDICATES.get(kind)
|
|
if handler is None:
|
|
continue
|
|
extra = handler(rule.match_spec, event.payload)
|
|
if extra is None:
|
|
continue
|
|
evidence = dict(base_evidence)
|
|
# Allow predicates to extend matched_headers without
|
|
# clobbering the base list.
|
|
extra_headers = extra.pop("matched_headers", None)
|
|
if isinstance(extra_headers, list):
|
|
merged = list(evidence.get("matched_headers", []))
|
|
merged.extend(h for h in extra_headers if isinstance(h, str))
|
|
evidence["matched_headers"] = sorted(set(merged))
|
|
evidence.update(extra)
|
|
evidence = _filter_evidence(evidence)
|
|
# Body sha is required by EmailEvidence; if upstream
|
|
# didn't supply one, derive from body_text (best-effort).
|
|
if not evidence.get("body_sha256"):
|
|
body_text = event.payload.get("body_text")
|
|
if isinstance(body_text, str) and body_text:
|
|
evidence["body_sha256"] = hashlib.sha256(
|
|
body_text.encode("utf-8", errors="replace"),
|
|
).hexdigest()
|
|
out.extend(emit_tags(rule, event, evidence))
|
|
return out
|
|
|
|
|
|
__all__ = ["EmailLifter"]
|