diff --git a/backend/Dockerfile b/backend/Dockerfile
index e4d1713..6391b28 100644
--- a/backend/Dockerfile
+++ b/backend/Dockerfile
@@ -30,7 +30,7 @@ ENV PYTHONDONTWRITEBYTECODE=1 \
 # Non-root user
 RUN groupadd --gid 10001 metamorph \
     && useradd --uid 10001 --gid metamorph --shell /usr/sbin/nologin --create-home metamorph \
-    && mkdir -p /data/evidence \
+    && mkdir -p /data/evidence /data/mitre \
     && chown -R metamorph:metamorph /data
 
 COPY --from=deps /opt/venv /opt/venv
diff --git a/backend/app/cli.py b/backend/app/cli.py
index c452d21..663eeed 100644
--- a/backend/app/cli.py
+++ b/backend/app/cli.py
@@ -56,10 +56,66 @@ def print_install_token(force: bool):
 
 
 @metamorph.command("seed-mitre")
-def seed_mitre():
-    """Placeholder for M4 — left so `make seed-mitre` doesn't crash."""
-    click.echo("MITRE seeding will land in M4. (no-op for now)", err=True)
-    sys.exit(0)
+@click.option(
+    "--source",
+    default=None,
+    help="STIX bundle source: local path or HTTPS URL. Defaults to the pinned MITRE Enterprise release.",
+)
+@click.option(
+    "--checksum-sha256",
+    "checksum_sha256",
+    default=None,
+    help="Expected sha256 of the bundle (required with a non-default --source URL unless --skip-checksum).",
+)
+@click.option(
+    "--skip-checksum",
+    is_flag=True,
+    help="Skip sha256 verification entirely (escape hatch for testing).",
+)
+def seed_mitre(source: str | None, checksum_sha256: str | None, skip_checksum: bool):
+    """Seed/refresh the MITRE ATT&CK Enterprise reference tables.
+
+    Upserts on `external_id`. Re-running with the same source updates the
+    name/description/url and re-applies the technique↔tactic mapping.
+    """
+    from app.services.mitre_seed import (
+        MITRE_DEFAULT_SHA256,
+        MITRE_DEFAULT_URL,
+        seed_mitre as seed_mitre_svc,
+    )
+
+    if skip_checksum:
+        expected_sha = None
+    elif checksum_sha256:
+        expected_sha = checksum_sha256
+    elif source is None or source == MITRE_DEFAULT_URL:
+        expected_sha = MITRE_DEFAULT_SHA256
+    else:
+        expected_sha = None  # let seed_mitre_svc decide whether to refuse
+
+    click.echo(
+        f"Seeding from {source or MITRE_DEFAULT_URL} "
+        f"(sha256 check: {'off' if skip_checksum else expected_sha or 'unverified'}) ...",
+        err=True,
+    )
+    try:
+        result = seed_mitre_svc(
+            source=source,
+            expected_sha256=expected_sha,
+            allow_unverified=skip_checksum,
+        )
+    except Exception as e:  # noqa: BLE001
+        click.echo(f"seed-mitre failed: {e}", err=True)
+        sys.exit(2)
+    click.echo(
+        f"  tactics: {result.tactics_upserted}, "
+        f"techniques: {result.techniques_upserted}, "
+        f"subtechniques: {result.subtechniques_upserted} "
+        f"(skipped orphans: {result.subtechniques_skipped_orphan}), "
+        f"links: {result.technique_tactic_links}, "
+        f"duration: {(result.finished_at - result.started_at).total_seconds():.1f}s",
+        err=True,
+    )
 
 
 app.cli.add_command(metamorph)
diff --git a/backend/app/services/mitre_seed.py b/backend/app/services/mitre_seed.py
new file mode 100644
index 0000000..ee42e43
--- /dev/null
+++ b/backend/app/services/mitre_seed.py
@@ -0,0 +1,478 @@
+"""MITRE ATT&CK Enterprise seed + sync.
+
+Parses a STIX 2.1 bundle into the `mitre_*` tables. Idempotent: re-running
+upserts on `external_id`, refreshes name/description/url, and re-applies the
+technique↔tactic mapping. Sub-techniques whose parent is missing in the
+bundle are skipped (with a WARNING log).
+
+Defaults pin a specific Enterprise release (see `MITRE_DEFAULT_*`). The pin
+is honored by the CLI (`flask metamorph seed-mitre`) and by the
+`POST /mitre/sync` admin endpoint; both accept a `--source` / `source_url`
+override for air-gapped operators.
+
+The bundle is downloaded with `urllib.request` (stdlib — no extra dep) and
+cached at `MITRE_BUNDLE_CACHE_PATH` (default `/data/mitre/<basename>.json`).
+Pass an absolute path as `source` to bypass the network entirely.
+"""
+
+from __future__ import annotations
+
+import hashlib
+import json
+import logging
+import os
+import re
+import urllib.parse
+import urllib.request
+from dataclasses import dataclass, field
+from datetime import datetime, timezone
+from pathlib import Path
+from typing import Iterable
+
+from sqlalchemy import delete, select
+
+from app.db.session import session_scope
+from app.models.mitre import (
+    MitreSubtechnique,
+    MitreTactic,
+    MitreTechnique,
+    MitreTechniqueTactic,
+)
+from app.models.setting import Setting
+
+log = logging.getLogger("metamorph.mitre.seed")
+
+# === Default pin =============================================================
+#
+# MITRE publishes versioned bundles at
+# `https://raw.githubusercontent.com/mitre-attack/attack-stix-data/master/enterprise-attack/enterprise-attack-<X.Y>.json`.
+# Update these three constants in lock-step when bumping the pin. The SHA256
+# is verified against the downloaded bytes — a mismatch aborts the seed.
+#
+MITRE_VERSION = "19.0"
+MITRE_DEFAULT_URL = (
+    "https://raw.githubusercontent.com/mitre-attack/attack-stix-data/master/"
+    "enterprise-attack/enterprise-attack-19.0.json"
+)
+MITRE_DEFAULT_SHA256 = "df520ea0775a57db7bff760145b02fed89290802913e056b7ed5970b02f3626a"
+
+MITRE_BUNDLE_CACHE_PATH = Path(os.environ.get("MITRE_CACHE_DIR", "/data/mitre"))
+MITRE_DOWNLOAD_TIMEOUT_SECONDS = 120
+
+# Settings keys used to expose the seed metadata to the operator UI/CLI.
+SETTING_LAST_SYNC = "mitre_last_sync"
+SETTING_VERSION = "mitre_version"
+SETTING_SOURCE_URL = "mitre_source_url"
+
+ATTACK_SOURCE_NAME = "mitre-attack"
+KILL_CHAIN_NAME = "mitre-attack"
+
+
+class MitreSeedError(Exception):
+    pass
+
+
+class MitreChecksumMismatch(MitreSeedError):
+    pass
+
+
+@dataclass
+class ParsedBundle:
+    tactics: list[dict] = field(default_factory=list)
+    techniques: list[dict] = field(default_factory=list)  # parent techniques
+    subtechniques: list[dict] = field(default_factory=list)
+    # Map: subtechnique attack-pattern STIX id -> parent technique STIX id
+    subtechnique_parents: dict[str, str] = field(default_factory=dict)
+    spec_version: str | None = None
+
+
+@dataclass
+class SeedResult:
+    tactics_upserted: int
+    techniques_upserted: int
+    subtechniques_upserted: int
+    subtechniques_skipped_orphan: int
+    technique_tactic_links: int
+    version: str | None
+    source: str
+    started_at: datetime
+    finished_at: datetime
+
+    def as_dict(self) -> dict:
+        return {
+            "tactics_upserted": self.tactics_upserted,
+            "techniques_upserted": self.techniques_upserted,
+            "subtechniques_upserted": self.subtechniques_upserted,
+            "subtechniques_skipped_orphan": self.subtechniques_skipped_orphan,
+            "technique_tactic_links": self.technique_tactic_links,
+            "version": self.version,
+            "source": self.source,
+            "started_at": self.started_at.isoformat(),
+            "finished_at": self.finished_at.isoformat(),
+            "duration_ms": int(
+                (self.finished_at - self.started_at).total_seconds() * 1000
+            ),
+        }
+
+
+# === I/O =====================================================================
+
+
+def _is_url(source: str) -> bool:
+    parsed = urllib.parse.urlparse(source)
+    return parsed.scheme in ("http", "https")
+
+
+def _sha256_of(path: Path) -> str:
+    h = hashlib.sha256()
+    with path.open("rb") as f:
+        for chunk in iter(lambda: f.read(1 << 16), b""):
+            h.update(chunk)
+    return h.hexdigest()
+
+
+def _download(url: str, dest: Path, *, expected_sha256: str | None = None) -> Path:
+    dest.parent.mkdir(parents=True, exist_ok=True)
+    tmp = dest.with_suffix(dest.suffix + ".part")
+    log.info("metamorph.mitre.download.start", extra={"url": url, "dest": str(dest)})
+    req = urllib.request.Request(url, headers={"User-Agent": "metamorph-mitre-seed/1.0"})
+    with urllib.request.urlopen(req, timeout=MITRE_DOWNLOAD_TIMEOUT_SECONDS) as resp:
+        with tmp.open("wb") as f:
+            for chunk in iter(lambda: resp.read(1 << 16), b""):
+                f.write(chunk)
+    if expected_sha256:
+        actual = _sha256_of(tmp)
+        if actual != expected_sha256:
+            tmp.unlink(missing_ok=True)
+            raise MitreChecksumMismatch(
+                f"sha256 mismatch for {url}: expected {expected_sha256}, got {actual}"
+            )
+    tmp.replace(dest)
+    log.info(
+        "metamorph.mitre.download.done",
+        extra={"url": url, "bytes": dest.stat().st_size},
+    )
+    return dest
+
+
+def resolve_source_to_path(
+    source: str | Path | None,
+    *,
+    cache_dir: Path = MITRE_BUNDLE_CACHE_PATH,
+    expected_sha256: str | None = MITRE_DEFAULT_SHA256,
+) -> tuple[Path, str]:
+    """Return (path, source_label). Downloads if `source` is an URL; otherwise
+    treats it as a local file. `None` → default URL.
+
+    `source_label` is what we persist in `settings.mitre_source_url`."""
+    if source is None:
+        source = MITRE_DEFAULT_URL
+    source_str = str(source)
+    if _is_url(source_str):
+        basename = source_str.rsplit("/", 1)[-1] or "enterprise-attack.json"
+        dest = cache_dir / basename
+        _download(source_str, dest, expected_sha256=expected_sha256)
+        return dest, source_str
+    path = Path(source_str)
+    if not path.exists():
+        raise MitreSeedError(f"source path does not exist: {path}")
+    return path, str(path)
+
+
+# === STIX parsing ============================================================
+
+
+def _attack_ref(obj: dict) -> dict | None:
+    for ref in obj.get("external_references") or ():
+        if ref.get("source_name") == ATTACK_SOURCE_NAME and ref.get("external_id"):
+            return ref
+    return None
+
+
+def parse_bundle(path: Path) -> ParsedBundle:
+    """Read the STIX bundle into normalized dicts ready for SQL upserts."""
+    with path.open("r", encoding="utf-8") as f:
+        bundle = json.load(f)
+    objs = bundle.get("objects") or []
+    parsed = ParsedBundle(spec_version=bundle.get("spec_version"))
+
+    parents_by_subtech: dict[str, str] = {}
+    for o in objs:
+        if (
+            o.get("type") == "relationship"
+            and o.get("relationship_type") == "subtechnique-of"
+            and not o.get("revoked")
+        ):
+            parents_by_subtech[o["source_ref"]] = o["target_ref"]
+    parsed.subtechnique_parents = parents_by_subtech
+
+    for o in objs:
+        if o.get("revoked") or o.get("x_mitre_deprecated"):
+            continue
+        kind = o.get("type")
+        if kind == "x-mitre-tactic":
+            ref = _attack_ref(o)
+            if not ref:
+                continue
+            parsed.tactics.append(
+                {
+                    "external_id": ref["external_id"],
+                    "name": o.get("name") or "",
+                    "short_name": o.get("x_mitre_shortname") or "",
+                    "description": o.get("description"),
+                    "url": ref.get("url"),
+                }
+            )
+        elif kind == "attack-pattern":
+            ref = _attack_ref(o)
+            if not ref:
+                continue
+            common = {
+                "external_id": ref["external_id"],
+                "name": o.get("name") or "",
+                "description": o.get("description"),
+                "url": ref.get("url"),
+            }
+            if o.get("x_mitre_is_subtechnique"):
+                parent_stix = parents_by_subtech.get(o["id"])
+                parsed.subtechniques.append(
+                    {**common, "stix_id": o["id"], "parent_stix_id": parent_stix}
+                )
+            else:
+                # Capture kill_chain_phases so we can map to tactics by short_name.
+                phases = [
+                    p.get("phase_name")
+                    for p in (o.get("kill_chain_phases") or ())
+                    if p.get("kill_chain_name") == KILL_CHAIN_NAME and p.get("phase_name")
+                ]
+                parsed.techniques.append(
+                    {**common, "stix_id": o["id"], "phase_names": phases}
+                )
+    return parsed
+
+
+# === DB upserts ==============================================================
+
+
+def _upsert_tactics(s, tactics: Iterable[dict]) -> tuple[dict, int]:
+    """Upsert tactics. Returns (short_name → tactic_id, n_upserted)."""
+    existing = {t.external_id: t for t in s.scalars(select(MitreTactic)).all()}
+    short_to_id: dict = {}
+    upserted = 0
+    for t in tactics:
+        row = existing.get(t["external_id"])
+        if row is None:
+            row = MitreTactic(
+                external_id=t["external_id"],
+                short_name=t["short_name"],
+                name=t["name"],
+                description=t["description"],
+                url=t["url"],
+            )
+            s.add(row)
+            s.flush()
+            upserted += 1
+        else:
+            row.short_name = t["short_name"]
+            row.name = t["name"]
+            row.description = t["description"]
+            row.url = t["url"]
+            upserted += 1
+        short_to_id[t["short_name"]] = row.id
+    return short_to_id, upserted
+
+
+def _upsert_techniques(
+    s, techniques: Iterable[dict], short_to_tactic_id: dict
+) -> tuple[dict, int, int]:
+    """Upsert techniques + their tactic links. Returns (stix_id→technique_id, n_upserted, n_links)."""
+    existing = {t.external_id: t for t in s.scalars(select(MitreTechnique)).all()}
+    stix_to_id: dict = {}
+    n_upserted = 0
+    n_links = 0
+
+    # We'll rebuild the technique↔tactic mapping for clarity (drop + add). This
+    # is O(techniques × tactics) but cheap relative to the parse itself.
+    s.execute(delete(MitreTechniqueTactic))
+
+    for t in techniques:
+        row = existing.get(t["external_id"])
+        if row is None:
+            row = MitreTechnique(
+                external_id=t["external_id"],
+                name=t["name"],
+                description=t["description"],
+                url=t["url"],
+            )
+            s.add(row)
+            s.flush()
+        else:
+            row.name = t["name"]
+            row.description = t["description"]
+            row.url = t["url"]
+        n_upserted += 1
+        stix_to_id[t["stix_id"]] = row.id
+        for phase in t.get("phase_names", []):
+            tac_id = short_to_tactic_id.get(phase)
+            if tac_id is None:
+                # Tactic referenced but not in bundle — log + skip.
+                log.warning(
+                    "metamorph.mitre.unknown_tactic_phase",
+                    extra={"technique": t["external_id"], "phase": phase},
+                )
+                continue
+            s.add(MitreTechniqueTactic(technique_id=row.id, tactic_id=tac_id))
+            n_links += 1
+    return stix_to_id, n_upserted, n_links
+
+
+def _upsert_subtechniques(
+    s,
+    subtechniques: Iterable[dict],
+    stix_to_tech_id: dict,
+) -> tuple[int, int]:
+    """Returns (n_upserted, n_skipped_orphans)."""
+    existing = {sb.external_id: sb for sb in s.scalars(select(MitreSubtechnique)).all()}
+    n_upserted = 0
+    n_skipped = 0
+    for sb in subtechniques:
+        parent_stix = sb.get("parent_stix_id")
+        parent_id = stix_to_tech_id.get(parent_stix) if parent_stix else None
+        if parent_id is None:
+            # Fall back to the dotted external_id convention (T1003.001 → T1003).
+            m = re.match(r"^(T\d+)\.\d+$", sb["external_id"])
+            if m:
+                parent_ext = m.group(1)
+                # We don't have a parent-by-external-id map here; query.
+                parent_row = next(
+                    iter(
+                        s.scalars(
+                            select(MitreTechnique).where(MitreTechnique.external_id == parent_ext)
+                        ).all()
+                    ),
+                    None,
+                )
+                parent_id = parent_row.id if parent_row else None
+        if parent_id is None:
+            log.warning(
+                "metamorph.mitre.orphan_subtechnique",
+                extra={"subtechnique": sb["external_id"]},
+            )
+            n_skipped += 1
+            continue
+        row = existing.get(sb["external_id"])
+        if row is None:
+            s.add(
+                MitreSubtechnique(
+                    external_id=sb["external_id"],
+                    name=sb["name"],
+                    description=sb["description"],
+                    url=sb["url"],
+                    technique_id=parent_id,
+                )
+            )
+        else:
+            row.name = sb["name"]
+            row.description = sb["description"]
+            row.url = sb["url"]
+            row.technique_id = parent_id
+        n_upserted += 1
+    return n_upserted, n_skipped
+
+
+def _upsert_setting(s, key: str, value: object) -> None:
+    row = s.scalar(select(Setting).where(Setting.key == key))
+    if row is None:
+        s.add(Setting(key=key, value=value))
+    else:
+        row.value = value
+
+
+# === Entry point =============================================================
+
+
+def seed_mitre(
+    *,
+    source: str | Path | None = None,
+    expected_sha256: str | None = MITRE_DEFAULT_SHA256,
+    cache_dir: Path = MITRE_BUNDLE_CACHE_PATH,
+    allow_unverified: bool = False,
+) -> SeedResult:
+    """Top-level seed. URL → download + verify + parse; path → just parse.
+
+    Custom URLs (anything other than `MITRE_DEFAULT_URL`) MUST be paired with
+    an `expected_sha256` for integrity, or with `allow_unverified=True` to opt
+    out explicitly. This avoids a silent integrity bypass when an operator
+    points the sync at a typo'd or attacker-controlled mirror.
+    """
+    started_at = datetime.now(tz=timezone.utc)
+    if source is not None and _is_url(str(source)) and str(source) != MITRE_DEFAULT_URL:
+        if expected_sha256 is None or expected_sha256 == MITRE_DEFAULT_SHA256:
+            # The caller passed a non-default URL but didn't override the hash:
+            # MITRE_DEFAULT_SHA256 would obviously not match → force an explicit
+            # decision rather than silently bypassing.
+            if not allow_unverified:
+                raise MitreSeedError(
+                    "custom URL requires an expected_sha256 (or allow_unverified=True)"
+                )
+            expected_sha256 = None
+
+    path, source_label = resolve_source_to_path(
+        source, cache_dir=cache_dir, expected_sha256=expected_sha256
+    )
+
+    parsed = parse_bundle(path)
+    log.info(
+        "metamorph.mitre.parsed",
+        extra={
+            "tactics": len(parsed.tactics),
+            "techniques": len(parsed.techniques),
+            "subtechniques": len(parsed.subtechniques),
+            "spec_version": parsed.spec_version,
+        },
+    )
+
+    with session_scope() as s:
+        short_to_tactic_id, n_tactics = _upsert_tactics(s, parsed.tactics)
+        stix_to_tech_id, n_techs, n_links = _upsert_techniques(
+            s, parsed.techniques, short_to_tactic_id
+        )
+        n_subs, n_orphan = _upsert_subtechniques(s, parsed.subtechniques, stix_to_tech_id)
+
+        finished_at = datetime.now(tz=timezone.utc)
+        _upsert_setting(s, SETTING_LAST_SYNC, finished_at.isoformat())
+        # If the URL is the pinned one, we know the version; otherwise leave None.
+        version = MITRE_VERSION if source_label == MITRE_DEFAULT_URL else None
+        if version:
+            _upsert_setting(s, SETTING_VERSION, version)
+        _upsert_setting(s, SETTING_SOURCE_URL, source_label)
+
+    result = SeedResult(
+        tactics_upserted=n_tactics,
+        techniques_upserted=n_techs,
+        subtechniques_upserted=n_subs,
+        subtechniques_skipped_orphan=n_orphan,
+        technique_tactic_links=n_links,
+        version=version,
+        source=source_label,
+        started_at=started_at,
+        finished_at=finished_at,
+    )
+    log.info("metamorph.mitre.seed_completed", extra=result.as_dict())
+    return result
+
+
+def read_status() -> dict:
+    """Return the persisted seed metadata for `GET /mitre/status`."""
+    keys = {SETTING_LAST_SYNC, SETTING_VERSION, SETTING_SOURCE_URL}
+    out = {k: None for k in keys}
+    with session_scope() as s:
+        for row in s.scalars(select(Setting).where(Setting.key.in_(keys))).all():
+            out[row.key] = row.value
+    return {
+        "last_sync": out[SETTING_LAST_SYNC],
+        "version": out[SETTING_VERSION],
+        "source_url": out[SETTING_SOURCE_URL],
+        "default_url": MITRE_DEFAULT_URL,
+        "default_version": MITRE_VERSION,
+    }
diff --git a/docker-compose.yml b/docker-compose.yml
index a2d11ba..e9da7d5 100644
--- a/docker-compose.yml
+++ b/docker-compose.yml
@@ -38,6 +38,7 @@ services:
       EVIDENCE_DIR: ${EVIDENCE_DIR}
     volumes:
       - metamorph_evidence:/data/evidence
+      - metamorph_mitre:/data/mitre
     depends_on:
       db:
         condition: service_healthy
@@ -76,6 +77,7 @@ services:
 volumes:
   metamorph_db:
   metamorph_evidence:
+  metamorph_mitre:
 
 networks:
   metamorph: