diff --git a/decnet/sniffer/fingerprint.py b/decnet/sniffer/fingerprint.py
index 322d3f0d..80dfec3f 100644
--- a/decnet/sniffer/fingerprint.py
+++ b/decnet/sniffer/fingerprint.py
@@ -71,9 +71,37 @@ _BUS_TRAFFIC_EVENTS: frozenset[str] = frozenset({
     "http_request_fingerprint",
     "http2_settings",
     "http3_settings",
+    "ipv6_link_local_leak",
 })
 
 
+def _ipv6_iid_classify(addr: str) -> tuple[str, str]:
+    """Return (iid_kind, mac_oui) for a link-local IPv6 address.
+
+    iid_kind: "eui64" | "stable_privacy" | "temporary" | "unknown"
+    mac_oui:  "aa:bb:cc" (first 3 octets, lowercase) or "" when not derivable.
+
+    EUI-64 IIDs embed the MAC: bytes 11-12 of the 128-bit address are 0xff 0xfe,
+    and bit 6 of byte 8 (the universal/local bit) is flipped.  All other IIDs
+    from RFC 7217 (stable-privacy) or RFC 4941 (temporary) have no embedded MAC.
+    """
+    try:
+        import ipaddress
+        packed = ipaddress.ip_address(addr).packed  # 16 bytes
+    except Exception:
+        return "unknown", ""
+
+    iid = packed[8:]  # 8-byte Interface Identifier
+    if iid[3] == 0xff and iid[4] == 0xfe:
+        # EUI-64: recover 3-byte OUI from bytes 0-2, flip U/L bit (bit 1 of byte 0)
+        oui_bytes = bytearray([iid[0] ^ 0x02, iid[1], iid[2]])
+        oui = ":".join(f"{b:02x}" for b in oui_bytes)
+        return "eui64", oui
+    # Stable-privacy (RFC 7217) and temporary (RFC 4941) are not distinguishable
+    # from the address alone; classify as stable_privacy by default.
+    return "stable_privacy", ""
+
+
 def _parse_ssh_banner(data: bytes) -> str | None:
     """
     Return the attacker's SSH identification string (RFC 4253 §4.2) if
@@ -1275,8 +1303,71 @@ class SnifferEngine:
             else:
                 self._flows.pop(key, None)
 
+    def _on_ipv6_packet(self, pkt: Any) -> None:
+        """Handle an IPv6 packet and emit ipv6_link_local_leak if fe80::/10 src."""
+        try:
+            from scapy.layers.inet6 import IPv6
+        except ImportError:
+            return
+
+        if not pkt.haslayer(IPv6):
+            return
+
+        ip6 = pkt[IPv6]
+        src: str = ip6.src
+        dst: str = ip6.dst
+
+        # Only care about link-local sources (fe80::/10).
+        if not src.lower().startswith("fe80:"):
+            return
+
+        # Correlate to a known attacker: check if the packet destination
+        # is a known decky IP (attacker→decky direction).
+        attacker_v4 = ""
+        node_name = self._ip_to_decky.get(dst)
+        if node_name is None:
+            # Also accept reverse: decky→attacker (e.g. NDP response to our NS)
+            node_name = self._ip_to_decky.get(src)
+            if node_name is None:
+                return
+        else:
+            # dst is a decky — the attacker is the src (link-local)
+            # Try to find the attacker's known v4 address from ip_to_decky reverse.
+            # We don't have an authoritative v4 here; leave it empty and let the
+            # lifter correlate by decky_name + timing.
+            attacker_v4 = ""
+
+        iid_kind, mac_oui = _ipv6_iid_classify(src)
+        # Scapy packets from sniff() carry the iface in pkt.sniffed_on when
+        # iface= was passed to sniff(); getattr handles absent attribute safely.
+        iface = getattr(pkt, "sniffed_on", None) or ""
+
+        from datetime import datetime, timezone
+        observed_at = datetime.now(timezone.utc).isoformat()
+
+        self._log(
+            node_name,
+            "ipv6_link_local_leak",
+            src_ip=src,
+            dst_ip=dst,
+            iid_kind=iid_kind,
+            mac_oui=mac_oui,
+            on_iface=iface,
+            attacker_v4=attacker_v4,
+            observed_at=observed_at,
+        )
+
     def on_packet(self, pkt: Any) -> None:
         """Process a single scapy packet. Called from the sniff thread."""
+        try:
+            from scapy.layers.inet6 import IPv6
+        except ImportError:
+            pass
+        else:
+            if pkt.haslayer(IPv6):
+                self._on_ipv6_packet(pkt)
+                return
+
         try:
             from scapy.layers.inet import IP, TCP
         except ImportError:
diff --git a/decnet/sniffer/worker.py b/decnet/sniffer/worker.py
index 7ac44574..084306b0 100644
--- a/decnet/sniffer/worker.py
+++ b/decnet/sniffer/worker.py
@@ -89,7 +89,7 @@ def _sniff_loop(
     log_path: Path,
     json_path: Path,
     stop_event: threading.Event,
-    bpf_filter: str = "tcp",
+    bpf_filter: str = "tcp or ip6",
     publish_fn: Callable[[str, str, dict[str, Any]], None] | None = None,
     engine: "SnifferEngine | None" = None,
 ) -> None:
diff --git a/tests/sniffer/test_ipv6_leak.py b/tests/sniffer/test_ipv6_leak.py
new file mode 100644
index 00000000..15ac30cb
--- /dev/null
+++ b/tests/sniffer/test_ipv6_leak.py
@@ -0,0 +1,135 @@
+"""Passive IPv6 link-local leak detection — sniffer unit tests.
+
+Tests SnifferEngine._on_ipv6_packet and _ipv6_iid_classify via direct
+packet injection (no sniff thread — per project constraint on scapy sniff
+threads in pytest teardown).
+"""
+from __future__ import annotations
+
+import pytest
+
+from scapy.layers.inet6 import ICMPv6ND_NS, IPv6
+
+from decnet.sniffer.fingerprint import SnifferEngine, _ipv6_iid_classify
+
+_DECKY_IP6 = "fe80::1"   # decky's own link-local (destination)
+_DECKY_IP4 = "10.0.0.5"  # corresponding v4 for ip_to_decky mapping
+_DECKY = "decky-a"
+
+# EUI-64 derived from MAC aa:bb:cc:dd:ee:ff →
+#   bytes: aa^0x02, bb, cc, ff, fe, dd, ee, ff
+# → IID: a8:bb:cc:ff:fe:dd:ee:ff → fe80::aabb:ccff:fedd:eeff
+_EUI64_ADDR = "fe80::aabb:ccff:fedd:eeff"
+_EUI64_OUI  = "a8:bb:cc"   # U/L bit flipped (aa XOR 0x02 = a8)
+
+# Stable-privacy / random IID — no fffe bytes at positions 3-4
+_STABLE_ADDR = "fe80::1234:5678:9abc:def0"
+
+
+def _engine(extra_map: dict[str, str] | None = None) -> tuple[SnifferEngine, list[str]]:
+    captured: list[str] = []
+    ip_map = {_DECKY_IP4: _DECKY}
+    if extra_map:
+        ip_map.update(extra_map)
+    engine = SnifferEngine(
+        ip_to_decky=ip_map,
+        write_fn=captured.append,
+        dedup_ttl=300.0,
+    )
+    return engine, captured
+
+
+# ── _ipv6_iid_classify ───────────────────────────────────────────────────────
+
+
+def test_iid_classify_eui64_returns_oui() -> None:
+    kind, oui = _ipv6_iid_classify(_EUI64_ADDR)
+    assert kind == "eui64"
+    assert oui == _EUI64_OUI
+
+
+def test_iid_classify_stable_privacy() -> None:
+    kind, oui = _ipv6_iid_classify(_STABLE_ADDR)
+    assert kind == "stable_privacy"
+    assert oui == ""
+
+
+def test_iid_classify_bad_addr_returns_unknown() -> None:
+    kind, oui = _ipv6_iid_classify("not-an-address")
+    assert kind == "unknown"
+    assert oui == ""
+
+
+# ── _on_ipv6_packet ─────────────────────────────────────────────────────────
+
+
+def _make_ndp_ns(src: str, dst: str) -> object:
+    """Craft a Neighbor Solicitation from attacker link-local to decky."""
+    return IPv6(src=src, dst=dst) / ICMPv6ND_NS(tgt=dst)
+
+
+def test_eui64_packet_emits_ipv6_leak_event() -> None:
+    engine, captured = _engine(extra_map={"fe80::1": _DECKY})
+    pkt = _make_ndp_ns(_EUI64_ADDR, _DECKY_IP6)
+    engine._on_ipv6_packet(pkt)
+    assert len(captured) == 1
+    line = captured[0]
+    assert "ipv6_link_local_leak" in line
+    assert _EUI64_ADDR in line
+    assert "eui64" in line
+    assert _EUI64_OUI in line
+
+
+def test_stable_privacy_packet_emits_event() -> None:
+    engine, captured = _engine(extra_map={"fe80::1": _DECKY})
+    pkt = _make_ndp_ns(_STABLE_ADDR, _DECKY_IP6)
+    engine._on_ipv6_packet(pkt)
+    assert len(captured) == 1
+    assert "stable_privacy" in captured[0]
+
+
+def test_non_link_local_src_is_ignored() -> None:
+    engine, captured = _engine(extra_map={"fe80::1": _DECKY})
+    # GUA source — not a link-local leak
+    pkt = IPv6(src="2001:db8::1", dst=_DECKY_IP6) / ICMPv6ND_NS(tgt=_DECKY_IP6)
+    engine._on_ipv6_packet(pkt)
+    assert captured == []
+
+
+def test_packet_to_unknown_decky_is_ignored() -> None:
+    engine, captured = _engine()  # ip_to_decky has no v6 entries
+    pkt = _make_ndp_ns(_EUI64_ADDR, "fe80::dead")
+    engine._on_ipv6_packet(pkt)
+    assert captured == []
+
+
+def test_on_packet_dispatches_ipv6_branch() -> None:
+    """on_packet() must route IPv6 packets to _on_ipv6_packet."""
+    engine, captured = _engine(extra_map={"fe80::1": _DECKY})
+    pkt = _make_ndp_ns(_EUI64_ADDR, _DECKY_IP6)
+    engine.on_packet(pkt)
+    assert any("ipv6_link_local_leak" in line for line in captured)
+
+
+def test_dedup_suppresses_repeat_emit() -> None:
+    engine, captured = _engine(extra_map={"fe80::1": _DECKY})
+    pkt = _make_ndp_ns(_EUI64_ADDR, _DECKY_IP6)
+    engine._on_ipv6_packet(pkt)
+    engine._on_ipv6_packet(pkt)
+    assert len(captured) == 1  # second identical packet deduped
+
+
+def test_publish_fn_fires_on_leak() -> None:
+    published: list[tuple[str, str, dict]] = []
+    engine = SnifferEngine(
+        ip_to_decky={"fe80::1": _DECKY},
+        write_fn=lambda _: None,
+        publish_fn=lambda node, event, payload: published.append((node, event, payload)),
+    )
+    pkt = _make_ndp_ns(_EUI64_ADDR, _DECKY_IP6)
+    engine._on_ipv6_packet(pkt)
+    assert len(published) == 1
+    node, event, payload = published[0]
+    assert event == "ipv6_link_local_leak"
+    assert payload["iid_kind"] == "eui64"
+    assert payload["mac_oui"] == _EUI64_OUI