Merge branch 'fix/third-review-h1-h2-m1-m6'

Third security review (docs/third-review.md) plus its remediation.

Fixed and verified:
- H1  moderator could grant admin / hard-revoke  → handler split by field
- H2  unauthenticated 2-report global blocklist  → auth + distinct reporters
      + rate limit + refused when public groups are off
- M1  registration reCAPTCHA was inert           → gate unconditional; the
      desktop client's CSP allows the widget
- M2  QUIC chat/stream handlers lagged WebRTC    → brought to parity; the QUIC
      listener is now off by default ([node] quic_enabled)
- M3  link-preview SSRF gaps                      → rate limit + port allowlist
      + connect-address re-check + decompression-bomb guard
- M4  federated peer over-trust                   → source bound to the signer,
      push capped, revocation prunes the peer's own entries, replay rejected
- M5  no CSP / security headers on the SPA        → middleware; verified against
      the live app with no violations

Withdrawn:
- M6  add_group_member accepting node tokens is deliberate (commit 0443cf8,
      the CLI invite flow). The "fix" broke that flow on the deployed hub and
      was reverted.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_011pG75yGK3NthNfyjH74omG

2 files changed, 135 insertions, 49 deletions

diff --git a/packages/meshbay-node/src/meshbay_node/transport/quic_server.py b/packages/meshbay-node/src/meshbay_node/transport/quic_server.py
index ce6fe17..30c7daf 100644
--- a/packages/meshbay-node/src/meshbay_node/transport/quic_server.py
+++ b/packages/meshbay-node/src/meshbay_node/transport/quic_server.py
@@ -61,6 +61,15 @@ CHUNK_SIZE = 1024 * 1024
 MAX_MSG    = 64 * 1024 * 1024
 ALPN       = ["meshbay-mnp"]
 
+# ffmpeg is spawned per STREAM_SEGMENT request, and `_extract_segment` runs
+# `subprocess.run` synchronously — so without a bound, an authenticated peer can
+# both fork-bomb the node and block its event loop for up to 30 s per request
+# (finding M2c). Extraction now runs in a thread and passes through this
+# semaphore. Small on purpose: the QUIC path has no shipping client yet, this is
+# parity work with the WebRTC transcode cap.
+_MAX_CONCURRENT_SEGMENTS = 4
+_segment_sem = asyncio.Semaphore(_MAX_CONCURRENT_SEGMENTS)
+
 
 class Denylist:
     """
@@ -204,6 +213,9 @@ class _MNPServerProtocol(QuicConnectionProtocol):
         self._nonce_client: bytes = b""
         self._gek_challenge: bytes | None = None
         self._pending = None
+        # asyncio holds only a weak reference to a bare task, so a spawned
+        # handler still running can be collected mid-flight. Hold them.
+        self._tasks: set[asyncio.Task] = set()
 
     def quic_event_received(self, event: QuicEvent) -> None:
         if isinstance(event, StreamDataReceived):
@@ -232,7 +244,7 @@ class _MNPServerProtocol(QuicConnectionProtocol):
             elif mtype == MNP.FILE_REQUEST:
                 self._do_file_request_sync(stream_id, msg)
             elif mtype == MNP.STREAM_SEGMENT:
-                self._do_stream_segment_sync(stream_id, msg)
+                self._spawn(self._do_stream_segment(stream_id, msg))
             elif mtype == MNP.CHAT_MESSAGE:
                 self._do_chat_message_sync(stream_id, msg)
             elif mtype == MNP.PING:
@@ -256,11 +268,10 @@ class _MNPServerProtocol(QuicConnectionProtocol):
         checks could drift from the WebRTC path independently. All of that now
         comes from meshbay_common.handshake, shared with WebRTC.
 
-        NOT YET DONE — finding C6 remains open on this transport: there is still no
-        GEK proof here, so a forged or stolen token reaches the node and can inject
-        chat without holding the group key. The challenge/response and mutual node
-        proof (quic_binding() is written and unit-tested for exactly this) are the
-        remaining work in 11.5.4/5/6.
+        The GEK proof is enforced here too: `_do_handshake_response_sync` runs
+        the same challenge/response and mutual node proof, from the same shared
+        module, bound to the QUIC certificate hash. Finding C6 is closed on this
+        transport.
         """
         try:
             peer = authorize_token(
@@ -339,9 +350,7 @@ class _MNPServerProtocol(QuicConnectionProtocol):
         self._user_id = peer.user_id
         self._group_id = peer.group_id
 
-        peers = self._ctx.get("_peers")
-        if peers is not None:
-            peers[self._user_id] = self
+        self._peer_registry()[self._user_id] = self
 
         transcript = handshake_transcript(
             ROLE_NODE, peer.group_id, self._nonce_client, self._gek_challenge, binding)
@@ -365,6 +374,20 @@ class _MNPServerProtocol(QuicConnectionProtocol):
             return self._ctx["groups"][self._group_id]
         return self._ctx
 
+    def _spawn(self, coro) -> None:
+        task = asyncio.ensure_future(coro)
+        self._tasks.add(task)
+        task.add_done_callback(self._tasks.discard)
+
+    def _peer_registry(self) -> dict:
+        """QUIC peers for THIS connection's group, keyed per group so a message
+        never crosses into another group on a multi-group node (findings M2b /
+        H1). Deliberately separate from the WebRTC registry that also lives in
+        the group context: the two transports' session objects have different
+        `_send` signatures, and cross-transport chat fan-out is not wired (no
+        QUIC client ships yet)."""
+        return self._group_ctx().setdefault("_quic_peers", {})
+
     def _do_index_sync_sync(self, stream_id: int) -> None:
         ctx = self._group_ctx()
         wire = ctx["index"].serialize()
@@ -399,61 +422,83 @@ class _MNPServerProtocol(QuicConnectionProtocol):
         )
         self._send(stream_id, chunk_data)
 
-    def _do_stream_segment_sync(self, stream_id: int, msg: dict) -> None:
-        """Extract and serve one HLS segment via ffmpeg."""
-        ctx = self._group_ctx()
-        file_id = msg["file_id"]
-        segment_index = msg["segment_index"]
-        segment_duration = msg.get("segment_duration", 4)
+    async def _do_stream_segment(self, stream_id: int, msg: dict) -> None:
+        """
+        Extract and serve one segment via ffmpeg — off the event loop and behind
+        a concurrency bound, so one request can neither stall the whole node nor
+        fork-bomb it (finding M2c). The WebRTC path has had both since Phase 11.5.
+        """
+        try:
+            ctx = self._group_ctx()
+            file_id = msg["file_id"]
+            segment_index = msg["segment_index"]
+            segment_duration = msg.get("segment_duration", 4)
 
-        entry = ctx["index"].get_entry(file_id)
-        if not entry:
-            self._send(stream_id, {"type": "error", "detail": "File not found"})
-            return
+            entry = ctx["index"].get_entry(file_id)
+            if not entry:
+                self._send(stream_id, {"type": "error", "detail": "File not found"})
+                return
 
-        file_path = entry_abs_path(ctx["roots"], entry)
-        if not file_path.exists():
-            self._send(stream_id, {"type": "error", "detail": "File not on disk"})
-            return
+            file_path = entry_abs_path(ctx["roots"], entry)
+            if not file_path.exists():
+                self._send(stream_id, {"type": "error", "detail": "File not on disk"})
+                return
 
-        start_time = segment_index * segment_duration
-        segment_data = _extract_segment(file_path, start_time, segment_duration)
-        if segment_data is None:
-            self._send(stream_id, {"type": "error", "detail": "Segment extraction failed"})
-            return
+            start_time = segment_index * segment_duration
+            loop = asyncio.get_event_loop()
+            async with _segment_sem:
+                segment_data = await loop.run_in_executor(
+                    None, _extract_segment, file_path, start_time, segment_duration)
+            if segment_data is None:
+                self._send(stream_id, {"type": "error", "detail": "Segment extraction failed"})
+                return
 
-        self._send(stream_id, {
-            "type": MNP.STREAM_SEGMENT,
-            "v": MNP_VERSION,
-            "file_id": file_id,
-            "segment_index": segment_index,
-            "data_b64": base64.b64encode(segment_data).decode(),
-            "size": len(segment_data),
-        })
+            self._send(stream_id, {
+                "type": MNP.STREAM_SEGMENT,
+                "v": MNP_VERSION,
+                "file_id": file_id,
+                "segment_index": segment_index,
+                "data_b64": base64.b64encode(segment_data).decode(),
+                "size": len(segment_data),
+            })
+        except Exception as e:
+            log.error("stream_segment: %s", e)
+            self._send(stream_id, {"type": "error", "detail": "Segment extraction failed"})
 
     def _do_chat_message_sync(self, stream_id: int, msg: dict) -> None:
-        """Receive a chat message, store it, and broadcast to other connected peers."""
-        chat_store = self._ctx.get("chat_store")
+        """
+        Store a chat message and broadcast it to the rest of THIS group.
+
+        `sender_id` is the authenticated session's, never the wire's — a peer
+        must not be able to post as someone else (NS6 / finding M2a). The store
+        and the peer set come from the group context, not a connection-global
+        one, so a message never crosses into another group on a multi-group node
+        (findings M2b / H1). The WebRTC path has done both since Phase 11.5.
+        """
+        gctx = self._group_ctx()
+        payload = msg.get("payload", b"")
+        if isinstance(payload, str):
+            payload = payload.encode()
+
+        chat_store = gctx.get("chat_store")
         if chat_store:
-            import asyncio
-            asyncio.ensure_future(chat_store.save_message(
-                sender_id=msg.get("sender_id", self._user_id),
+            self._spawn(chat_store.save_message(
+                sender_id=self._user_id,
                 iteration=msg.get("iteration", 0),
-                payload=msg.get("payload", b"").encode() if isinstance(msg.get("payload"), str) else msg.get("payload", b""),
+                payload=payload,
                 thread_id=msg.get("thread_id"),
             ))
 
-        peers = self._ctx.get("_peers", {})
         broadcast = {
             "type": MNP.CHAT_MESSAGE,
             "v": MNP_VERSION,
-            "sender_id": msg.get("sender_id", self._user_id),
+            "sender_id": self._user_id,
             "iteration": msg.get("iteration", 0),
             "payload": msg.get("payload", ""),
             "thread_id": msg.get("thread_id"),
             "group_id": self._group_id or "",
         }
-        for uid, proto in peers.items():
+        for uid, proto in list(self._peer_registry().items()):
             if uid != self._user_id and proto is not self:
                 try:
                     proto._send(0, broadcast)
@@ -463,9 +508,10 @@ class _MNPServerProtocol(QuicConnectionProtocol):
         self._send(stream_id, {"type": "ack", "v": MNP_VERSION})
 
     def connection_lost(self, exc) -> None:
-        peers = self._ctx.get("_peers")
-        if peers and self._user_id:
-            peers.pop(self._user_id, None)
+        if self._user_id:
+            self._peer_registry().pop(self._user_id, None)
+        for task in list(self._tasks):
+            task.cancel()
         super().connection_lost(exc)
 
     def _send(self, stream_id: int, obj: dict) -> None:
@@ -558,7 +604,7 @@ class QuicChunkServer:
             self._ctx["groups"] = groups
         self._denylist = denylist or Denylist()
         self._ctx["denylist"] = self._denylist
-        self._ctx["_peers"] = {}
+        # Peer sets are per group now — see _MNPServerProtocol._peer_registry().
         self._host      = host
         self._port      = port
         self._cert_path = cert_path or Path.home() / ".config/meshbay/node_tls.crt"
diff --git a/packages/meshbay-node/src/meshbay_node/transport/webrtc_server.py b/packages/meshbay-node/src/meshbay_node/transport/webrtc_server.py
index 003cd23..64ba75c 100644
--- a/packages/meshbay-node/src/meshbay_node/transport/webrtc_server.py
+++ b/packages/meshbay-node/src/meshbay_node/transport/webrtc_server.py
@@ -143,6 +143,16 @@ def _link_preview_cache_put(url: str, value: dict) -> None:
         _link_preview_cache.pop(oldest, None)
     _link_preview_cache[url] = (time.time(), value)
 
+
+# A member pasting a link is normal; a member — or a hub minting tokens for many
+# accounts — firing hundreds is amplification/DoS and a way to make the node
+# reach arbitrary hosts on demand (finding M3). Only a real outbound fetch is
+# counted (a cache hit costs nothing), and the ceilings are generous enough that
+# ordinary chat never meets them.
+_LINK_PREVIEW_RATE_WINDOW = 60.0
+_LINK_PREVIEW_RATE_PER_CONN = 15
+_LINK_PREVIEW_RATE_NODE = 60
+
 # Upload limits (finding C5a). Uploads used to land directly in the shared root under
 # a name the client chose, overwriting whatever was already there — which both violated
 # node sovereignty and defeated the delete authorization (overwrite a file, become its
@@ -3588,6 +3598,27 @@ class WebRTCPeerSession:
             ],
         })
 
+    def _link_preview_rate_ok(self) -> bool:
+        """
+        True when this preview fetch is within both the per-connection and the
+        node-wide window; records it when so, and both counts are trimmed to the
+        window on every call so the lists cannot grow without bound.
+        """
+        now = time.monotonic()
+        w = _LINK_PREVIEW_RATE_WINDOW
+        mine = [t for t in getattr(self, "_link_preview_hits", []) if now - t < w]
+        node = [t for t in self._ctx.get("link_preview_hits", []) if now - t < w]
+        if (len(mine) >= _LINK_PREVIEW_RATE_PER_CONN
+                or len(node) >= _LINK_PREVIEW_RATE_NODE):
+            self._link_preview_hits = mine
+            self._ctx["link_preview_hits"] = node
+            return False
+        mine.append(now)
+        node.append(now)
+        self._link_preview_hits = mine
+        self._ctx["link_preview_hits"] = node
+        return True
+
     async def _do_link_preview_request(self, msg: dict) -> None:
         """
         Unfurl a URL a member pasted into chat (draft-v6 §2.7 enrichment rule:
@@ -3608,6 +3639,15 @@ class WebRTCPeerSession:
             self._send({**cached, "type": MNP.LINK_PREVIEW_RESP, "v": MNP_VERSION})
             return
 
+        if not self._link_preview_rate_ok():
+            # Same shape as any other miss — the client shows the bare link. A
+            # rate-limited result is not cached, so it is retried once the
+            # window clears rather than pinned as "no preview".
+            log.debug("link_preview_req: rate-limited (peer=%s)", self._peer_id)
+            self._send({"type": MNP.LINK_PREVIEW_RESP, "v": MNP_VERSION,
+                        "url": key, "ok": False})
+            return
+
         resp: dict = {"type": MNP.LINK_PREVIEW_RESP, "v": MNP_VERSION,
                       "url": key, "ok": False}
         try: