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Withdrawn: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

10 files changed, 263 insertions, 64 deletions

diff --git a/packages/meshbay-hub/src/meshbay_hub/api/admin.py b/packages/meshbay-hub/src/meshbay_hub/api/admin.py
index ab6fa07..4960674 100644
--- a/packages/meshbay-hub/src/meshbay_hub/api/admin.py
+++ b/packages/meshbay-hub/src/meshbay_hub/api/admin.py
@@ -14,7 +14,7 @@ from sqlalchemy import func, select
 from sqlalchemy.ext.asyncio import AsyncSession
 
 from meshbay_hub.auth import decrypt_email
-from meshbay_hub.api.deps import require_admin, require_moderator
+from meshbay_hub.api.deps import require_admin, require_moderator, user_is_admin
 from meshbay_hub.api.revocation import get_connected_node_count, is_node_connected
 from meshbay_hub.db.engine import get_db
 from meshbay_hub.db.models import Group, GroupMember, IPLog, Node, User
@@ -196,6 +196,25 @@ async def admin_patch_user(
     if user.id == current_user.id:
         raise HTTPException(status_code=400, detail="Cannot modify your own account")
 
+    # A moderator suspends and restores accounts — reversible content moderation.
+    # Changing what someone *is* (their role), and the one irreversible status
+    # (`revoked`, which is signed and broadcast to every node), are administrative.
+    # Without this split a moderator could promote an accomplice to admin, or
+    # revoke every admin, entirely from the moderation role. `admin_delete_user`
+    # already draws this exact line for the same reason.
+    privileged = body.role is not None or body.status == "revoked"
+    if privileged and not user_is_admin(current_user):
+        raise HTTPException(
+            status_code=403,
+            detail="Changing a role, or revoking an account, requires admin rights")
+
+    # An admin's account is not a moderator's to touch at all — not their role,
+    # not their status.
+    if user_is_admin(user) and not user_is_admin(current_user):
+        raise HTTPException(
+            status_code=403,
+            detail="Only an admin can change another admin's account")
+
     from meshbay_hub.api.notifications import create_notification
 
     if body.role is not None:
diff --git a/packages/meshbay-hub/src/meshbay_hub/api/deps.py b/packages/meshbay-hub/src/meshbay_hub/api/deps.py
index 1bf57a4..907a481 100644
--- a/packages/meshbay-hub/src/meshbay_hub/api/deps.py
+++ b/packages/meshbay-hub/src/meshbay_hub/api/deps.py
@@ -72,11 +72,21 @@ async def require_user_scope(
     return current_user
 
 
+def user_is_admin(user: User) -> bool:
+    """Admin by DB role or by the config allow-list. Use inside a handler that
+    already depends on `require_moderator` but has to draw the admin line for
+    one field (see `admin_patch_user`)."""
+    return user.role == "admin" or user.username in _admin_usernames
+
+
+def user_is_moderator(user: User) -> bool:
+    return user.role in ("moderator", "admin") or user.username in _admin_usernames
+
+
 async def require_moderator(
     current_user: User = Depends(get_current_user),
 ) -> User:
-    if current_user.role not in ("moderator", "admin") \
-       and current_user.username not in _admin_usernames:
+    if not user_is_moderator(current_user):
         raise HTTPException(status_code=status.HTTP_403_FORBIDDEN,
                             detail="Moderator access required")
     return current_user
@@ -85,8 +95,7 @@ async def require_moderator(
 async def require_admin(
     current_user: User = Depends(get_current_user),
 ) -> User:
-    if current_user.role != "admin" \
-       and current_user.username not in _admin_usernames:
+    if not user_is_admin(current_user):
         raise HTTPException(status_code=status.HTTP_403_FORBIDDEN,
                             detail="Admin access required")
     return current_user
diff --git a/packages/meshbay-hub/src/meshbay_hub/api/federation.py b/packages/meshbay-hub/src/meshbay_hub/api/federation.py
index 7f1262d..b1e0e30 100644
--- a/packages/meshbay-hub/src/meshbay_hub/api/federation.py
+++ b/packages/meshbay-hub/src/meshbay_hub/api/federation.py
@@ -27,7 +27,7 @@ import uuid
 import jwt
 from fastapi import APIRouter, Depends, HTTPException, Header
 from pydantic import BaseModel
-from sqlalchemy import select
+from sqlalchemy import func, select
 from sqlalchemy.ext.asyncio import AsyncSession
 
 from meshbay_common import MHP_VERSION
@@ -41,6 +41,17 @@ log = logging.getLogger(__name__)
 
 router = APIRouter(prefix="/mhp", tags=["federation"])
 
+# One push may not dump the world, and one peer may not fill the table.
+MAX_FEDERATED_GROUPS_PER_PUSH = 500
+MAX_FEDERATED_GROUPS_PER_PEER = 2000
+
+# Seen `jti` values for the state-changing MHP endpoints, pruned lazily. The
+# sending side that would set an `aud` claim is unbuilt, so audience binding is
+# not available; this stops a captured POST /mhp/directory or /mhp/revoke from
+# being replayed inside the token's short TTL. GET /mhp/directory is idempotent
+# and not covered.
+_seen_mhp_jti: dict[str, float] = {}
+
 
 def _issue_mhp_token(target_hub_id: str) -> str:
     """Issue a short-lived JWT for authenticating to a peer hub."""
@@ -57,9 +68,15 @@ def _issue_mhp_token(target_hub_id: str) -> str:
 
 
 async def _verify_mhp_token(
-    token: str, db: AsyncSession, expected_aud: str | None = None,
+    token: str, db: AsyncSession, *, single_use: bool = False,
 ) -> dict:
-    """Verify a JWT from a peer hub using DB-stored public key."""
+    """
+    Verify a JWT from a peer hub against its DB-stored public key and return the
+    payload.
+
+    `single_use=True` (the state-changing endpoints) additionally rejects a
+    replayed `jti` within the token's lifetime.
+    """
     unverified = jwt.decode(token, options={"verify_signature": False})
     sender_id = unverified.get("iss")
 
@@ -67,15 +84,22 @@ async def _verify_mhp_token(
     if not peer:
         raise PermissionError(f"Unknown hub: {sender_id!r}. Register as peer first.")
 
-    options = {}
-    if expected_aud:
-        options["audience"] = expected_aud
-
     decoded = jwt.decode(
         token, peer.pk_hub_pem.encode(),
         algorithms=["EdDSA"],
-        options=options,
+        options={"require": ["exp", "iss"]},
     )
+
+    if single_use:
+        now = time.time()
+        for j, exp in list(_seen_mhp_jti.items()):
+            if exp < now:
+                _seen_mhp_jti.pop(j, None)
+        jti = decoded.get("jti", "")
+        if not jti or jti in _seen_mhp_jti:
+            raise PermissionError("MHP token replay")
+        _seen_mhp_jti[jti] = float(decoded.get("exp", now + 300))
+
     return decoded
 
 
@@ -142,30 +166,50 @@ async def receive_directory(
     db: AsyncSession = Depends(get_db),
 ):
     try:
-        await _verify_mhp_token(authorization.removeprefix("Bearer "), db)
+        payload = await _verify_mhp_token(
+            authorization.removeprefix("Bearer "), db, single_use=True)
     except Exception as e:
         raise HTTPException(status_code=401, detail=str(e))
 
+    # `source_hub` is the signer of this request, never `body.hub_id` — a peer
+    # does not get to relay or spoof a third hub's groups into our directory.
+    sender = payload["iss"]
+    if len(body.groups) > MAX_FEDERATED_GROUPS_PER_PUSH:
+        raise HTTPException(status_code=413, detail="Too many groups in one push")
+
     from datetime import datetime, timezone
     now = datetime.now(timezone.utc)
+    have = await db.scalar(
+        select(func.count()).select_from(FederatedGroup)
+        .where(FederatedGroup.source_hub == sender)) or 0
+
     count = 0
     for g in body.groups:
-        existing = await db.get(FederatedGroup, g["id"])
-        if existing:
-            existing.name        = g.get("name", existing.name)
-            existing.join_policy = g.get("join_policy", existing.join_policy)
-            existing.updated_at  = now
+        gid = str(g.get("id", ""))[:36]
+        name = str(g.get("name", ""))[:128]
+        jp = g.get("join_policy", "invite")
+        if not gid or jp not in ("invite", "open"):
+            continue
+        # A federated id must never shadow a real local group.
+        if await db.get(Group, gid):
+            log.warning("Federated id %s collides with a local group — skipped", gid[:8])
+            continue
+        row = await db.get(FederatedGroup, gid)
+        if row:
+            if row.source_hub != sender:
+                continue          # only the hub that advertised it may update it
+            row.name        = name or row.name
+            row.join_policy = jp
+            row.updated_at  = now
         else:
+            if have + count >= MAX_FEDERATED_GROUPS_PER_PEER:
+                break
             db.add(FederatedGroup(
-                id=g["id"],
-                name=g.get("name", ""),
-                source_hub=body.hub_id,
-                join_policy=g.get("join_policy", "invite"),
-            ))
+                id=gid, name=name, source_hub=sender, join_policy=jp))
         count += 1
     await db.commit()
-    log.info("Persisted %d groups from hub %s", count, body.hub_id[:16])
-    return {"accepted": count, "from_hub": body.hub_id}
+    log.info("Persisted %d groups from hub %s", count, sender[:16])
+    return {"accepted": count, "from_hub": sender}
 
 
 # ── Revocation propagation ────────────────────────────────────────────────────
@@ -179,15 +223,43 @@ async def receive_revocation(
     authorization: str = Header(...),
     db: AsyncSession = Depends(get_db),
 ):
+    """
+    Act on a revocation from a peer hub.
+
+    This does **not** reach local nodes: nothing here hosts a federated group,
+    and a local node would reject a token signed by another hub's key anyway
+    (that path was a silent no-op). What a peer may legitimately revoke is a
+    group *it advertised to us* — so this prunes our copy of the peer's
+    directory. Users are per-hub; a peer does not get to revoke ours.
+    """
     try:
-        await _verify_mhp_token(authorization.removeprefix("Bearer "), db)
+        payload = await _verify_mhp_token(
+            authorization.removeprefix("Bearer "), db, single_use=True)
     except Exception as e:
         raise HTTPException(status_code=401, detail=str(e))
 
-    from meshbay_hub.api.revocation import broadcast_revocation
-    sent = await broadcast_revocation(body.token)
-    log.info("Propagated revocation to %d local nodes", sent)
-    return {"propagated_to": sent}
+    sender = payload["iss"]
+    peer = await db.get(HubPeer, sender)
+    try:
+        inner = jwt.decode(
+            body.token, peer.pk_hub_pem.encode(), algorithms=["EdDSA"],
+            options={"verify_exp": False})
+    except Exception as e:
+        raise HTTPException(status_code=400, detail=f"Bad revocation token: {e}")
+
+    if inner.get("type") != "revocation" or inner.get("target") != "group":
+        return {"pruned": 0, "note": "federation may only revoke groups it advertised"}
+
+    target_id = inner.get("target_id", "")
+    row = await db.get(FederatedGroup, target_id)
+    pruned = 0
+    if row and row.source_hub == sender:
+        await db.delete(row)
+        await db.commit()
+        pruned = 1
+    log.info("Federated group %s revoked by %s (pruned=%d)",
+             target_id[:8], sender[:16], pruned)
+    return {"pruned": pruned}
 
 
 # ── Peer management (admin) ───────────────────────────────────────────────────
diff --git a/packages/meshbay-hub/src/meshbay_hub/api/groups.py b/packages/meshbay-hub/src/meshbay_hub/api/groups.py
index fdb0444..07d3ec0 100644
--- a/packages/meshbay-hub/src/meshbay_hub/api/groups.py
+++ b/packages/meshbay-hub/src/meshbay_hub/api/groups.py
@@ -589,6 +589,13 @@ async def add_group_member(
     current_user: User = Depends(get_current_user),
     db: AsyncSession = Depends(get_db),
 ):
+    # `get_current_user`, not `require_user_scope`: the node calls this after a
+    # CLI `member invite` so the group becomes visible in the invitee's SPA
+    # (commit 0443cf8). The node authenticates with a node-scoped token, and the
+    # `group.admin_id == current_user.id` check below is the real guard — a node
+    # can only touch its own operator's groups, adding an already-registered
+    # account. (Third-review M6 proposed tightening this to `require_user_scope`;
+    # that broke the CLI invite flow and was reverted — see the review doc.)
     group = await db.get(Group, group_id)
     if not group:
         raise HTTPException(status_code=404, detail="Group not found")
diff --git a/packages/meshbay-hub/src/meshbay_hub/api/moderation.py b/packages/meshbay-hub/src/meshbay_hub/api/moderation.py
index 853f255..0939eec 100644
--- a/packages/meshbay-hub/src/meshbay_hub/api/moderation.py
+++ b/packages/meshbay-hub/src/meshbay_hub/api/moderation.py
@@ -1,13 +1,16 @@
 """
 MeshBay Hub — moderation endpoints.
 
-Public reporting flow:
-  POST /v1/reports            — report a content hash (no auth required)
+Reporting flow:
+  POST /v1/reports            — report a content hash (sign-in required)
 
-  Thresholds:
-    1st report  → logged, node admin notified (future: push notification)
-    2nd report  → content hash added to blocklist automatically
-    3rd+ report → logged as repeat offense (escalation for human review)
+  Thresholds (counted as DISTINCT reporting accounts, not raw rows):
+    < AUTO_BLOCK_THRESHOLD distinct reporters → logged
+    >= AUTO_BLOCK_THRESHOLD distinct reporters → hash added to the blocklist
+
+  The flow only runs while the hub brokers public content: with public groups
+  switched off instance-wide there is nothing here to serve a reported hash from,
+  so it is refused rather than left open as an unauthenticated write surface.
 
 Admin endpoints:
   GET  /v1/admin/blocklist    — list blocked hashes
@@ -27,7 +30,9 @@ from pydantic import BaseModel
 from sqlalchemy import func, select
 from sqlalchemy.ext.asyncio import AsyncSession
 
+from meshbay_hub import hub_settings
 from meshbay_hub.api.deps import get_current_user, require_admin
+from meshbay_hub.api.middleware import limiter
 from meshbay_hub.api.netutil import client_ip
 from meshbay_hub.db.engine import get_db
 from meshbay_hub.db.models import ContentBlocklist, ContentReport, User
@@ -36,7 +41,11 @@ log = logging.getLogger(__name__)
 
 router = APIRouter(tags=["moderation"])
 
-AUTO_BLOCK_THRESHOLD = 2   # reports before automatic block
+# Distinct reporting accounts before a hash is auto-blocked. Kept low for a
+# responsive community signal, but note it is only as strong as account
+# creation: while a bot can register freely (see the reCAPTCHA gap), the real
+# control is the admin reviewing `GET /v1/admin/blocklist` and the audit log.
+AUTO_BLOCK_THRESHOLD = 3
 
 
 # ── Models ────────────────────────────────────────────────────────────────────
@@ -56,34 +65,58 @@ class BlocklistAddRequest(BaseModel):
 # ── Public endpoints ──────────────────────────────────────────────────────────
 
 @router.post("/v1/reports", status_code=201)
+@limiter.limit("10/hour")
 async def report_content(
     body: ReportRequest,
     request: Request,
+    current_user: User = Depends(get_current_user),
     db: AsyncSession = Depends(get_db),
 ):
-    """Report a content hash. No authentication required."""
+    """
+    Report a public content hash for moderation.
+
+    Sign-in is required. It used to be anonymous, which made it a censorship
+    primitive: two unauthenticated POSTs naming any blake3 id auto-added it to
+    the blocklist that nodes enforce, network-wide, with manual admin removal the
+    only undo. The threshold now counts *distinct reporting accounts*, one vote
+    per account per hash.
+
+    Refused entirely when the hub has public groups switched off: nothing here
+    brokers public content then, nothing syncs the blocklist, and an open write
+    endpoint would only be abuse surface.
+    """
+    if not await hub_settings.public_groups_allowed(db):
+        raise HTTPException(
+            status_code=403,
+            detail="This hub does not broker public content, so there is nothing to report here.")
+
     if len(body.content_hash) != 64 or not all(c in "0123456789abcdef" for c in body.content_hash):
         raise HTTPException(status_code=422, detail="content_hash must be 64 hex chars (blake3)")
 
-    ip = client_ip(request)
+    # One vote per account per hash — a single reporter must not be able to walk
+    # the threshold up on their own by posting repeatedly.
+    already = await db.scalar(
+        select(ContentReport.id).where(
+            ContentReport.content_hash == body.content_hash,
+            ContentReport.reporter_id == current_user.id))
 
-    # Count existing reports for this hash
-    count_result = await db.execute(
-        select(func.count()).where(ContentReport.content_hash == body.content_hash))
-    count = count_result.scalar_one()
+    if not already:
+        db.add(ContentReport(
+            content_hash=body.content_hash,
+            reporter_id=current_user.id,
+            group_id=body.group_id,
+            reason=body.reason,
+            detail=body.detail,
+            ip_address=client_ip(request),
+        ))
+        await db.flush()
 
-    report = ContentReport(
-        content_hash=body.content_hash,
-        group_id=body.group_id,
-        reason=body.reason,
-        detail=body.detail,
-        ip_address=ip,
-    )
-    db.add(report)
+    distinct_reporters = await db.scalar(
+        select(func.count(func.distinct(ContentReport.reporter_id)))
+        .where(ContentReport.content_hash == body.content_hash)) or 0
 
-    action = "logged"
-    if count + 1 >= AUTO_BLOCK_THRESHOLD:
-        # Check if already blocked
+    action = "already_reported" if already else "logged"
+    if distinct_reporters >= AUTO_BLOCK_THRESHOLD:
         existing = await db.get(ContentBlocklist, body.content_hash)
         if not existing:
             db.add(ContentBlocklist(
@@ -92,13 +125,14 @@ async def report_content(
                 added_by="auto",
             ))
             action = "auto_blocked"
-            log.warning("Content auto-blocked after %d reports: %s", count + 1, body.content_hash[:16])
+            log.warning("Content auto-blocked after %d distinct reporters: %s",
+                        distinct_reporters, body.content_hash[:16])
 
     await db.commit()
     return {
         "status":       action,
         "content_hash": body.content_hash,
-        "report_count": count + 1,
+        "report_count": distinct_reporters,
         "threshold":    AUTO_BLOCK_THRESHOLD,
     }
 
diff --git a/packages/meshbay-hub/src/meshbay_hub/api/users.py b/packages/meshbay-hub/src/meshbay_hub/api/users.py
index 559cfa6..9b70b59 100644
--- a/packages/meshbay-hub/src/meshbay_hub/api/users.py
+++ b/packages/meshbay-hub/src/meshbay_hub/api/users.py
@@ -150,8 +150,13 @@ async def register(
             return {"user_id": found.id, "email_verification_required": True}
         raise HTTPException(status_code=409, detail="Username already taken")
 
-    # Captcha gate — web path only (native clients send auth_key)
-    if _cfg and _cfg.captcha.enabled and not body.auth_key:
+    # Captcha gate — every fresh registration when a captcha is configured, with
+    # no client carve-out. The earlier `and not body.auth_key` exempted anything
+    # that sent an `auth_key`, which is *every* real client (the browser sends it
+    # too, from the password split) — so the check was off for everyone, and a
+    # bot skipped it by sending the field. The desktop client is Chromium and
+    # renders the same widget, so it has no need of an exemption either.
+    if _cfg and _cfg.captcha.enabled:
         await _verify_captcha_or_raise(body.captcha_token, request)
 
     # Email uniqueness (only active or pending accounts)
diff --git a/packages/meshbay-hub/src/meshbay_hub/api/webapp.py b/packages/meshbay-hub/src/meshbay_hub/api/webapp.py
index 0821809..6dfd3ed 100644
--- a/packages/meshbay-hub/src/meshbay_hub/api/webapp.py
+++ b/packages/meshbay-hub/src/meshbay_hub/api/webapp.py
@@ -22,7 +22,8 @@ STATIC_DIR = Path(__file__).parent.parent / "static"
 router = APIRouter(tags=["webapp"])
 
 # Assets the shell pulls in, in load order. Everything else is imported by
-# app.js and rides on the same query string via window.__MB_ASSET_V.
+# app.js from a relative path, which inherits the `/a/<hash>/` prefix the shell
+# loaded app.js under — so the whole module graph moves together.
 # Every module the page loads. A file missing from here is a file whose change
 # does not move the URL, so a browser holding the old one never asks for it —
 # which is the failure this list exists to prevent, and it is silent.
@@ -77,6 +78,33 @@ ASSET_V = _asset_version()
 _NO_STORE = {"Cache-Control": "no-store"}
 
 
+# Content-Security-Policy for the whole hub, applied by a middleware in app.py.
+#
+# This is the *same* policy the desktop client's protocol handler already sends
+# for these exact UI files (`meshbay-client/src/main.js`), plus the two reCAPTCHA
+# hosts the sign-up widget loads its script, challenge iframe and images from.
+# `'unsafe-inline'` is style-only — htm/preact set inline `style=` attributes
+# everywhere; nothing inline executes, and the shell below carries no inline
+# `<script>`. `'wasm-unsafe-eval'` is required for the Argon2id WASM. The hub's
+# own origin is deliberately absent from `script-src`: a response it returns is
+# never executed, which is the point of T3.
+_RECAPTCHA_SRC = "https://www.google.com https://www.gstatic.com"
+CSP = "; ".join([
+    "default-src 'none'",
+    f"script-src 'self' 'wasm-unsafe-eval' {_RECAPTCHA_SRC}",
+    "style-src 'self' 'unsafe-inline'",
+    f"img-src 'self' data: blob: {_RECAPTCHA_SRC}",
+    "media-src 'self' blob:",
+    "font-src 'self'",
+    "connect-src 'self' https: wss:",
+    "worker-src 'self'",
+    f"frame-src {_RECAPTCHA_SRC}",
+    "frame-ancestors 'none'",
+    "base-uri 'none'",
+    "form-action 'none'",
+])
+
+
 @router.get("/app", response_class=HTMLResponse)
 async def app_root():
     return HTMLResponse(_HTML, headers=_NO_STORE)
@@ -109,7 +137,6 @@ _HTML = """\
        and app.js's own relative imports inherit the prefix, which is the only
        way the module graph is guaranteed not to be a mixture of two builds.
        See _asset_version() and VersionedStatics. -->
-  <script>window.__MB_ASSET_V = "{v}";</script>
   <!-- Argon2id (WebAssembly, inlined) — WebCrypto has no memory-hard KDF, and the
        keypair bundle needs one: it is protected by the passphrase alone and sits
        on every node its owner joins (C4). Vendored, see static/vendor/PROVENANCE.md -->
diff --git a/packages/meshbay-hub/src/meshbay_hub/app.py b/packages/meshbay-hub/src/meshbay_hub/app.py
index 76d7ec0..2daa55b 100644
--- a/packages/meshbay-hub/src/meshbay_hub/app.py
+++ b/packages/meshbay-hub/src/meshbay_hub/app.py
@@ -35,7 +35,7 @@ from meshbay_hub.api.relay       import router as relay_router
 from meshbay_hub.api.signaling   import router as signaling_router
 from meshbay_hub.api.admin       import router as admin_router
 from meshbay_hub.api.notifications import router as notifications_router
-from meshbay_hub.api.webapp     import router as webapp_router, STATIC_DIR, ASSET_V
+from meshbay_hub.api.webapp     import router as webapp_router, STATIC_DIR, ASSET_V, CSP
 from meshbay_hub.api.middleware import limiter
 
 
@@ -142,6 +142,21 @@ def create_app(cfg: HubConfig | None = None) -> FastAPI:
     app.state.limiter = limiter
     app.add_exception_handler(RateLimitExceeded, _rate_limit_exceeded_handler)
 
+    @app.middleware("http")
+    async def _security_headers(request, call_next):
+        """
+        Second-review L5, third-review M5: the SPA shell and its assets went out
+        with no CSP and no other protective headers. This adds them everywhere —
+        `webapp.CSP` is the same policy the desktop client already enforces on
+        these exact files. `setdefault` so a route that sets its own wins.
+        """
+        response = await call_next(request)
+        response.headers.setdefault("Content-Security-Policy", CSP)
+        response.headers.setdefault("X-Content-Type-Options", "nosniff")
+        response.headers.setdefault("Referrer-Policy", "strict-origin-when-cross-origin")
+        response.headers.setdefault("X-Frame-Options", "DENY")
+        return response
+
     # Routers (webapp last — catches / before API routes)
     app.include_router(hub_router)
     app.include_router(users_router)
diff --git a/packages/meshbay-hub/src/meshbay_hub/static/auth-page.js b/packages/meshbay-hub/src/meshbay_hub/static/auth-page.js
index df08bc4..4c00137 100644
--- a/packages/meshbay-hub/src/meshbay_hub/static/auth-page.js
+++ b/packages/meshbay-hub/src/meshbay_hub/static/auth-page.js
@@ -237,8 +237,11 @@ export function RegisterPage() {
         const rk = window.MeshBayKeys.generateRecoveryKey();
         // `name` (trimmed), not the raw field: the hub stores the trimmed
         // username and every key derivation must fold in the same string.
+        // `captcha.token` rides along — the submit button is already disabled
+        // until it is set when a captcha is configured (see the form below).
         await window.MeshBayKeys.registerUser(
-          name, email, password, emailRecovery ? rk.mnemonic : null);
+          name, email, password, emailRecovery ? rk.mnemonic : null,
+          captcha.token);
         setRecoveryMnemonic(rk.mnemonic);
         session.recoveryKey =
           await window.MeshBayKeys.deriveRecoveryKey(rk.mnemonic, name);
@@ -257,6 +260,10 @@ export function RegisterPage() {
       }
     } catch (err) {
       setError(err.message);
+      // A reCAPTCHA token is single-use: after a failed attempt (name taken,
+      // e-mail in use…) it is spent, so clear it and make the user solve a
+      // fresh one before the next try. No-op when no captcha is configured.
+      captcha.reset();
     } finally {
       setLoading(false);
     }
diff --git a/packages/meshbay-hub/src/meshbay_hub/static/keyderive.js b/packages/meshbay-hub/src/meshbay_hub/static/keyderive.js
index 0aaa6a5..a540a94 100644
--- a/packages/meshbay-hub/src/meshbay_hub/static/keyderive.js
+++ b/packages/meshbay-hub/src/meshbay_hub/static/keyderive.js
@@ -276,7 +276,7 @@ async function decryptBundle(bundleB64, password, username) {
  *
  * Returns the raw private keys for immediate use after registration.
  */
-async function registerUser(username, email, password, recoveryMnemonic) {
+async function registerUser(username, email, password, recoveryMnemonic, captchaToken) {
   // No keypair here any more. Identity keys are per node: one is generated the
   // first time this account joins a given node, encrypted under the passphrase,
   // and left with that node. So an operator who cracks what sits on their own
@@ -291,6 +291,10 @@ async function registerUser(username, email, password, recoveryMnemonic) {
   // appends it to the verification e-mail and stores it nowhere
   // (docs/auth-confirm.md §4.4). Omitted when they chose to save it themselves.
   if (recoveryMnemonic) payload.recovery_key = recoveryMnemonic;
+  // reCAPTCHA response, when the hub has a captcha configured. The widget lives
+  // in RegisterPage (auth-page.js); this function just forwards its token. A
+  // hub with no captcha configured sends nothing and the server does not check.
+  if (captchaToken) payload.captcha_token = captchaToken;
 
   const resp = await hubCall('/v1/users/register', {
     method: 'POST',