fix: bound pending admin challenges and sign every value an op acts on

Any member could make a node hold unbounded challenge requests; a
connection now keeps at most 8, 64 KiB each. root_add, group_attach,
invite_create and tmdb_config signed less than they did; their subjects
are now canonical JSON of every value (the TMDB token by SHA-256).
MNP 5.0, floor kept at 4.0.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

8 files changed, 276 insertions, 94 deletions

diff --git a/packages/meshbay-node/src/meshbay_node/transport/webrtc/admin.py b/packages/meshbay-node/src/meshbay_node/transport/webrtc/admin.py
index f42d8e8..daaee62 100644
--- a/packages/meshbay-node/src/meshbay_node/transport/webrtc/admin.py
+++ b/packages/meshbay-node/src/meshbay_node/transport/webrtc/admin.py
@@ -5,6 +5,7 @@ import base64
 import os
 import time
 
+import msgpack
 from cryptography.hazmat.primitives.asymmetric.ed25519 import Ed25519PublicKey
 from meshbay_common import MNP_VERSION
 from meshbay_common.adminop import (
@@ -42,6 +43,16 @@ from meshbay_common.adminop import (
 from meshbay_common.crypto import pk_to_b64
 from meshbay_common.protocol import MNP
 
+# What one connection may have waiting for a signature. Anyone authenticated can
+# ask for a challenge — the signature is what is checked, and it comes later — so
+# without a bound a member who never answers makes the node keep every request,
+# payload and all, for the life of the connection (§13.5b). A person signs one
+# operation at a time; a handful covers a settings page saving several at once.
+MAX_PENDING_ADMIN_OPS = 8
+# The subject and payload of one pending operation, packed. A path is at most a
+# few KiB, and the largest field a legitimate request carries is a directory list.
+MAX_ADMIN_OP_BYTES = 64 * 1024
+
 # Which executor runs each signed operation once its signature has been
 # checked. Every one runs as a task of the session.
 _ADMIN_EXECUTORS = {
@@ -98,8 +109,22 @@ class AdminMixin:
         (e.g. root management from a NodePage connection).
         """
         gid = group_id if group_id is not None else (self._group_id or "")
+        now = time.time()
+        for op_id, pending in list(self._admin_ops.items()):
+            if now - pending["ts"] > ADMIN_CHALLENGE_TTL:
+                del self._admin_ops[op_id]
+        if len(self._admin_ops) >= MAX_PENDING_ADMIN_OPS:
+            self._send({"type": "error", "detail": "Too many operations waiting for a "
+                        "signature", "code": "too_many_pending"})
+            self._audit("admin_pending_flood", op)
+            return
+        if len(msgpack.packb([subject, payload or {}], use_bin_type=True)) \
+                > MAX_ADMIN_OP_BYTES:
+            self._send({"type": "error", "detail": "Request too large",
+                        "code": "too_large"})
+            return
         nonce = os.urandom(32)
-        ts = int(time.time())
+        ts = int(now)
         op_id = base64.b64encode(os.urandom(16)).decode()
         self._admin_ops[op_id] = {
             "op": op, "subject": subject, "nonce": nonce, "ts": ts,
diff --git a/packages/meshbay-node/src/meshbay_node/transport/webrtc/admission.py b/packages/meshbay-node/src/meshbay_node/transport/webrtc/admission.py
index b02e59a..e678a13 100644
--- a/packages/meshbay-node/src/meshbay_node/transport/webrtc/admission.py
+++ b/packages/meshbay-node/src/meshbay_node/transport/webrtc/admission.py
@@ -8,7 +8,12 @@ import time
 
 from cryptography.hazmat.primitives.asymmetric.ed25519 import Ed25519PublicKey
 from meshbay_common import MNP_VERSION
-from meshbay_common.adminop import OP_INVITE_CANCEL, OP_INVITE_CREATE, OP_INVITE_LINK_CREATE
+from meshbay_common.adminop import (
+    OP_INVITE_CANCEL,
+    OP_INVITE_CREATE,
+    OP_INVITE_LINK_CREATE,
+    invite_create_subject,
+)
 from meshbay_common.crypto import wrap_gek_aes
 from meshbay_common.device import (
     DEVICE_TTL,
@@ -71,11 +76,13 @@ class AdmissionMixin:
             })
             return
 
-        self._issue_admin_challenge(OP_INVITE_CREATE, invitee_id, {
-            "group_id": group_id,
-            "user_id": invitee_id,
-            "username": str(msg.get("username", ""))[:64],
-        })
+        username = str(msg.get("username", ""))[:64]
+        self._issue_admin_challenge(
+            OP_INVITE_CREATE, invite_create_subject(invitee_id, username), {
+                "group_id": group_id,
+                "user_id": invitee_id,
+                "username": username,
+            })
 
     def _do_invite_link_create(self, msg: dict) -> None:
         """
diff --git a/packages/meshbay-node/src/meshbay_node/transport/webrtc/apps/video_meta.py b/packages/meshbay-node/src/meshbay_node/transport/webrtc/apps/video_meta.py
index 834bac4..a9b35dc 100644
--- a/packages/meshbay-node/src/meshbay_node/transport/webrtc/apps/video_meta.py
+++ b/packages/meshbay-node/src/meshbay_node/transport/webrtc/apps/video_meta.py
@@ -11,6 +11,7 @@ from meshbay_common.adminop import (
     OP_TMDB_ENABLED,
     OP_TMDB_OVERRIDE,
     OP_TMDB_REMATCH,
+    tmdb_config_subject,
 )
 from meshbay_common.protocol import MNP
 
@@ -60,12 +61,12 @@ class VideoMetaMixin:
         if not self._has_admin_authority():
             self._send({"type": "error", "detail": "No authorized key for this"})
             return
-        # The subject is the signed, audited, human-shown string — it must
-        # never contain the token itself (it would end up in the audit log
-        # in plaintext). The actual token travels only in `payload`, which
-        # is node-side context, never re-sent or re-verified from the wire.
-        # The language is not a secret, so it travels in the subject itself.
-        subject = f"custom_token={'yes' if token else 'no'},language={language or 'default'}"
+        # The subject is the signed, audited string, so it must never contain
+        # the token itself (it would end up in the audit log in plaintext); it
+        # carries the token's SHA-256 instead, which binds the signature to this
+        # token without writing it down. `None` (unchanged) and `""` (clear)
+        # stay distinct, for the token and the language alike.
+        subject = tmdb_config_subject(token, language)
         self._issue_admin_challenge(
             OP_TMDB_CONFIG, subject,
             payload={"token": token, "language": language},
diff --git a/packages/meshbay-node/src/meshbay_node/transport/webrtc/node_ops.py b/packages/meshbay-node/src/meshbay_node/transport/webrtc/node_ops.py
index 9d58d8c..f7bbbfa 100644
--- a/packages/meshbay-node/src/meshbay_node/transport/webrtc/node_ops.py
+++ b/packages/meshbay-node/src/meshbay_node/transport/webrtc/node_ops.py
@@ -14,6 +14,8 @@ from meshbay_common.adminop import (
     OP_ROOT_UPDATE,
     OP_SET_SCAN_SETTINGS,
     OP_TRANSFER_LIMITS,
+    group_attach_subject,
+    root_add_subject,
 )
 from meshbay_common.protocol import MNP
 
@@ -246,10 +248,11 @@ class NodeOpsMixin:
         # is ignored rather than obeyed: on load it forces every other root
         # read-only, which is the model the RO/RW one replaced. A second
         # writable directory is `root_add` with `writable`.
+        writable = bool(msg.get("writable", True))
+        # The directory being exposed is signed, not only the group's name.
         self._issue_admin_challenge(
-            OP_GROUP_ATTACH, name,
-            payload={"name": name, "shared_dir": shared_dir,
-                     "writable": bool(msg.get("writable", True))},
+            OP_GROUP_ATTACH, group_attach_subject(name, shared_dir, writable),
+            payload={"name": name, "shared_dir": shared_dir, "writable": writable},
             group_id="")
 
     async def _admin_exec_group_attach(
@@ -342,16 +345,20 @@ class NodeOpsMixin:
         if not self._has_admin_authority():
             self._send({"type": "error", "detail": "No authorized key for this"})
             return
+        payload = {
+            "group_id": target_group, "path": path,
+            "name": str(msg.get("name", ""))[:128],
+            "kind": str(msg.get("kind", "generic"))[:16],
+            "writable": bool(msg.get("writable", msg.get("upload", False))),
+            "removable": bool(msg.get("removable", False)),
+        }
+        # Everything the executor acts on is signed — `writable` decides whether
+        # every member may write there. The group is in the transcript itself.
         self._issue_admin_challenge(
-            OP_ROOT_ADD, path,
-            payload={
-                "group_id": target_group, "path": path,
-                "name": str(msg.get("name", ""))[:128],
-                "kind": str(msg.get("kind", "generic"))[:16],
-                "writable": bool(msg.get("writable", msg.get("upload", False))),
-                "removable": bool(msg.get("removable", False)),
-            },
-            group_id=target_group)
+            OP_ROOT_ADD,
+            root_add_subject(path, payload["name"], payload["kind"],
+                             payload["writable"], payload["removable"]),
+            payload=payload, group_id=target_group)
 
     async def _admin_exec_root_add(
         self, pending: dict, transcript: bytes, sig: bytes,
diff --git a/packages/meshbay-node/tests/golden/dispatch.json b/packages/meshbay-node/tests/golden/dispatch.json
index 4767996..a7bb543 100644
--- a/packages/meshbay-node/tests/golden/dispatch.json
+++ b/packages/meshbay-node/tests/golden/dispatch.json
@@ -2068,7 +2068,7 @@
      "subject": "x",
      "ts": "<volatile>",
      "type": "admin_challenge",
-     "v": "4.0"
+     "v": "5.0"
     }
    ],
    "spawned": []
@@ -2391,7 +2391,7 @@
      "subject": "gggggggggggggggggggggggggggggggg",
      "ts": "<volatile>",
      "type": "admin_challenge",
-     "v": "4.0"
+     "v": "5.0"
     }
    ],
    "spawned": []
@@ -2410,7 +2410,7 @@
      "subject": "['x']",
      "ts": "<volatile>",
      "type": "admin_challenge",
-     "v": "4.0"
+     "v": "5.0"
     }
    ],
    "spawned": []
@@ -2429,7 +2429,7 @@
      "subject": "7",
      "ts": "<volatile>",
      "type": "admin_challenge",
-     "v": "4.0"
+     "v": "5.0"
     }
    ],
    "spawned": []
@@ -2448,7 +2448,7 @@
      "subject": "x",
      "ts": "<volatile>",
      "type": "admin_challenge",
-     "v": "4.0"
+     "v": "5.0"
     }
    ],
    "spawned": []
@@ -2718,7 +2718,7 @@
      "messages": [],
      "req_id": 4242,
      "type": "chat_hist_resp",
-     "v": "4.0"
+     "v": "5.0"
     }
    ],
    "spawned": []
@@ -2732,7 +2732,7 @@
      "messages": [],
      "req_id": 4242,
      "type": "chat_hist_resp",
-     "v": "4.0"
+     "v": "5.0"
     }
    ],
    "spawned": []
@@ -2746,7 +2746,7 @@
      "messages": [],
      "req_id": 4242,
      "type": "chat_hist_resp",
-     "v": "4.0"
+     "v": "5.0"
     }
    ],
    "spawned": []
@@ -2760,7 +2760,7 @@
      "messages": [],
      "req_id": 4242,
      "type": "chat_hist_resp",
-     "v": "4.0"
+     "v": "5.0"
     }
    ],
    "spawned": []
@@ -2774,7 +2774,7 @@
      "messages": [],
      "req_id": 4242,
      "type": "chat_hist_resp",
-     "v": "4.0"
+     "v": "5.0"
     }
    ],
    "spawned": []
@@ -2788,7 +2788,7 @@
      "messages": [],
      "req_id": 4242,
      "type": "chat_hist_resp",
-     "v": "4.0"
+     "v": "5.0"
     }
    ],
    "spawned": []
@@ -2802,7 +2802,7 @@
      "messages": [],
      "req_id": 4242,
      "type": "chat_hist_resp",
-     "v": "4.0"
+     "v": "5.0"
     }
    ],
    "spawned": []
@@ -2816,7 +2816,7 @@
      "messages": [],
      "req_id": 4242,
      "type": "chat_hist_resp",
-     "v": "4.0"
+     "v": "5.0"
     }
    ],
    "spawned": []
@@ -8535,7 +8535,7 @@
      "subject": "gggggggggggggggggggggggggggggggg",
      "ts": "<volatile>",
      "type": "admin_challenge",
-     "v": "4.0"
+     "v": "5.0"
     }
    ],
    "spawned": []
@@ -8554,7 +8554,7 @@
      "subject": "['x']",
      "ts": "<volatile>",
      "type": "admin_challenge",
-     "v": "4.0"
+     "v": "5.0"
     }
    ],
    "spawned": []
@@ -8573,7 +8573,7 @@
      "subject": "7",
      "ts": "<volatile>",
      "type": "admin_challenge",
-     "v": "4.0"
+     "v": "5.0"
     }
    ],
    "spawned": []
@@ -8592,7 +8592,7 @@
      "subject": "x",
      "ts": "<volatile>",
      "type": "admin_challenge",
-     "v": "4.0"
+     "v": "5.0"
     }
    ],
    "spawned": []
@@ -8924,10 +8924,10 @@
      "op": "group_attach",
      "op_id": "<volatile>",
      "req_id": 4242,
-     "subject": "['x']",
+     "subject": "{\"name\":\"['x']\",\"shared_dir\":\"['x']\",\"writable\":true}",
      "ts": "<volatile>",
      "type": "admin_challenge",
-     "v": "4.0"
+     "v": "5.0"
     }
    ],
    "spawned": []
@@ -8943,10 +8943,10 @@
      "op": "group_attach",
      "op_id": "<volatile>",
      "req_id": 4242,
-     "subject": "7",
+     "subject": "{\"name\":\"7\",\"shared_dir\":\"7\",\"writable\":true}",
      "ts": "<volatile>",
      "type": "admin_challenge",
-     "v": "4.0"
+     "v": "5.0"
     }
    ],
    "spawned": []
@@ -8962,10 +8962,10 @@
      "op": "group_attach",
      "op_id": "<volatile>",
      "req_id": 4242,
-     "subject": "x",
+     "subject": "{\"name\":\"x\",\"shared_dir\":\"x\",\"writable\":true}",
      "ts": "<volatile>",
      "type": "admin_challenge",
-     "v": "4.0"
+     "v": "5.0"
     }
    ],
    "spawned": []
@@ -9300,7 +9300,7 @@
      "subject": "['x']",
      "ts": "<volatile>",
      "type": "admin_challenge",
-     "v": "4.0"
+     "v": "5.0"
     }
    ],
    "spawned": []
@@ -9319,7 +9319,7 @@
      "subject": "7",
      "ts": "<volatile>",
      "type": "admin_challenge",
-     "v": "4.0"
+     "v": "5.0"
     }
    ],
    "spawned": []
@@ -9338,7 +9338,7 @@
      "subject": "x",
      "ts": "<volatile>",
      "type": "admin_challenge",
-     "v": "4.0"
+     "v": "5.0"
     }
    ],
    "spawned": []
@@ -11641,7 +11641,7 @@
      "subject": "link:gggggggggggggggggggggggggggggggg",
      "ts": "<volatile>",
      "type": "admin_challenge",
-     "v": "4.0"
+     "v": "5.0"
     }
    ],
    "spawned": []
@@ -13740,7 +13740,7 @@
      "subject": "['x']",
      "ts": "<volatile>",
      "type": "admin_challenge",
-     "v": "4.0"
+     "v": "5.0"
     }
    ],
    "spawned": []
@@ -13759,7 +13759,7 @@
      "subject": "7",
      "ts": "<volatile>",
      "type": "admin_challenge",
-     "v": "4.0"
+     "v": "5.0"
     }
    ],
    "spawned": []
@@ -13778,7 +13778,7 @@
      "subject": "x",
      "ts": "<volatile>",
      "type": "admin_challenge",
-     "v": "4.0"
+     "v": "5.0"
     }
    ],
    "spawned": []
@@ -14113,7 +14113,7 @@
      "subject": "['x']",
      "ts": "<volatile>",
      "type": "admin_challenge",
-     "v": "4.0"
+     "v": "5.0"
     }
    ],
    "spawned": []
@@ -14132,7 +14132,7 @@
      "subject": "7",
      "ts": "<volatile>",
      "type": "admin_challenge",
-     "v": "4.0"
+     "v": "5.0"
     }
    ],
    "spawned": []
@@ -14151,7 +14151,7 @@
      "subject": "x",
      "ts": "<volatile>",
      "type": "admin_challenge",
-     "v": "4.0"
+     "v": "5.0"
     }
    ],
    "spawned": []
@@ -16212,7 +16212,7 @@
      "req_id": 4242,
      "token": null,
      "type": "pong",
-     "v": "4.0"
+     "v": "5.0"
     }
    ],
    "spawned": []
@@ -16227,7 +16227,7 @@
       "x"
      ],
      "type": "pong",
-     "v": "4.0"
+     "v": "5.0"
     }
    ],
    "spawned": []
@@ -16240,7 +16240,7 @@
      "req_id": 4242,
      "token": 7,
      "type": "pong",
-     "v": "4.0"
+     "v": "5.0"
     }
    ],
    "spawned": []
@@ -16253,7 +16253,7 @@
      "req_id": 4242,
      "token": "x",
      "type": "pong",
-     "v": "4.0"
+     "v": "5.0"
     }
    ],
    "spawned": []
@@ -16266,7 +16266,7 @@
      "req_id": 4242,
      "token": null,
      "type": "pong",
-     "v": "4.0"
+     "v": "5.0"
     }
    ],
    "spawned": []
@@ -16281,7 +16281,7 @@
       "x"
      ],
      "type": "pong",
-     "v": "4.0"
+     "v": "5.0"
     }
    ],
    "spawned": []
@@ -16294,7 +16294,7 @@
      "req_id": 4242,
      "token": 7,
      "type": "pong",
-     "v": "4.0"
+     "v": "5.0"
     }
    ],
    "spawned": []
@@ -16307,7 +16307,7 @@
      "req_id": 4242,
      "token": "x",
      "type": "pong",
-     "v": "4.0"
+     "v": "5.0"
     }
    ],
    "spawned": []
@@ -16639,10 +16639,10 @@
      "op": "root_add",
      "op_id": "<volatile>",
      "req_id": 4242,
-     "subject": "['x']",
+     "subject": "{\"kind\":\"['x']\",\"name\":\"['x']\",\"path\":\"['x']\",\"removable\":true,\"writable\":true}",
      "ts": "<volatile>",
      "type": "admin_challenge",
-     "v": "4.0"
+     "v": "5.0"
     }
    ],
    "spawned": []
@@ -16658,10 +16658,10 @@
      "op": "root_add",
      "op_id": "<volatile>",
      "req_id": 4242,
-     "subject": "7",
+     "subject": "{\"kind\":\"7\",\"name\":\"7\",\"path\":\"7\",\"removable\":true,\"writable\":true}",
      "ts": "<volatile>",
      "type": "admin_challenge",
-     "v": "4.0"
+     "v": "5.0"
     }
    ],
    "spawned": []
@@ -16677,10 +16677,10 @@
      "op": "root_add",
      "op_id": "<volatile>",
      "req_id": 4242,
-     "subject": "x",
+     "subject": "{\"kind\":\"x\",\"name\":\"x\",\"path\":\"x\",\"removable\":true,\"writable\":true}",
      "ts": "<volatile>",
      "type": "admin_challenge",
-     "v": "4.0"
+     "v": "5.0"
     }
    ],
    "spawned": []
@@ -17015,7 +17015,7 @@
      "subject": "['x']",
      "ts": "<volatile>",
      "type": "admin_challenge",
-     "v": "4.0"
+     "v": "5.0"
     }
    ],
    "spawned": []
@@ -17034,7 +17034,7 @@
      "subject": "7",
      "ts": "<volatile>",
      "type": "admin_challenge",
-     "v": "4.0"
+     "v": "5.0"
     }
    ],
    "spawned": []
@@ -17053,7 +17053,7 @@
      "subject": "x",
      "ts": "<volatile>",
      "type": "admin_challenge",
-     "v": "4.0"
+     "v": "5.0"
     }
    ],
    "spawned": []
@@ -17388,7 +17388,7 @@
      "subject": "['x']",
      "ts": "<volatile>",
      "type": "admin_challenge",
-     "v": "4.0"
+     "v": "5.0"
     }
    ],
    "spawned": []
@@ -17407,7 +17407,7 @@
      "subject": "7",
      "ts": "<volatile>",
      "type": "admin_challenge",
-     "v": "4.0"
+     "v": "5.0"
     }
    ],
    "spawned": []
@@ -17426,7 +17426,7 @@
      "subject": "x",
      "ts": "<volatile>",
      "type": "admin_challenge",
-     "v": "4.0"
+     "v": "5.0"
     }
    ],
    "spawned": []
@@ -17761,7 +17761,7 @@
      "subject": "['x']",
      "ts": "<volatile>",
      "type": "admin_challenge",
-     "v": "4.0"
+     "v": "5.0"
     }
    ],
    "spawned": []
@@ -17780,7 +17780,7 @@
      "subject": "7",
      "ts": "<volatile>",
      "type": "admin_challenge",
-     "v": "4.0"
+     "v": "5.0"
     }
    ],
    "spawned": []
@@ -17799,7 +17799,7 @@
      "subject": "x",
      "ts": "<volatile>",
      "type": "admin_challenge",
-     "v": "4.0"
+     "v": "5.0"
     }
    ],
    "spawned": []
@@ -18134,7 +18134,7 @@
      "subject": "['x']:rw=on,rem=on",
      "ts": "<volatile>",
      "type": "admin_challenge",
-     "v": "4.0"
+     "v": "5.0"
     }
    ],
    "spawned": []
@@ -18153,7 +18153,7 @@
      "subject": "7:rw=on,rem=on",
      "ts": "<volatile>",
      "type": "admin_challenge",
-     "v": "4.0"
+     "v": "5.0"
     }
    ],
    "spawned": []
@@ -18172,7 +18172,7 @@
      "subject": "x:rw=on,rem=on",
      "ts": "<volatile>",
      "type": "admin_challenge",
-     "v": "4.0"
+     "v": "5.0"
     }
    ],
    "spawned": []
@@ -21116,10 +21116,10 @@
      "op": "tmdb_config",
      "op_id": "<volatile>",
      "req_id": 4242,
-     "subject": "custom_token=no,language=default",
+     "subject": "{\"language\":null,\"token\":null}",
      "ts": "<volatile>",
      "type": "admin_challenge",
-     "v": "4.0"
+     "v": "5.0"
     }
    ],
    "spawned": []
@@ -21159,10 +21159,10 @@
      "op": "tmdb_config",
      "op_id": "<volatile>",
      "req_id": 4242,
-     "subject": "custom_token=yes,language=x",
+     "subject": "{\"language\":\"x\",\"token\":\"sha256:2d711642b726b04401627ca9fbac32f5c8530fb1903cc4db02258717921a4881\"}",
      "ts": "<volatile>",
      "type": "admin_challenge",
-     "v": "4.0"
+     "v": "5.0"
     }
    ],
    "spawned": []
@@ -23049,7 +23049,7 @@
      "subject": "d=7,u=7",
      "ts": "<volatile>",
      "type": "admin_challenge",
-     "v": "4.0"
+     "v": "5.0"
     }
    ],
    "spawned": []
diff --git a/packages/meshbay-node/tests/test_admin_challenge_bounds.py b/packages/meshbay-node/tests/test_admin_challenge_bounds.py
new file mode 100644
index 0000000..fd3b2f1
--- /dev/null
+++ b/packages/meshbay-node/tests/test_admin_challenge_bounds.py
@@ -0,0 +1,135 @@
+"""
+What a connection may leave waiting for a signature (docs/MESHBAY_DESIGN.md §13.5b).
+
+Anyone authenticated can ask for an admin challenge — the signature is checked
+later — so a member who never answers must not make the node keep every request.
+Measured before the bound: 200 `root_add` of 1 MiB each from a plain member held
+200 pending operations and ~400 MiB for the life of the connection.
+"""
+
+import struct
+import time
+
+import msgpack
+from cryptography.hazmat.primitives.asymmetric.ed25519 import Ed25519PrivateKey
+from meshbay_node.transport.webrtc.admin import MAX_ADMIN_OP_BYTES, MAX_PENDING_ADMIN_OPS
+from meshbay_node.transport.webrtc_server import WebRTCPeerSession
+
+GROUP = "g" * 32
+
+
+class _Channel:
+    readyState = "open"
+
+    def __init__(self):
+        self.sent = []
+
+    def send(self, data: bytes) -> None:
+        (n,) = struct.unpack(">I", data[:4])
+        self.sent.append(msgpack.unpackb(data[4:4 + n], raw=False))
+
+
+class _PC:
+    connectionState = "connected"
+    iceConnectionState = "connected"
+    remoteDescription = None
+    localDescription = None
+    sctp = None
+
+
+def _member_session():
+    """An authenticated member — not the operator — on a node that has one."""
+    ctx = {"sk_node": Ed25519PrivateKey.from_private_bytes(b"\x01" * 32),
+           "groups": {GROUP: {}}, "has_admin_authority": True}
+    s = WebRTCPeerSession(_PC(), ctx, peer_id="peer")
+    s._channel = _Channel()
+    s._audit = lambda *a, **k: None
+    s._user_id, s._group_id = "member-1", GROUP
+    return s
+
+
+def _root_add(s, path: str) -> dict:
+    s._dispatch_message({"type": "root_add", "group_id": GROUP, "path": path})
+    return s._channel.sent[-1]
+
+
+def test_a_member_cannot_pile_up_challenges():
+    s = _member_session()
+    for i in range(MAX_PENDING_ADMIN_OPS):
+        assert _root_add(s, f"/srv/{i}")["type"] == "admin_challenge"
+    refused = _root_add(s, "/srv/one-too-many")
+    assert refused["type"] == "error" and refused["code"] == "too_many_pending"
+    assert len(s._admin_ops) == MAX_PENDING_ADMIN_OPS
+
+
+def test_an_oversized_request_is_not_kept():
+    s = _member_session()
+    refused = _root_add(s, "x" * (MAX_ADMIN_OP_BYTES + 1))
+    assert refused["type"] == "error" and refused["code"] == "too_large"
+    assert s._admin_ops == {}
+
+
+def test_an_expired_challenge_frees_its_place():
+    s = _member_session()
+    for i in range(MAX_PENDING_ADMIN_OPS):
+        _root_add(s, f"/srv/{i}")
+    for pending in s._admin_ops.values():
+        pending["ts"] -= 10_000
+    assert _root_add(s, "/srv/after-expiry")["type"] == "admin_challenge"
+    assert len(s._admin_ops) == 1
+
+
+def test_answering_a_challenge_frees_its_place():
+    s = _member_session()
+    for i in range(MAX_PENDING_ADMIN_OPS):
+        _root_add(s, f"/srv/{i}")
+    op_id = next(iter(s._admin_ops))
+    s._dispatch_message({"type": "admin_response", "op_id": op_id, "signature": "!!"})
+    assert len(s._admin_ops) == MAX_PENDING_ADMIN_OPS - 1
+    assert _root_add(s, "/srv/next")["type"] == "admin_challenge"
+    assert all(time.time() - p["ts"] < 5 for p in s._admin_ops.values())
+
+
+# ── What a challenge covers (docs/MESHBAY_DESIGN.md §5.4) ────────────────────
+#
+# The signature covers the subject and nothing else of a request, so every value
+# the executor acts on has to be in it.
+
+def test_root_add_signs_whether_members_may_write():
+    from meshbay_common.adminop import root_add_subject
+    s = _member_session()
+    s._dispatch_message({"type": "root_add", "group_id": GROUP, "path": "/srv/drop",
+                         "name": "Drop", "writable": True, "removable": False})
+    challenge = s._channel.sent[-1]
+    assert challenge["subject"] == root_add_subject("/srv/drop", "Drop", "generic",
+                                                    True, False)
+    assert challenge["subject"] != root_add_subject("/srv/drop", "Drop", "generic",
+                                                    False, False)
+
+
+def test_group_attach_signs_the_directory_it_exposes():
+    from meshbay_common.adminop import group_attach_subject
+    s = _member_session()
+    s._dispatch_message({"type": "group_attach", "name": "photos",
+                         "shared_dir": "/home/me/Photos"})
+    assert s._channel.sent[-1]["subject"] == group_attach_subject(
+        "photos", "/home/me/Photos", True)
+
+
+def test_invite_create_signs_the_name_it_records():
+    from meshbay_common.adminop import invite_create_subject
+    s = _member_session()
+    s._ctx["roster"] = object()     # only its presence is checked before the challenge
+    s._dispatch_message({"type": "invite_create", "group_id": GROUP,
+                         "user_id": "u-1", "username": "alice"})
+    assert s._channel.sent[-1]["subject"] == invite_create_subject("u-1", "alice")
+
+
+def test_tmdb_config_signs_the_token_without_writing_it():
+    from meshbay_common.adminop import tmdb_config_subject
+    s = _member_session()
+    s._dispatch_message({"type": "tmdb_config", "token": "secret-token",
+                         "language": "fr-FR"})
+    subject = s._channel.sent[-1]["subject"]
+    assert subject == tmdb_config_subject("secret-token", "fr-FR")
+    assert "secret-token" not in subject
diff --git a/packages/meshbay-node/tests/test_tmdb_config_policy.py b/packages/meshbay-node/tests/test_tmdb_config_policy.py
index 6a51eb0..c671ac8 100644
--- a/packages/meshbay-node/tests/test_tmdb_config_policy.py
+++ b/packages/meshbay-node/tests/test_tmdb_config_policy.py
@@ -22,7 +22,7 @@ from pathlib import Path
 
 import pytest
 from cryptography.hazmat.primitives.asymmetric.ed25519 import Ed25519PrivateKey
-from meshbay_common.adminop import OP_TMDB_CONFIG
+from meshbay_common.adminop import OP_TMDB_CONFIG, tmdb_config_subject
 from meshbay_node.indexer.group_index import GroupIndex
 from meshbay_node.roster import Roster
 from meshbay_node.transport.webrtc_server import WebRTCPeerSession
@@ -134,7 +134,9 @@ async def test_subject_reflects_whether_a_token_was_supplied(tmp_path):
     session._do_tmdb_config({"token": "x"})
 
     _, subject, _, _ = issued[0]
-    assert subject == "custom_token=yes,language=default"
+    # The token is bound by its digest and never written into the subject.
+    assert subject == tmdb_config_subject("x", None)
+    assert "sha256:" in subject and tmdb_config_subject("y", None) != subject
 
 
 async def test_subject_says_no_custom_token_when_none_given(tmp_path):
@@ -146,7 +148,10 @@ async def test_subject_says_no_custom_token_when_none_given(tmp_path):
     session._do_tmdb_config({})
 
     _, subject, _, _ = issued[0]
-    assert subject == "custom_token=no,language=default"
+    # Nothing given means both unchanged — distinct from clearing either.
+    assert subject == tmdb_config_subject(None, None)
+    assert subject != tmdb_config_subject("", None)
+    assert subject != tmdb_config_subject(None, "")
 
 
 async def test_subject_reflects_a_configured_language(tmp_path):
@@ -158,7 +163,7 @@ async def test_subject_reflects_a_configured_language(tmp_path):
     session._do_tmdb_config({"language": "fr-FR"})
 
     _, subject, payload, _ = issued[0]
-    assert subject == "custom_token=no,language=fr-FR"
+    assert subject == tmdb_config_subject(None, "fr-FR")
     assert payload["language"] == "fr-FR"
 
 
diff --git a/packages/meshbay-node/tests/test_webrtc_transport.py b/packages/meshbay-node/tests/test_webrtc_transport.py
index 4542817..7a6f517 100644
--- a/packages/meshbay-node/tests/test_webrtc_transport.py
+++ b/packages/meshbay-node/tests/test_webrtc_transport.py
@@ -34,6 +34,7 @@ from meshbay_common.adminop import (
     OP_INVITE_CREATE,
     OP_INVITE_LINK_CREATE,
     admin_transcript,
+    invite_create_subject,
 )
 from meshbay_common.crypto import (
     generate_gek,
@@ -1335,7 +1336,8 @@ async def test_invite_then_join_delivers_the_gek(sk_node, sk_hub, gek, shared_di
     challenge_msg = await asyncio.wait_for(q_admin.get(), timeout=5.0)
     assert challenge_msg["type"] == MNP.ADMIN_CHALLENGE
     assert challenge_msg["op"] == OP_INVITE_CREATE
-    assert challenge_msg["subject"] == "user-002"
+    # The name the invitation records is signed with the account it is for.
+    assert challenge_msg["subject"] == invite_create_subject("user-002", "bob")
 
     ch_admin.send(_pack({
         "type": MNP.ADMIN_RESPONSE, "v": MNP_VERSION,