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>

1 files changed, 7 insertions, 6 deletions

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},