Strip "join_authorised_via_users_server" from join events which do not need it. (#10933)

This fixes a "Event not signed by authorising server" error when
transition room member from join -> join, e.g. when updating a
display name or avatar URL for restricted rooms.

1 files changed, 9 insertions, 1 deletions

diff --git a/synapse/handlers/room_member.py b/synapse/handlers/room_member.py
index 02103f6c9a..29b3e41cc9 100644
--- a/synapse/handlers/room_member.py
+++ b/synapse/handlers/room_member.py
@@ -573,6 +573,14 @@ class RoomMemberHandler(metaclass=abc.ABCMeta):
                 errcode=Codes.BAD_JSON,
             )
 
+        # The event content should *not* include the authorising user as
+        # it won't be properly signed. Strip it out since it might come
+        # back from a client updating a display name / avatar.
+        #
+        # This only applies to restricted rooms, but there should be no reason
+        # for a client to include it. Unconditionally remove it.
+        content.pop(EventContentFields.AUTHORISING_USER, None)
+
         effective_membership_state = action
         if action in ["kick", "unban"]:
             effective_membership_state = "leave"
@@ -939,7 +947,7 @@ class RoomMemberHandler(metaclass=abc.ABCMeta):
         # be included in the event content in order to efficiently validate
         # the event.
         content[
-            "join_authorised_via_users_server"
+            EventContentFields.AUTHORISING_USER
         ] = await self.event_auth_handler.get_user_which_could_invite(
             room_id,
             current_state_ids,