Check that the canonical room alias actually points to the room

synapse/handlers/_base.py

@@ -18,7 +18,7 @@ from twisted.internet import defer
 from synapse.api.errors import LimitExceededError, SynapseError
 from synapse.crypto.event_signing import add_hashes_and_signatures
 from synapse.api.constants import Membership, EventTypes
-from synapse.types import UserID
+from synapse.types import UserID, RoomAlias
 
 from synapse.util.logcontext import PreserveLoggingContext
 
@@ -130,6 +130,22 @@ class BaseHandler(object):
                         returned_invite.signatures
                     )
 
+        if event.type == EventTypes.CanonicalAlias:
+            # Check the alias is acually valid (at this time at least)
+            room_alias_str = event.content.get("alias", None)
+            if room_alias_str:
+                room_alias = RoomAlias.from_string(room_alias_str)
+                directory_handler = self.hs.get_handlers().directory_handler
+                mapping = yield directory_handler.get_association(room_alias)
+
+                if mapping["room_id"] != event.room_id:
+                    raise SynapseError(
+                        400,
+                        "Room alias %s does not point to the room" % (
+                            room_alias_str,
+                        )
+                    )
+
         destinations = set(extra_destinations)
         for k, s in context.current_state.items():
             try: