Validate the alt_aliases property of canonical alias events (#6971)

1 files changed, 7 insertions, 7 deletions

diff --git a/synapse/handlers/directory.py b/synapse/handlers/directory.py
index 0b23ca919a..61eb49059b 100644
--- a/synapse/handlers/directory.py
+++ b/synapse/handlers/directory.py
@@ -13,8 +13,6 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
-
-import collections
 import logging
 import string
 from typing import List
@@ -307,15 +305,17 @@ class DirectoryHandler(BaseHandler):
             send_update = True
             content.pop("alias", "")
 
-        # Filter alt_aliases for the removed alias.
-        alt_aliases = content.pop("alt_aliases", None)
-        # If the aliases are not a list (or not found) do not attempt to modify
-        # the list.
-        if isinstance(alt_aliases, collections.Sequence):
+        # Filter the alt_aliases property for the removed alias. Note that the
+        # value is not modified if alt_aliases is of an unexpected form.
+        alt_aliases = content.get("alt_aliases")
+        if isinstance(alt_aliases, (list, tuple)) and alias_str in alt_aliases:
             send_update = True
             alt_aliases = [alias for alias in alt_aliases if alias != alias_str]
+
             if alt_aliases:
                 content["alt_aliases"] = alt_aliases
+            else:
+                del content["alt_aliases"]
 
         if send_update:
             yield self.event_creation_handler.create_and_send_nonmember_event(