diff --git a/changelog.d/15038.bugfix b/changelog.d/15038.bugfix
new file mode 100644
index 0000000000..4695a09756
--- /dev/null
+++ b/changelog.d/15038.bugfix
@@ -0,0 +1 @@
+Fix a long-standing bug where the room aliases returned could be corrupted.
diff --git a/synapse/handlers/directory.py b/synapse/handlers/directory.py
index 2ea52257cb..d31b0fbb17 100644
--- a/synapse/handlers/directory.py
+++ b/synapse/handlers/directory.py
@@ -485,7 +485,8 @@ class DirectoryHandler:
                 )
             )
             if canonical_alias:
-                room_aliases.append(canonical_alias)
+                # Ensure we do not mutate room_aliases.
+                room_aliases = room_aliases + [canonical_alias]
 
             if not self.config.roomdirectory.is_publishing_room_allowed(
                 user_id, room_id, room_aliases