diff --git a/changelog.d/12743.bugfix b/changelog.d/12743.bugfix
new file mode 100644
index 000000000000..1227e583ca16
--- /dev/null
+++ b/changelog.d/12743.bugfix
@@ -0,0 +1,2 @@
+Fix a long-standing bug wherein remote display names or avatar URLs containing null bytes caused updating the user directory to fail resulting in high CPU on the main process. Contributed 
+by Nick @ Beeper.
diff --git a/synapse/handlers/user_directory.py b/synapse/handlers/user_directory.py
index 74f7fdfe6ce5..ea03ac959722 100644
--- a/synapse/handlers/user_directory.py
+++ b/synapse/handlers/user_directory.py
@@ -464,6 +464,7 @@ async def _handle_possible_remote_profile_change(
 
         prev_name = prev_event.content.get("displayname")
         new_name = event.content.get("displayname")
+
         # If the new name is an unexpected form, do not update the directory.
         if not isinstance(new_name, str):
             new_name = prev_name
diff --git a/synapse/storage/databases/main/events.py b/synapse/storage/databases/main/events.py
index 42d484dc98d9..c2ea2d199397 100644
--- a/synapse/storage/databases/main/events.py
+++ b/synapse/storage/databases/main/events.py
@@ -47,6 +47,7 @@
 from synapse.storage.databases.main.events_worker import EventCacheEntry
 from synapse.storage.databases.main.search import SearchEntry
 from synapse.storage.engines.postgres import PostgresEngine
+from synapse.storage.util import non_null_str_or_none
 from synapse.storage.util.id_generators import AbstractStreamIdGenerator
 from synapse.storage.util.sequence import SequenceGenerator
 from synapse.types import JsonDict, StateMap, get_domain_from_id
@@ -1728,9 +1729,6 @@ def _store_room_members_txn(
                 not affect the current local state.
         """
 
-        def non_null_str_or_none(val: Any) -> Optional[str]:
-            return val if isinstance(val, str) and "\u0000" not in val else None
-
         self.db_pool.simple_insert_many_txn(
             txn,
             table="room_memberships",
diff --git a/synapse/storage/databases/main/user_directory.py b/synapse/storage/databases/main/user_directory.py
index df772d472102..59b86e6cd457 100644
--- a/synapse/storage/databases/main/user_directory.py
+++ b/synapse/storage/databases/main/user_directory.py
@@ -42,6 +42,7 @@
 from synapse.storage.databases.main.state import StateFilter
 from synapse.storage.databases.main.state_deltas import StateDeltasStore
 from synapse.storage.engines import PostgresEngine, Sqlite3Engine
+from synapse.storage.util import non_null_str_or_none
 from synapse.types import (
     JsonDict,
     UserProfile,
@@ -470,10 +471,8 @@ async def update_profile_in_user_dir(
         Update or add a user's profile in the user directory.
         """
         # If the display name or avatar URL are unexpected types, overwrite them.
-        if not isinstance(display_name, str):
-            display_name = None
-        if not isinstance(avatar_url, str):
-            avatar_url = None
+        display_name = non_null_str_or_none(display_name)
+        avatar_url = non_null_str_or_none(avatar_url)
 
         def _update_profile_in_user_dir_txn(txn: LoggingTransaction) -> None:
             self.db_pool.simple_upsert_txn(
diff --git a/synapse/storage/util/__init__.py b/synapse/storage/util/__init__.py
index 5e83dba2ed6f..04d5b42911b6 100644
--- a/synapse/storage/util/__init__.py
+++ b/synapse/storage/util/__init__.py
@@ -11,3 +11,9 @@
 # WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 # See the License for the specific language governing permissions and
 # limitations under the License.
+
+from typing import Any, Optional
+
+
+def non_null_str_or_none(val: Any) -> Optional[str]:
+    return val if isinstance(val, str) and "\u0000" not in val else None