diff --git a/channels/security/websocket.py b/channels/security/websocket.py
index 15bc4fa96..0d14b4b9c 100644
--- a/channels/security/websocket.py
+++ b/channels/security/websocket.py
@@ -140,7 +140,7 @@ def AllowedHostsOriginValidator(application):
     """
     allowed_hosts = settings.ALLOWED_HOSTS
     if settings.DEBUG and not allowed_hosts:
-        allowed_hosts = ["localhost", "127.0.0.1", "[::1]"]
+        allowed_hosts = [".localhost", "127.0.0.1", "[::1]"]
     return OriginValidator(application, allowed_hosts)
 
 
diff --git a/tests/security/test_websocket.py b/tests/security/test_websocket.py
index 1444ea829..93b1eab18 100644
--- a/tests/security/test_websocket.py
+++ b/tests/security/test_websocket.py
@@ -1,10 +1,42 @@
 import pytest
+from django.test import override_settings
 
 from channels.generic.websocket import AsyncWebsocketConsumer
-from channels.security.websocket import OriginValidator
+from channels.security.websocket import (
+    AllowedHostsOriginValidator,
+    OriginValidator,
+)
 from channels.testing import WebsocketCommunicator
 
 
+@pytest.mark.django_db(transaction=True)
+@pytest.mark.asyncio
+async def test_allowed_hosts_origin_validator():
+    """
+    Tests that AllowedHostsOriginValidator correctly allows/denies connections.
+    """
+    with override_settings(
+        DEBUG=True,
+        ALLOWED_HOSTS=[],
+    ):
+        # Make our test application
+        application = AllowedHostsOriginValidator(AsyncWebsocketConsumer())
+        # Test a subdomain of localhost
+        communicator = WebsocketCommunicator(
+            application, "/", headers=[(b"origin", b"http://subdomain.localhost:8000")]
+        )
+        connected, _ = await communicator.connect()
+        assert connected
+        await communicator.disconnect()
+        # Test a bad connection
+        communicator = WebsocketCommunicator(
+            application, "/", headers=[(b"origin", b"http://bad-domain.com")]
+        )
+        connected, _ = await communicator.connect()
+        assert not connected
+        await communicator.disconnect()
+
+
 @pytest.mark.django_db(transaction=True)
 @pytest.mark.asyncio
 async def test_origin_validator():