diff --git a/docs/source/getting-started/authenticators-users-basics.md b/docs/source/getting-started/authenticators-users-basics.md
index 903ea9e5..ad075ba0 100644
--- a/docs/source/getting-started/authenticators-users-basics.md
+++ b/docs/source/getting-started/authenticators-users-basics.md
@@ -16,6 +16,10 @@ c.Authenticator.allowed_users = {'mal', 'zoe', 'inara', 'kaylee'}
 Users in the `allowed_users` set are added to the Hub database when the Hub is
 started.
 
+```{warning}
+If this configuration value is not set, then **all users will be allowed into your hub**.
+```
+
 ## Configure admins (`admin_users`)
 
 ```{note}