The default behavior of Factorio is to allow anybody onto your server, this also applies if a player knows or guesses your server password. It is advisable to whitelist which players can connect to your server to protect from griefers.

Note: If no players are present in your whitelist, anyone can join your server. At least one player must be present for the whitelist to be enforced.

Using ServerFlex

  1. Go to your server on the Control Panel.
  2. Click settings in the server navigation bar
  3. Click whitelist on the left navigation bar
  4. Enter the username into the input box
  5. Click Add Player and you're done.

If at any point you wish to open your server publicly simply remove all players from the whitelist and restart your server.