The Rogue Connections
By default, macOS is designed to keep you connected to the internet as seamlessly as possible. When you walk into a coffee shop, an airport, or a hotel where you have previously connected to the Wi-Fi, your Mac will automatically rejoin that network the moment it comes into range. While convenient, this is a significant security risk if the network is open and unencrypted. Furthermore, it can be highly annoying if you are tethered to your iPhone’s secure cellular connection and your Mac decides to automatically drop that tether to join a weak, barely-functional public hotspot simply because it recognizes the SSID.
Requiring Permission to Join
To maintain control over your connectivity and stop macOS from automatically joining known Wi-Fi networks without your explicit consent, you must toggle the “Ask to join” settings.
Click the Apple logo in the top-left corner and select System Settings. In the left-hand navigation sidebar, click on Wi-Fi. Scroll down to the bottom of the right-hand panel and click the Advanced… button. This opens your list of Known Networks. First, locate the toggle switch at the top labelled Ask to join networks and ensure it is turned On. This forces your Mac to prompt you before connecting to any network it isn’t currently using. Secondly, scroll through the “Known Networks” list, find any specific public hotspots you want to restrict, click the More (three dots) icon next to them, and uncheck Auto-Join to ensure they are ignored until manually clicked.