Port 8123: Home Assistant
Port 8123 is the default web interface and API port for Home Assistant.
What runs on port 8123
Home Assistant, the open-source smart-home platform, serves its dashboard, mobile-app API and integrations on TCP 8123. Remote access is commonly set up through its paid Nabu Casa cloud relay, a VPN, or a reverse proxy with HTTPS, rather than by forwarding 8123 directly.
On a home network
Raspberry Pis, mini PCs, NAS boxes and Home Assistant Green or Yellow devices running Home Assistant listen on 8123.
If port 8123 is reachable from the internet
Home Assistant requires a login and is designed to be reachable, so exposure is not reckless in itself, but it controls locks, cameras and alarms, so a breach has physical consequences. Exposed directly on plain HTTP, the password travels unencrypted. Use Nabu Casa, a VPN, or HTTPS with multi-factor login, and keep it updated.
How to check whether it is open on your network
In Home Assistant, check Settings, System, Network for its external URL, and your router for forwards to 8123. RECON's Exposure Check shows whether it answers from outside.
To find out whether port 8123 is open on something in your home, and whether the internet can reach it, the app scans your Wi-Fi from your phone and checks your public address from the outside. It says what it checked and what it could not. Get RECON for iPhone — one-time purchase, no account, nothing leaves the device.