Privacy and security

System Bridge has no accounts, telemetry, analytics or crash reporting, and it never sends your system data anywhere. Clients on your machine or network fetch it from System Bridge, and only with your API token. To be found on your local network, System Bridge does share your hostname and IP address there without a token (see Network discovery).

This page lists every way the app communicates, so you can decide what to allow.

Summary

Connection	Direction	Where	When
API server	Incoming	Port 9170	Always
Network discovery	Both	Port 1900, mDNS	Always
Update check	Outgoing	api.github.com	Up to 12 an hour
DNS lookups	Outgoing	Your DNS server	Every minute
Discord	Outgoing	discord.com	If set up

“Always” means whenever the backend runs. The API server and network discovery are reachable from your local network. Apart from Discord, which only runs if you set it up, none of these can currently be turned off.

Nothing else leaves your machine unless you ask for it, for example by opening a URL.

API server

The backend listens on port 9170 on every network interface, so other devices on your network can reach it. Change the port with SYSTEM_BRIDGE_PORT.

The API uses plain HTTP and WebSocket, without TLS. Anyone who can see your network traffic can read the data and your API token as they pass. Use it on networks you trust, or put it behind a TLS proxy or VPN.

What needs the API token

Your API token is a random UUID, created on first start and stored in the token file in the settings directory. Only your user can read it.

These need no token, and only show that System Bridge is running:

• /api: a fixed status message.
• /api/health: the status, current time and System Bridge version.
• The web client files. The web client asks for your token before it shows any data.

Everything else needs the token:

Endpoint	How the token is sent
/api/data/{module}	X-API-Token or token header
/api/media/file/data	token query parameter
/api/websocket	token field in every message. The connection closes if no valid message arrives within 10 seconds.
/api/mcp	token query parameter or Authorization: Bearer header
/api/quit	X-API-Token header

The WebSocket and MCP endpoints accept connections from any web origin, so the token is their only protection.

What a client with your token can do

Treat the token like a password. A client with it can:

• Read every data module. These include your hostname, IP and MAC addresses, logged-in users, running processes, what’s playing, which apps are using your camera or microphone, and your Discord voice state.
• List files and read file details anywhere your user can access, starting from your user directories and any media directories you add.
• Download media files from your user directories and media directories.
• Open any file your user can access, open URLs, send notifications, and type keys and text.
• Run commands from your command allowlist, and nothing else. The allowlist is empty by default.
• Lock, sleep, restart or shut down your machine, or log you out.
• Read and change System Bridge settings.
• Mute, deafen and change volumes in Discord, if you’ve set it up.

System Bridge only sends system data to clients that have connected with the token and asked for it.

Network discovery

System Bridge announces itself on your local network so apps like Home Assistant can find it. These messages stay on your local network and aren’t routed to the internet.

SSDP

• Every 30 seconds, System Bridge sends an SSDP NOTIFY message to the multicast address 239.255.255.250:1900. It contains your hostname, local IP address and API port.
• It answers SSDP searches for System Bridge, or for all devices, with the same details.
• It serves a device description at http://{host}:1900/description.xml, with no token needed. The description contains your hostname, local IP address and API port.

Change the SSDP port with SYSTEM_BRIDGE_SSDP_PORT.

mDNS

System Bridge advertises the _system-bridge._tcp service with your hostname, IP addresses and API port, and answers mDNS queries for it.

The system-bridge client discovery command also sends mDNS queries for _system-bridge._tcp and _http._tcp to find other System Bridge instances.

Update check

The system module reports whether a newer version is available. To find out, System Bridge requests the latest release from:

https://api.github.com/repos/timmo001/system-bridge/releases/latest

• The request is a plain GET with Go’s default User-Agent. It sends no version, identifier or system data.
• The result is cached for 5 minutes, so the request runs about 12 times an hour at most, with a hard limit of 30.
• GitHub sees your public IP address, as with any web request.
• The request uses the proxy in your HTTPS_PROXY environment variable, if set.

DNS lookups

When it updates the system module, System Bridge looks up your machine’s full domain name: it looks up your hostname, then does a reverse lookup on the addresses it gets back. These lookups go to the DNS server your system uses.

To find your main IPv4 and IPv6 addresses, System Bridge asks the operating system which local address it would use to reach 8.8.8.8 and 2001:4860:4860::8888. It does this with a UDP connect, which sends no packets, so nothing reaches those addresses.

Local-only features

These only talk to your operating system and other apps on your machine:

• Media status and control use your system’s media controls.
• Notifications use your desktop’s notification system.
• Keyboard input, power actions, and opening files and URLs go through your operating system. Opening a URL launches your default browser, which then loads the page.
• The Documentation item in the system tray opens this site in your browser.
• The CLI only connects to the backend on 127.0.0.1.

Data stored on your machine

System Bridge keeps these files in the settings directory:

File	Contents
settings.json	Your settings, including the command allowlist and media directories
token	Your API token, readable only by your user
data/{module}.json	The latest data for each module, rewritten on every update
discord.json	Your Discord application’s credentials, if you set up Discord
discord-token.json	Your Discord access token, readable only by your user

The web client stores the host, port and API token you connect with in your browser’s local storage. When you open the web client from the system tray, these are passed in the URL.

Logs

System Bridge writes one log file a day to the logs directory. Only your user can read them, and they’re deleted after 7 days.

The default WARN level only logs warnings and errors. If you raise it to INFO, logs also include:

• Your API token, logged at startup.
• The web client URL, including your token, when you open it from the system tray.
• The contents of notification, open, command, power and settings requests.

At DEBUG, logs also include the contents of other requests, such as the text in keyboard text requests.

Logs stay on your machine. Before you share them in a bug report, check them and remove your token.

Optional: Discord

Nothing happens until you create discord.json in the settings directory. Until then, System Bridge only checks for that file every 30 seconds. It doesn’t connect to Discord, and the discord module doesn’t exist.

Once you’ve set it up:

• System Bridge connects to the Discord desktop app through its local socket, or a named pipe on Windows. This connection stays on your machine. System Bridge uses it to read and change your mute, deafen and volume settings.
• When you first authorise, and whenever the access token expires, System Bridge sends a request to https://discord.com/api/oauth2/token. The request contains your application’s client ID and client secret, plus the authorisation code or refresh token. Discord replies with an access token.
• The access token is stored in discord-token.json in the settings directory, and only your user can read it. Delete it to authorise again.

What Discord does with these requests is covered by Discord’s privacy policy.