Why on earth would you use a web interface and expose your HTTP port in order to do a coinjoin?
That's good point. While i don't play to try Jam for now, it could prevented by using firewall with advance rule.
In fact, any decent firewall should block all ports by default. On my own full node, everything is only accessible locally from the machine itself, by default.
If you open a port on the local firewall, it becomes available on your LAN, but it's still going to be safe from anyone not on this network. Only by opening a port on the device's firewall
and port-forwarding it on your router, it would actually be 'exposed'.
Web GUIs allow you to e.g. run JoinMarket on a headless server in your LAN and access it from your various PCs and laptops, and they do tend to look more modern than something built on QT. Although personally, as a long-term Bitcoin-Qt user, I prefer the JoinMarket-Qt interface.