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man qbittorrent-nox says that, "Web UI is accessible as a default on http://localhost:8080"

However, when I start qbittorrent-nox, te UI actually listens on  http://0.0.0.0:8080, which means that it is accessible over the network.

How can I change the UI to listen on http://127.0.0.1:8080 only ?

0.0.0.0 means all IP addresses on the local machine
Its network accessable if someone knows your local IP address, it isnt internet accessible unless you port-forward.

Lol

Just to confirm what kitkat said, from an internet isolated machine:
root@xxxx:~# ping 0.0.0.0
PING 0.0.0.0 (127.0.0.1) 56(84) bytes of data.
64 bytes from 127.0.0.1: icmp_seq=1 ttl=64 time=0.046 ms

Just realized i didn't answer his question.

To make that change you'd have to recompile the application with the change made as its hardcoded.
Not exactly sure where the code responsible is or whether its even changeable as i havent looked at the WebUI codebase and a quick search didn't return results.
The reason for that hardcode is because it means the client listens on localhost:8080, 127.0.0.1:8080 <yourlocalip>:8080 which makes it LAN accessible (run qbittorent nox on a local server, how do i access it without been on the server!?)

A toggle would be fairly meaningless as if you're that concerned about someone on the LAN accessing your (passworded) WebUI you can setup local firewall rules to block LAN-to-LAN traffic on port :8080 as no essential system services use that port for any OS's i can think of.. (exempt the router or you may have issues browsing the internet)