alt Hacker NewsIf I recall correctly Figma uses it to connect to the locally installed app, and Discord definitely uses it to check if its desktop app is installed by scanning ports (6463-6472).
I'm aware of two blockers for LAN intrusions from public internet domains, uBlock Origin has a filter list called "Block Outsider Intrusion into LAN" [0] under the "Privacy" filters, and there's a cool Firefox extension called Port Authority [1][2] that does pretty much the same thing yet more specifically targeted and without the exclusions allowed by the uBlock filterlist (stuff like Figma's use is allowed through, as you can see in [0]). I've contributed some to Port Authority, too :)
0: https://github.com/uBlockOrigin/uAssets/blob/master/filters/...
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The native messaging feature is a much better way to talk to native apps, that should be used instead for modern browsers.
There are surely other ways to achieve this. If you are logged into an app and the site at tbe same time they can use the server to communicate. Discord doesn't need to know if the app is installed to work. That sounds sketchy.