Example case: https://lock.cmpxchg8b.com/utorrent-crash-test.html (background: https://bugs.chromium.org/p/project-zero/issues/detail?id=1524)
Page uses img src attribute to make requests to localhost and this goes through uMatrix default settings (* * image allow). Tested on Firefox and Chrome. Now I added to my rules:
* 127.0.0.1 * block
* localhost * block
* my-LAN-IP * block
127.0.0.1 127.0.0.1 * inherit
localhost localhost * inherit
my-LAN-IP my-LAN-IP * inherit
But maybe 127.0.0.1 and localhost rules are good enough for default? This would make silent attacks like above harder, but won't break services that are running on localhost and intentionally visited by user in the browser (on localhost:port and 127.0.0.1:port). Or there are legitimate cases when remote web page needs to request localhost?