Intent to prototype & ship: Treat localhost addresses as "Potentially Trustworthy"

[Frédéric Wang]

Hi,

I'm going to try and land a patch for bug 1220810 today, which makes
localhost addresses secure contexts. It seems there were attempts to
land this change 7 months ago and again 3 months ago, but I can't find
any intent email, so I'm sending this one.

Summary: Ensure that localhost addresses resolve to a loopback address,
thereby ensuring that we can safely treat `http://localhost/` and
`http://*.localhost/` as "Potentially Trustworthy". This addresses
various bug reports from developers and aligns with specifications.

Bug: https://bugzilla.mozilla.org/show_bug.cgi?id=1488740

Standards:
https://w3c.github.io/webappsec-secure-contexts/#localhost
https://tools.ietf.org/html/draft-west-let-localhost-be-localhost

Platform coverage: All

Preference: This will ship enabled by default (existing
network.proxy.allow_hijacking_localhost preference can be used to
disable the hardcoded loopback address and resolve proxy for localhost
but I think it's mostly for internal testing).

DevTools bug: N/A

Other browsers:
Chromium: Shipped since version 83
(https://bugs.chromium.org/p/chromium/issues/detail?id=589141#c15)
WebKit: Considering (https://bugs.webkit.org/show_bug.cgi?id=171934#c73)

web-platform-tests:
This is covered by internal Gecko tests, but I opened
https://bugzilla.mozilla.org/show_bug.cgi?id=1672323 as a follow-up.

--
Frédéric Wang