validate_caldav_url resolves and vets the initial host, but caldav's
niquests session follows 3xx redirects by default, so a validated public
URL can be redirected at request time to loopback/link-local/private
space, re-opening the SSRF the host check closes. The existing redirect
guard only covered the settings test-connection path.
Add a shared _build_dav_client helper that pins the session to zero
redirects (any 3xx then raises instead of silently following an
attacker-chosen Location), and route both the pull (_sync_blocking) and
write-back (_writeback_blocking) paths through it. Mirrors the
follow_redirects=False already used on the test-connection path.
Tests exercise the real DAVClient request path (a 302 toward an internal
host is refused, the sink is never contacted; the PROPFIND is asserted to
reach the public server first so the check can't pass vacuously), confirm
the helper disables redirects on the installed client, guard against a
raw DAVClient creeping back in, cover mixed public/internal DNS results
in both orderings, and add the resolves-to-no-usable-records fail-closed
branch.
Joeseph Grey
f78539ba15