render_unauthorized built the post-login return URL directly from the
attacker-influenceable X-Forwarded-Host / X-Forwarded-Uri headers, stored it
in the session, and reflected it into the signin `rd`. After authentication
that URL is followed with allow_other_host, so a spoofed host was an open
redirect.
Now the forwarded URL is only honoured if it resolves to a known, active
forward-auth application (via validate_redirect_url); otherwise it falls back
to a validated `rd` or the IdP's base URL. Once render_unauthorized only ever
stores a validated value, the sessions_controller "supplement, don't replace"
behaviour is safe, so no change is needed there.
Two integration tests were asserting the old behaviour by reflecting
unregistered hosts (grafana.example.com, app.example.com); they now register
those domains as forward-auth apps so they exercise the real feature. Adds a
regression test that a spoofed X-Forwarded-Host is neither stored nor
reflected.
Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
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