backchannel_logout_uri was validated only for scheme/HTTPS, so an admin (or a
compromised admin account) could point it at internal infrastructure — cloud
metadata (169.254.169.254), loopback, or RFC1918 hosts — and every user logout
would fire a server-side POST there.
Add PrivateAddressCheck (app/lib) and apply it as defense-in-depth:
- Application validation rejects URIs whose host is, or is a literal, internal
address (loopback / private / link-local / 0.0.0.0 / localhost / metadata
hostnames). Fast, DNS-free, immediate admin feedback.
- BackchannelLogoutJob re-checks at request time WITH DNS resolution and aborts
(no retry) if the host resolves to a non-public address — covering URIs that
predate the validation and public hostnames pointed at internal IPs.
Tests cover the address classification, the model validation, and updates an
existing test that used a localhost logout URI.
Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
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