CVE-2026-88044: rclone: RC per-server auth-proxy bypass

serve/start accepts protocol options in a per-server proxyOpt object. The FTP and S3 RC adapters parse that object and pass it to their server constructors, but the constructors decide whether proxy authentication is enabled by checking the process-global proxy.Opt.AuthProxy instead of the supplied proxyOpt.AuthProxy.

When the process-global option is empty—the normal case when only the RC request configures the server—the supplied authentication proxy is silently ignored. FTP falls back to its fixed-backend mode, whose defaults accept username anonymous with any password, exposing read, write, and delete operations without the authentication the operator configured. S3 falls back to the fixed filesystem: with an auth_key, any holder of that key reaches the fixed RC fs instead of the backend selected by the auth proxy.

The S3 no-auth_key mode is explicitly documented as anonymous and is not part of this vulnerability claim. The confirmed S3 impact is proxy-based authorization/backend routing being ignored when S3 authentication is otherwise enabled.

Confirmed affected versions are v1.70.0 through v1.75.0, plus development commit 5629f2668c69149bf3d9d8e2a25bb32a2648606e. The dedicated CLI commands use the process-global option and are not affected by this configuration mismatch.