rclone: WebDAV Credentials Survive a Same-Host HTTPS-to-HTTP Redirect

## 1. Summary WebDAV's default redirect handling can replay Basic authorization and configured Cookie headers over plaintext HTTP after a same-host HTTPS-to-HTTP redirect. This was reproduced through the real backend. Unlike the low-impact STS token in rclone's published S3 redirect advisory, Basic passwords and session cookies are complete reusable credentials, supporting a High rating when they grant normal WebDAV read/write access. The credible threat requires a legitimate endpoint, gateway, or accelerator to emit an unsafe redirect and an adjacent/on-path actor to observe the plaintext hop. A report should not rely on a malicious original WebDAV endpoint because that endpoint already receives the credentials. ## 2. Affected Assets & Attack Surface - Backend configuration/authentication: `backend/webdav/webdav.go:127-139`, `170-206`, `440-530` - Shared redirect callback: `lib/rest/rest.go:218-231` - HTTP client: `fs/fshttp/http.go:311-329` - Credentials: Basic passwords, bearer authorization, SharePoint/session cookies, and configured secret headers - Confirmed affected version: `<= v1.74.0-240` ## 3. Technical Root Cause Analysis `PreserveMethodRedirectFn` limits redirect count and restores the original method, but it does not reject a transport downgrade or compare the full origin tuple. The client therefore relies on Go's hostname-oriented sensitive-header forwarding rules. Those rules can preserve `Authorization` and Cookie on a same-host redirect even when the new sch