GHSA-8mxv-9xhp-86h4

## 1. Summary

The S3 redirect callback strips `X-Amz-Security-Token` when a redirect changes scheme or host, but it does not strip IBM IAM bearer authorization or customer-provided encryption keys. Two independently validated paths remain:

- a same-host HTTPS-to-HTTP redirect preserves `Authorization: Bearer ...` and exposes a reusable IBM IAM token to the plaintext network path; - a cross-origin redirect preserves SSE-C and copy-source SSE-C key headers.

The High rating is driven by the reusable IBM IAM bearer token. The SSE-C cross-origin disclosure is a secondary confidentiality issue. The meaningful threat is a trusted endpoint, gateway, or accelerator that emits an unsafe redirect, followed by an adjacent/on-path observer; describing the originally configured endpoint itself as the attacker would be weak because that endpoint already receives the request secrets.

## 2. Affected Assets & Attack Surface

- S3 redirect policy: `backend/s3/s3.go:1345-1379` - IBM IAM signer: `backend/s3/ibm_signer.go:28-40` - SSE-C key preparation: `backend/s3/s3.go:1821-1837` - Affected operations: requests carrying IBM IAM authorization, SSE-C keys, or copy-source SSE-C keys - Confirmed affected version: `<= v1.74.0-240-ga0c09f138`

## 3. Technical Root Cause Analysis

`s3CheckRedirect` applies a one-header denylist:

```go if s3RedirectCrossesHost(req, via) { req.Header.Del("X-Amz-Security-Token") } ```

Go removes `Authorization` on some hostname changes, but preserves it for a same-host redirect and does not treat a scheme downgrade as sufficient reason to remove it. Go also has no generic knowledge that the SSE-C headers contain raw encryption keys. The rclone callback recognizes the STS token but not these additional origin-bound secrets.

## 4. Proof-of-Concept & Evidence

Using the actual redirect callback:

1. An HTTPS endpoint redirected to HTTP on the same hostname. 2. The plaintext destination received the planted IBM bearer token and SSE-C headers. 3. A separate redirect to an unrelated hostname caused Go to remove `Authorization`, but the destination still received both SSE-C key headers. 4. In both cases, rclone removed the planted STS token, proving that the S3-specific callback executed while omitting the other secret classes.

The related [GHSA-gx4c-2hqx-cw2r](https://github.com/rclone/rclone/security/advisories/GHSA-gx4c-2hqx-cw2r) covers the STS downgrade path and confirms that rclone treats scheme changes as a credential boundary. It does not cover the IBM bearer or SSE-C variants retained here.

## 5. Impact Assessment

A captured IBM bearer token can authorize reads, writes, and deletes within its IAM scope. A disclosed SSE-C key can expose corresponding ciphertext available to the recipient; copy-source keys can expose protected source objects. The exact impact is limited by token policy and the attacker's access to encrypted objects.

## 6. Remediation Guidance

- Reject every HTTPS-to-HTTP redirect before replay. - Do not automatically follow secret-bearing cross-origin redirects. - On any scheme, host, or effective-port change, remove all authorization, cookies, session tokens, SSE-C fields, copy-source SSE-C fields, and provider-specific credentials. - Where redirects are required, allowlist exact destinations and reconstruct/re-sign a new request. - Add redirect tests for every secret header class and for scheme, hostname, subdomain, and port changes.