rclone: S3 Redirect Sanitization Omits IBM IAM Bearer Tokens and SSE-C Keys

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:

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 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.