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:
```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.
