rclone: S3 backend does not strip X-Amz-Security-Token on a same-host HTTPS->HTTP redirect

Vulnerability Details

File: backend/s3/s3.go Lines: 1359-1380 (functions s3CheckRedirect / s3RedirectCrossesHost)

Root Cause

Commit e7b1eb774 (released in v1.74.3) added a CheckRedirect policy for the S3 HTTP client whose purpose is to strip the X-Amz-Security-Token header (the AWS STS session token) whenever a redirect chain "crosses a host", so the token isn't forwarded to an unintended origin.

s3RedirectCrossesHost decides this purely by comparing url.URL.Host (hostname[:port]); it never looks at url.URL.Scheme. A redirect that keeps the exact same host:port but changes the scheme from https:// to http:// therefore compares as "same host" and X-Amz-Security-Token is not stripped — it is sent again, this time over plaintext HTTP.

func s3RedirectCrossesHost(req *http.Request, via []*http.Request) bool {
	if len(via) == 0 {
		return false
	}
	host := via[0].URL.Host
	for _, redirect := range via[1:] {
		if redirect.URL.Host != host {
			return true
		}
	}
	return host != req.URL.Host
}

Attack Scenario

1. The user configures an s3 remote (or --s3-endpoint pointing at a self-hosted/third-party S3-compatible service) using temporary credentials that include an STS session_token (common for assumed-role / CI / Kubernetes IRSA setups).
2. The configured endpoint responds to a request with a 3xx redirect to the same host:port but with http:// instead of https:// (TLS-front misconfiguration, maintenance redirect, or a malicious/compromised storage provider trying to harvest the token).
3. rclone's S3 HTTP client follows the redirect and re-sends the request, including X-Amz-Security-Token, over the now-unencrypted connection to that same host.
4. Any passive observer on that now-plaintext network path can read the STS session token from the request headers.

Impact

Disclosure of the AWS STS session token (X-Amz-Security-Token) in cleartext for the remainder of its validity window. This is the exact class of leak that e7b1eb774 was written to close — it just doesn't cover the scheme-downgrade axis of "crossing a host".

Vulnerable Code

func s3RedirectCrossesHost(req *http.Request, via []*http.Request) bool {
	if len(via) == 0 {
		return false
	}
	host := via[0].URL.Host
	for _, redirect := range via[1:] {
		if redirect.URL.Host != host {
			return true
		}
	}
	return host != req.URL.Host
}