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
}

Recommended Fix

Also compare URL.Scheme, so a scheme downgrade on the same host is treated
the same as a host change:

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

Verification

Added a unit test (backend/s3/redirect_scheme_test.go) that calls the real,
unmodified s3RedirectCrossesHost / s3CheckRedirect with an
https://bucket.example.com -> http://bucket.example.com redirect chain.

On unpatched code (commit 16091ce365, current master / v1.74.3):

• s3RedirectCrossesHost returns false
• s3CheckRedirect leaves X-Amz-Security-Token: SECRET-SESSION-TOKEN
  intact on the outgoing (plaintext) request.

=== RUN   TestSchemeDowngradeNotDetectedAsCrossHost
    redirect_scheme_test.go:23: initial=https://bucket.example.com final=http://bucket.example.com s3RedirectCrossesHost=false
--- PASS: TestSchemeDowngradeNotDetectedAsCrossHost (0.00s)

After applying the one-line fix above (also adding scheme comparison), the
token is correctly stripped and all existing redirect tests
(TestClientRemovesSecurityTokenOnCrossHostRedirect,
TestClientDoesNotRestoreSecurityTokenAfterCrossHostRedirect,
TestClientKeepsSecurityTokenOnSameHostRedirect,
TestClientStopsAfterTenRedirects) continue to pass.

A minimal fix commit is ready and can be pushed to a private fork once this
report is acknowledged.