CVE-2026-79782

rclone before 1.74.4 fails to strip the X-Amz-Security-Token header when an S3 redirect changes scheme from HTTPS to HTTP on the same host. Attackers can intercept plaintext HTTP traffic to capture AWS STS session tokens sent in request headers.

rclone before 1.74.4 fails to strip the X-Amz-Security-Token header when an S3 redirect changes scheme from HTTPS to HTTP on the same host. Attackers can intercept plaintext HTTP traffic to capture AWS STS session tokens sent in request headers.

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. …

rclone before 1.74.4 fails to strip the X-Amz-Security-Token header when an S3 redirect changes scheme from HTTPS to HTTP on the same host. Attackers can intercept plaintext HTTP traffic to capture AWS STS session tokens sent in request headers.

rclone before 1.74.4 fails to strip the X-Amz-Security-Token header when an S3 redirect changes scheme from HTTPS to HTTP on the same host. Attackers can intercept plaintext HTTP traffic to capture AWS STS session tokens sent in request headers.

rclone before 1.74.4 fails to strip the X-Amz-Security-Token header when an S3 redirect changes scheme from HTTPS to HTTP on the same host. Attackers can intercept plaintext HTTP traffic to capture AWS STS session tokens sent in request headers.