PT-2026-75972 · Go · Github.Com/Rclone/Rclone

Published

2026-08-05

·

Updated

2026-08-05

CVSS v3.1

3.1

Low

VectorAV:A/AC:H/PR:N/UI:N/S:U/C:L/I:N/A:N

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.
go
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

go
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:
go
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.

Fix

Insufficiently Protected Credentials

Cleartext Transmission of Sensitive Information

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Weakness Enumeration

CWE-522CWE-319

Related Identifiers

GHSA-GX4C-2HQX-CW2R

Affected Products

Github.Com/Rclone/Rclone