CVE-2026-79782

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).

1. 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).

1. 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.

1. 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/redirectschemetest.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.