GHSA-gx4c-2hqx-cw2r: Low severity go/github.com/rclone/rclone vulnerability

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 sessiontoken (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/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 redirectschemetest.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.