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. ```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.

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.

S3 session token leakage on HTTPS to HTTP redirect in github.com/rclone/rclone