rclone: S3 backend does not strip X-Amz-Security-Token on a same-host HTTPS->HTTP redirect — github.com/rclone/rclone
Cleartext Transmission of Sensitive InformationVulnerability 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
- The user configures an
s3remote (or--s3-endpointpointing at a self-hosted/third-party S3-compatible service) using temporary credentials that include an STSsession_token(common for assumed-role / CI / Kubernetes IRSA setups). - The configured endpoint responds to a request with a 3xx redirect to the
same host:port but with
http://instead ofhttps://(TLS-front misconfiguration, maintenance redirect, or a malicious/compromised storage provider trying to harvest the token). - 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. - 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):
s3RedirectCrossesHostreturnsfalses3CheckRedirectleavesX-Amz-Security-Token: SECRET-SESSION-TOKENintact 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.
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