rclone serve s3: --auth-proxy without --auth-key authenticates nobody - full SigV4 signature bypass

Exploitation

No confirmed exploitation recorded. Patch Next marks exploitation only from CISA KEV.

CVSS

9.8 · CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H

EPSS

• CVE-2026-88018: 0.51% estimated exploitation activity in the next 30 days; percentile 43.61 (FIRST EPSS, 2026-11-03). This is a model estimate, not proof of exploitation or customer exposure.

As published by the source; long descriptions are shortened.

Summary rclone serve s3's handler chain, when --auth-proxy is configured, is (outermost first): authPairMiddleware - proxyAuthMiddleware - gofakes3's own SigV4-verifying handler. authPairMiddleware parses the accessKeyID straight out of the incoming request's own Authorization header (entirely client-controlled) and registers {accessKey: ws.s3Secret} into gofakes3's shared credential store via AddAuthKeys, for EVERY access key any client presents - not just ones previously known to the server. ws.s3Secret defaults to "" whenever --auth-key is not set, which the --auth-proxy documentation (and the reference bin/testproxy.py) presents as a complete, standalone authentication mechanism requiring no other flag - matching how it's used for serve webdav/ftp/sftp. gofakes3's SigV4 verification then checks the request's signature against exactly the secret authPairMiddleware just registered for that same client-chosen key. An empty string is a valid HMAC key, so a caller can trivially…

Published by the source

10 Sep 2026, 22:49 UTC

Last modified by the source

17 Sep 2026, 17:41 UTC

Source last checked

26 Oct 2026, 15:29 UTC

1. Revision 3 · amended · 26 Oct 2026

   Amended from ghsa record GHSA-xwwr-4h3p-r22c

2. Revision 2 · amended · 24 Oct 2026

   Amended from osv record GO-2026-6462

3. Revision 1 · created · 24 Oct 2026

   Published from osv record GHSA-xwwr-4h3p-r22c

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# Patch Next advisory brief: CVE-2026-88018

Advisory: rclone serve s3: --auth-proxy without --auth-key authenticates nobody - full SigV4 signature bypass
IDs: CVE-2026-88018, GHSA-xwwr-4h3p-r22c
Record: https://patchnext.com/advisories/cve-2026-88018 (revision 3, source checked 26 Oct 2026, 15:29 UTC)
Inventory: none attached. This brief comes from a public page and does not say whether any deployment is affected.
Severity: Critical (CVSS 9.8)
Exploitation: no confirmed exploitation recorded. Patch Next confirms exploitation only from CISA KEV.
Vendor response: Vendor fix available; fixed versions by package: github.com/rclone/rclone 1.75.1. A vendor fix does not mean a deployment is fixed.

## Affected components
- Go github.com/rclone/rclone: affected < 1.75.1; fixed in 1.75.1

## Suggested inspection (read-only)
- Go github.com/rclone/rclone: check the installed version with your package manager.

## Supported action
Upgrade each affected component to a fixed version on its release line. For a transitive dependency, upgrade the parent package or add an override or constraint, then regenerate the lockfile. Redeploy every environment that ran an affected version.

## Required regression checks
- Run the test suite and a production build.
- Exercise the features that depend on github.com/rclone/rclone.
- Repeat the inspection and confirm that only fixed versions remain installed.

## Rollback considerations
- Keep the previous lockfile and deployment artifact. Rolling back reinstates the vulnerable version, so apply any documented mitigation while you fix forward.

## Unresolved questions
- Which deployments run an affected version?
- Exploitation status can change. Check again before lowering the priority.
- Mark the work resolved only after the fixed version is verified in production.

## Sources
- Verified: https://osv.dev/vulnerability/GHSA-xwwr-4h3p-r22c (OSV record GHSA-xwwr-4h3p-r22c lists affected ranges: Go github.com/rclone/rclone >=0 <1.75.1 (fixed))
- Verified: https://osv.dev/vulnerability/GO-2026-6462 (OSV record GO-2026-6462 lists affected ranges: Go github.com/rclone/rclone >=0 <1.75.1 (fixed))
- Verified: https://github.com/advisories/GHSA-xwwr-4h3p-r22c (GitHub-reviewed advisory GHSA-xwwr-4h3p-r22c lists affected ranges: Go github.com/rclone/rclone >=0 <1.75.1)

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