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CVE-2026-88044 – Authentication Bypass – rclone RC Serve FTP/S3 (v1.70.0–1.75.0)

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Summary

CVE-2026-88044 is a critical authorization vulnerability (CWE-863: Incorrect Authorization) in rclone, the widely used command-line tool for syncing files and directories with cloud storage providers. When FTP or S3 servers are started through rclone’s remote control (RC) interface with a per-server authentication proxy configured, the underlying constructors incorrectly consult a process-global proxy setting instead of the request-specific one, causing the configured authentication proxy to be silently ignored. The flaw carries a CVSS v3.1 base score of 9.1 (Critical) and is remotely exploitable without authentication or user interaction.

Technical details

  • Root cause: the FTP and S3 serve constructors reference the process-global proxy.Opt.AuthProxy value rather than the per-server proxyOpt.AuthProxy value supplied when a server is started via the RC serve/start command.
  • Trigger conditions: an RC-started FTP or S3 server configured with a per-server authentication proxy, where the global AuthProxy option is left unset (the typical RC deployment scenario).
  • Attack vector: network, no privileges and no user interaction required (AV:N/AC:L/PR:N/UI:N).
  • Impact — FTP: the server falls back to a fixed-backend mode that accepts the username anonymous with any password, exposing read, write, and delete operations without the intended authentication.
  • Impact — S3: when configured with an auth_key, proxy-based backend authorization is bypassed, granting access to the fixed filesystem rather than the backend the proxy would have selected.
  • Servers started directly via CLI commands using the global proxy option are not affected; only servers started through the RC interface with per-server proxy options are vulnerable.

Affected software

  • rclone versions >= 1.70.0 and < 1.75.1 (confirmed affected: v1.70.0 through v1.75.0), including development commit 5629f2668c69149bf3d9d8e2a25bb32a2648606e.

Severity

  • CVSS v3.1 Base Score: 9.1 (Critical)
  • Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:N

Mitigation and recommended actions

  • Immediate: upgrade rclone to version 1.75.1 or later, which corrects the constructors to use the per-server AuthProxy setting.
  • If patching is not immediately possible: avoid starting FTP or S3 servers through the RC interface with per-server authentication proxy configurations; instead configure and start these servers via CLI with the global proxy option, or disable RC-based serve/start functionality until the upgrade can be applied.
  • Restrict network exposure of rclone RC, FTP, and S3 serve endpoints to trusted networks, and audit for anonymous FTP access or unexpected S3 filesystem exposure on any instance running an affected version.

References

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