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CVE-2026-88018 – Authentication Bypass / SigV4 Signature Forgery – rclone serve s3 (versions prior t

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Summary

CVE-2026-88018 is a critical authentication bypass vulnerability (CVSS 9.8) affecting rclone serve s3 when run with the --auth-proxy option but without --auth-key. The flaw allows an unauthenticated network attacker to register an arbitrary, self-chosen access key ID with an empty secret and forge a valid AWS SigV4 request signature, bypassing authentication entirely. Any organization exposing rclone serve s3 with this configuration is at risk of full compromise of the backend storage the proxy resolves.

Technical details

  • Root cause: when --auth-proxy is configured without --auth-key, rclone’s authPairMiddleware registers each client-chosen accessKeyID in its credential store paired with an empty secret (ws.s3Secret).
  • The embedded S3-compatible server then validates the request’s SigV4 signature against that same empty secret, which is cryptographically valid for HMAC computation.
  • The auth-proxy script only receives the access key ID, not a real secret, so it cannot distinguish a legitimate client from an attacker who picks the same identifier.
  • Attack vector: network, no authentication or user interaction required — an attacker simply picks any access key ID, signs the S3 API request using an empty secret, and the signature validates successfully.
  • Impact: unauthorized access to whatever backend the auth-proxy resolves for the chosen identity, with full read/write capability — resulting in high impact to confidentiality, integrity, and availability.

Affected software

  • rclone versions prior to 1.75.1, when run as rclone serve s3 with --auth-proxy enabled and --auth-key not set.
  • Fixed in rclone 1.75.1.

Severity

  • CVSS v3.1 Base Score: 9.8 (Critical)
  • Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H

Mitigation and recommended actions

  • Immediate: Upgrade rclone to version 1.75.1 or later, which refuses to start serve s3 when --auth-proxy is configured without --auth-key.
  • If patching is not immediately possible: always specify --auth-key alongside --auth-proxy when running rclone serve s3, and audit any exposed serve s3 instances for this configuration.
  • Restrict network exposure of rclone serve s3 endpoints to trusted networks until patched, and review access logs for anomalous access key IDs indicating exploitation attempts.

References

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