CVE-2026-88018: rclone serve s3: --auth-proxy without --auth-key authenticates nobody - full SigV4 signature bypass

Severity: critical (CVSS 9.8 v3.1)
Exploited in the wild: not listed in CISA KEV
EPSS: 0.5% probability of exploitation in the next 30 days
Public exploit or detection template: none known
Published: 2026-09-10 · updated 2026-09-14
Weakness: CWE-287

Description

rclone is a command-line program to sync files and directories to and from different cloud storage providers. Prior to 1.75.1, rclone serve s3 configured with --auth-proxy but without --auth-key allows authPairMiddleware to register any client-chosen accessKeyID with an empty ws.s3Secret. gofakes3 then verifies the request’s SigV4 signature against that same empty secret, while Server.auth passes the access key identifier as both the user and authentication value to the proxy without an independent per-identity secret. An unauthenticated network attacker can therefore choose an arbitrary access key, sign with an empty secret, and reach whatever backend the auth-proxy script resolves for that identity. This issue is fixed in version 1.75.1.

Affected products

rclone: before 1.75.1 — fixed in 1.75.1

How to fix

Upgrade to a fixed version: rclone 1.75.1. Check the vendor advisory for mitigations.

References

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