CVE-2026-88018
Received Received - Intake

Authentication Bypass in Rclone S3 Auth Proxy

Publication date: 2026-09-10

Last updated on: 2026-09-10

Assigner: GitHub, Inc.

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.

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Meta Information

Published
2026-09-10
Last Modified
2026-09-10
Generated
2026-09-10
AI Q&A
2026-09-10
EPSS Evaluated
N/A
NVD
CVE-2026-88018
EUVD
EUVD-2026-75585

Affected Vendors & Products

Showing 3 associated CPEs
Vendor Product Version / Range
rclone rclone to 1.75.1 (exc)
-rclone rclone 1.75.1
rclone rclone 1.75.1

Helpful Resources

Exploitability

CWE
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KEV
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CWE ID Description
CWE-306 The product does not perform any authentication for functionality that requires a provable user identity or consumes a significant amount of resources.
CWE-287 When an actor claims to have a given identity, the product does not prove or insufficiently proves that the claim is correct.

Attack-Flow Graph

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Executive Summary

CVE-2026-88018 is a critical authentication bypass vulnerability in rclone's S3 server when using the --auth-proxy flag without --auth-key. It allows unauthenticated attackers to register any access key ID with an empty secret and sign requests using an empty secret, bypassing authentication entirely. The issue occurs because the proxy does not enforce a real secret for verification, making it impossible to distinguish legitimate users.

Detection Guidance

Check if rclone is running with the vulnerable configuration by inspecting running processes for 'rclone serve s3' with '--auth-proxy' but without '--auth-key'. Verify the version of rclone installed using 'rclone version' and compare it against 1.75.1 or later.

Impact Analysis

This vulnerability allows attackers to gain full access to the S3 service as an authenticated user without any credentials. They can read, modify, or delete data, and perform any operations permitted by the backend service. The impact includes unauthorized data access, data manipulation, and potential service disruption.

Compliance Impact

This vulnerability can severely impact compliance with GDPR and HIPAA by enabling unauthorized access to sensitive data. GDPR requires strict access controls and data protection measures, while HIPAA mandates secure handling of protected health information. A breach could result in legal penalties, reputational damage, and loss of trust.

Mitigation Strategies

Upgrade rclone to version 1.75.1 or later immediately. If using '--auth-proxy', ensure '--auth-key' is also configured or migrate to the new proxy protocol where the proxy provides secrets. Disable any exposed S3 services using the vulnerable configuration until patched.

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