Rclone < 1.75.1 S3 Serve SigV4 Signature Bypass (CVE-2026-88018)

critical Nessus Plugin ID 346829

Synopsis

An application installed on the remote host is affected by an authentication bypass vulnerability.

Description

The version of Rclone installed on the remote host is prior to 1.75.1. It is, therefore, affected by an authentication bypass vulnerability:

- When rclone serve s3 is configured with --auth-proxy and without --auth-key, the authPairMiddleware handler parses the access key ID out of the client-controlled Authorization header and registers that key with the server's S3 secret, which defaults to an empty string when --auth-key is not set. Because an empty string is a valid HMAC key, an unauthenticated attacker can choose an arbitrary access key ID, compute a valid SigV4 signature against an empty secret, and pass signature verification, reaching whatever backend the auth-proxy script resolves the chosen identity to. No credentials, prior access, or user interaction are required. (CVE-2026-88018)

Note that Nessus has not tested for this issue but has instead relied only on the application's self-reported version number.

Solution

Upgrade to Rclone version 1.75.1 or later.

Plugin Details

Severity: Critical

ID: 346829

File Name: rclone_CVE-2026-88018.nasl

Version: 1.2

Type: Local

Agent: windows, macosx, unix

Family: Misc.

Published: 9/17/2026

Updated: 9/18/2026

Configuration: Enable thorough checks (optional)

Supported Sensors: Nessus Agent, Nessus

Risk Information

VPR

Risk Factor: Medium

Score: 4.9

Percentile: 58.22

CVSS v2

Risk Factor: Critical

Base Score: 10

Temporal Score: 7.4

Vector: CVSS2#AV:N/AC:L/Au:N/C:C/I:C/A:C

CVSS Score Source: CVE-2026-88018

CVSS v3

Risk Factor: Critical

Base Score: 9.8

Temporal Score: 8.5

Vector: CVSS:3.0/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H

Temporal Vector: CVSS:3.0/E:U/RL:O/RC:C

Vulnerability Information

CPE: cpe:/a:rclone:rclone

Required KB Items: installed_sw/rclone

Exploit Ease: No known exploits are available

Patch Publication Date: 9/4/2026

Vulnerability Publication Date: 9/10/2026

Reference Information

CVE: CVE-2026-88018

Detections

Analytics