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

#%NASL_MIN_LEVEL 80900
##
# (C) Tenable, Inc.
##

include('compat.inc');

if (description)
{
  script_id(346829);
  script_version("1.2");
  script_set_attribute(attribute:"plugin_modification_date", value:"2026/09/18");

  script_cve_id("CVE-2026-88018");

  script_name(english:"Rclone < 1.75.1 S3 Serve SigV4 Signature Bypass (CVE-2026-88018)");

  script_set_attribute(attribute:"synopsis", value:
"An application installed on the remote host is affected by an authentication bypass vulnerability.");
  script_set_attribute(attribute:"description", value:
"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.");
  # https://github.com/rclone/rclone/security/advisories/GHSA-xwwr-4h3p-r22c
  script_set_attribute(attribute:"see_also", value:"http://www.nessus.org/u?0ca8c13c");
  script_set_attribute(attribute:"see_also", value:"https://github.com/rclone/rclone/releases/tag/v1.75.1");
  script_set_attribute(attribute:"solution", value:
"Upgrade to Rclone version 1.75.1 or later.");
  script_set_attribute(attribute:"agent", value:"all");
  script_set_cvss_base_vector("CVSS2#AV:N/AC:L/Au:N/C:C/I:C/A:C");
  script_set_cvss_temporal_vector("CVSS2#E:U/RL:OF/RC:C");
  script_set_cvss3_base_vector("CVSS:3.0/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H");
  script_set_cvss3_temporal_vector("CVSS:3.0/E:U/RL:O/RC:C");
  script_set_attribute(attribute:"cvss_score_source", value:"CVE-2026-88018");

  script_set_attribute(attribute:"exploitability_ease", value:"No known exploits are available");
  script_set_attribute(attribute:"exploit_available", value:"false");

  script_set_attribute(attribute:"vuln_publication_date", value:"2026/09/10");
  script_set_attribute(attribute:"patch_publication_date", value:"2026/09/04");
  script_set_attribute(attribute:"plugin_publication_date", value:"2026/09/17");

  script_set_attribute(attribute:"plugin_type", value:"local");
  script_set_attribute(attribute:"cpe", value:"cpe:/a:rclone:rclone");
  script_set_attribute(attribute:"thorough_tests", value:"true");
  script_end_attributes();

  script_category(ACT_GATHER_INFO);
  script_family(english:"Misc.");

  script_copyright(english:"This script is Copyright (C) 2026 and is owned by Tenable, Inc. or an Affiliate thereof.");

  script_dependencies("rclone_win_installed.nbin", "rclone_macos_installed.nbin", "rclone_nix_installed.nbin");
  script_require_keys("installed_sw/rclone");

  exit(0);
}

include('vdf.inc');

# @tvdl-content
var vuln_data = {
  'metadata': {'spec_version': '1.0'},
  'checks': [
    {
      'product': {'name': 'rclone', 'type': 'app'},
      'check_algorithm': 'default',
      'constraints': [
        {'fixed_version': '1.75.1'}
      ]
    }
  ]
};

var result = vdf::check_and_report(vuln_data:vuln_data, severity:SECURITY_HOLE);
vdf::handle_check_and_report_errors(vdf_result:result);