Weak Key Exchange (KEX) Algorithm(s) Supported (SSH)

# SPDX-FileCopyrightText: 2021 Greenbone AG
# Some text descriptions might be excerpted from (a) referenced
# source(s), and are Copyright (C) by the respective right holder(s).
#
# SPDX-License-Identifier: GPL-2.0-only

if(description)
{
  script_oid("1.3.6.1.4.1.25623.1.0.150713");
  script_version("2024-06-14T05:05:48+0000");
  script_tag(name:"last_modification", value:"2024-06-14 05:05:48 +0000 (Fri, 14 Jun 2024)");
  script_tag(name:"creation_date", value:"2021-09-14 08:36:00 +0000 (Tue, 14 Sep 2021)");
  script_tag(name:"cvss_base", value:"4.6");
  script_tag(name:"cvss_base_vector", value:"AV:A/AC:H/Au:N/C:C/I:N/A:N");
  script_tag(name:"severity_vector", value:"CVSS:3.1/AV:A/AC:H/PR:N/UI:N/S:U/C:H/I:N/A:N");
  script_tag(name:"severity_origin", value:"Greenbone");
  script_tag(name:"severity_date", value:"2021-11-23 10:58:59 +0000 (Tue, 23 Nov 2021)");

  script_name("Weak Key Exchange (KEX) Algorithm(s) Supported (SSH)");

  script_tag(name:"qod_type", value:"remote_banner");
  script_copyright("Copyright (C) 2021 Greenbone AG");
  script_tag(name:"solution_type", value:"Mitigation");
  script_category(ACT_GATHER_INFO);
  script_family("SSL and TLS");
  script_dependencies("gb_ssh_algos.nasl");
  script_require_ports("Services/ssh", 22);
  script_mandatory_keys("ssh/algos_available");

  script_tag(name:"summary", value:"The remote SSH server is configured to allow / support weak key
  exchange (KEX) algorithm(s).");

  script_tag(name:"vuldetect", value:"Checks the supported KEX algorithms of the remote SSH server.

  Currently weak KEX algorithms are defined as the following:

  - non-elliptic-curve Diffie-Hellmann (DH) KEX algorithms with 1024-bit MODP group / prime

  - ephemerally generated key exchange groups uses SHA-1

  - using RSA 1024-bit modulus key");

  script_tag(name:"insight", value:"- 1024-bit MODP group / prime KEX algorithms:

  Millions of HTTPS, SSH, and VPN servers all use the same prime numbers for Diffie-Hellman key
  exchange. Practitioners believed this was safe as long as new key exchange messages were generated
  for every connection. However, the first step in the number field sieve-the most efficient
  algorithm for breaking a Diffie-Hellman connection-is dependent only on this prime.

  A nation-state can break a 1024-bit prime.");

  script_tag(name:"impact", value:"An attacker can quickly break individual connections.");

  script_tag(name:"solution", value:"Disable the reported weak KEX algorithm(s)

  - 1024-bit MODP group / prime KEX algorithms:

  Alternatively use elliptic-curve Diffie-Hellmann in general, e.g. Curve 25519.");

  script_xref(name:"URL", value:"https://weakdh.org/sysadmin.html");
  script_xref(name:"URL", value:"https://www.rfc-editor.org/rfc/rfc9142");
  script_xref(name:"URL", value:"https://www.rfc-editor.org/rfc/rfc9142#name-summary-guidance-for-implem");
  script_xref(name:"URL", value:"https://www.rfc-editor.org/rfc/rfc6194");
  # nb: The link below is only showing some "basic" info around this topic but is included as an
  # additional reference.
  script_xref(name:"URL", value:"https://www.rfc-editor.org/rfc/rfc4253#section-6.5");

  exit(0);
}

include("ssh_func.inc");
include("list_array_func.inc");
include("port_service_func.inc");
include("misc_func.inc");
include("host_details.inc");

# From https://tools.ietf.org/id/draft-ietf-curdle-ssh-kex-sha2-09.html
weak_kex_algos = make_array( "diffie-hellman-group1-sha1", "Using Oakley Group 2 (a 1024-bit MODP group) and SHA-1",
                             "diffie-hellman-group-exchange-sha1", "Using SHA-1",
                             "rsa1024-sha1", "Using RSA 1024-bit modulus key and SHA-1",

                             # nb: Both are listed as gss-gex-sha1-* / gss-group1-sha1-* on draft-ietf-curdle-ssh-kex-sha2-09.html
                             # This is because of the following in https://datatracker.ietf.org/doc/html/rfc4462#section-2.5:
                             # The method name for each method is the concatenation of the string "gss-gex-sha1-" with the Base64 encoding
                             # of the MD5 hash [MD5] of the ASN.1 DER encoding [ASN1] of the underlying GSS-API mechanism's OID.
                             "gss-gex-sha1-", "Using SHA-1",
                             "gss-group1-sha1-", "Using Oakley Group 2 (a 1024-bit MODP group) and SHA-1",
                             "gss-group14-sha1-", "Deprecated algorithm, e.g. using SHA-1" );

port = ssh_get_port( default:22 );

if( ! supported_kex_algos = get_kb_list( "ssh/" + port + "/kex_algorithms" ) )
  exit( 0 );

found_weak_kex = FALSE;
weak_kex_algos_report = make_array();

foreach weak_kex_algo( keys( weak_kex_algos ) ) {
  if( in_array( search:weak_kex_algo, array:supported_kex_algos, part_match:TRUE ) ) {
    weak_kex_algos_report[weak_kex_algo] = weak_kex_algos[weak_kex_algo];
    found_weak_kex = TRUE;
  }
}

if( found_weak_kex ) {

  # nb:
  # - Store the reference from this one to gb_ssh_algos.nasl to show a cross-reference within the
  #   reports
  # - We don't want to use get_app_* functions as we're only interested in the cross-reference here
  register_host_detail( name:"detected_by", value:"1.3.6.1.4.1.25623.1.0.105565" ); # gb_ssh_algos.nasl
  register_host_detail( name:"detected_at", value:port + "/tcp" );

  report = '\n\n' + text_format_table( array:weak_kex_algos_report, sep:" | ", columnheader:make_list( "KEX algorithm", "Reason" ) );
  security_message( port:port, data:"The remote SSH server supports the following weak KEX algorithm(s):" + report );
  exit( 0 );
}

exit( 99 );