Siemens SIMATIC S7-1500 Improper Restriction of XML External Entity Reference (CVE-2018-20843)

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

include('compat.inc');

if (description)
{
  script_id(504083);
  script_version("1.3");
  script_set_attribute(attribute:"plugin_modification_date", value:"2026/02/14");

  script_cve_id("CVE-2018-20843");
  script_xref(name:"ICSA", value:"23-348-10");
  script_xref(name:"CEA-ID", value:"CEA-2021-0025");

  script_name(english:"Siemens SIMATIC S7-1500 Improper Restriction of XML External Entity Reference (CVE-2018-20843)");

  script_set_attribute(attribute:"synopsis", value:
"The remote OT asset is affected by a vulnerability.");
  script_set_attribute(attribute:"description", value:
"In libexpat in Expat before 2.2.7, XML input including XML names that
contain a large number of colons could make the XML parser consume a
high amount of RAM and CPU resources while processing (enough to be
usable for denial-of-service attacks).

This plugin only works with Tenable.ot.
Please visit https://www.tenable.com/products/tenable-ot for more information.");
  script_set_attribute(attribute:"see_also", value:"https://cert-portal.siemens.com/productcert/html/ssa-398330.html");
  script_set_attribute(attribute:"see_also", value:"https://support.industry.siemens.com/cs/ww/en/view/109478459/");
  script_set_attribute(attribute:"see_also", value:"https://www.cisa.gov/news-events/ics-advisories/icsa-23-348-10");
  script_set_attribute(attribute:"solution", value:
"The following text was originally created by the Cybersecurity and Infrastructure Security Agency (CISA). The original
can be found at CISA.gov.

Siemens has identified the following specific workarounds and mitigations users can apply to reduce risk:

- SIMATIC S7-1500 CPU 1518-4 PN/DP MFP (6ES7518-4AX00-1AB0): Only build and run applications from trusted sources.
Currently no fix is available.
- SIMATIC S7-1500 CPU 1518-4 PN/DP MFP (6ES7518-4AX00-1AC0): Only build and run applications from trusted sources.
Currently no fix is available.
- SIMATIC S7-1500 CPU 1518F-4 PN/DP MFP (6ES7518-4FX00-1AB0): Only build and run applications from trusted sources.
Currently no fix is available.
- SIMATIC S7-1500 CPU 1518F-4 PN/DP MFP (6ES7518-4FX00-1AC0): Only build and run applications from trusted sources.
Currently no fix is available.
- SIPLUS S7-1500 CPU 1518-4 PN/DP MFP (6AG1518-4AX00-4AC0): Only build and run applications from trusted sources.
Currently no fix is available.

Note: This SSA advises vulnerabilities for firmware version V3.1 only; for versions prior to V3.1 refer to
SiemensSecurity Bulletin SSB-439005.

As a general security measure, Siemens recommends protecting network access to devices with appropriate mechanisms. To
operate the devices in a protected IT environment, Siemens recommends configuring the environment according to Siemens'
operational guidelines for industrial security and following recommendations in the product manuals.

Additional information on industrial security by Siemens can be found on the Siemens industrial security webpage.

For more information see the associated Siemens security advisory SSA-398330 in HTML and CSAF.");
  script_set_cvss3_base_vector("CVSS:3.0/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H");
  script_set_cvss3_temporal_vector("CVSS:3.0/E:P/RL:O/RC:C");
  script_set_attribute(attribute:"cvss_score_source", value:"CVE-2018-20843");

  script_set_attribute(attribute:"exploitability_ease", value:"No known exploits are available");
  script_cwe_id(611);

  script_set_attribute(attribute:"vuln_publication_date", value:"2023/12/12");
  script_set_attribute(attribute:"patch_publication_date", value:"2023/12/12");
  script_set_attribute(attribute:"plugin_publication_date", value:"2025/11/13");

  script_set_attribute(attribute:"plugin_type", value:"remote");
  script_set_attribute(attribute:"cpe", value:"cpe:/o:siemens:simatic_s7-1500_cpu_firmware:3.1.5");
  script_set_attribute(attribute:"cpe", value:"cpe:/o:siemens:siplus_s7-1500_cpu_firmware:3.1.5");
  script_set_attribute(attribute:"generated_plugin", value:"former");
  script_end_attributes();

  script_category(ACT_GATHER_INFO);
  script_family(english:"Tenable.ot");

  script_copyright(english:"This script is Copyright (C) 2025-2026 and is owned by Tenable, Inc. or an Affiliate thereof.");

  script_dependencies("tenable_ot_api_integration.nasl");
  script_require_keys("Tenable.ot/Siemens");

  exit(0);
}


include('tenable_ot_cve_funcs.inc');

get_kb_item_or_exit('Tenable.ot/Siemens');

var asset = tenable_ot::assets::get(vendor:'Siemens');

var vuln_cpes = {
    "cpe:/o:siemens:simatic_s7-1500_cpu_firmware:3.1.5" :
        {"versionEndExcluding" : "3.1.5", "versionStartIncluding" : "3.1.0", "family" : "S71500", "orderNumbers" : ['6ES7518-4AX00-1AB0', '6ES7518-4FX00-1AC0', '6ES7518-4AX00-1AC0', '6ES7518-4FX00-1AB0']},
    "cpe:/o:siemens:siplus_s7-1500_cpu_firmware:3.1.5" :
        {"versionEndExcluding" : "3.1.5", "versionStartIncluding" : "3.1.0", "family" : "S71500", "orderNumbers" : ['6AG1518-4AX00-4AC0']}
};

tenable_ot::cve::compare_and_report(asset:asset, cpes:vuln_cpes, severity:SECURITY_HOLE);