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AUSCERT External Security Bulletin Redistribution
Advisory (icsa-21-103-14) Siemens Nucleus DNS
14 January 2022
AusCERT Security Bulletin Summary
Product: Siemens Nucleus DNS
Operating System: Network Appliance
Impact/Access: Provide Misleading Information -- Remote/Unauthenticated
Reduced Security -- Remote/Unauthenticated
CVE Names: CVE-2021-27393
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ICS Advisory (ICSA-21-103-14)
Siemens Nucleus DNS
Original release date: January 13, 2022
All information products included in https://us-cert.cisa.gov/ics are provided
"as is" for informational purposes only. The Department of Homeland Security
(DHS) does not provide any warranties of any kind regarding any information
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is governed by the Traffic Light Protocol (TLP) marking in the header. For more
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1. EXECUTIVE SUMMARY
o CVSS v3 5.3
o ATTENTION: Exploitable remotely/low attack complexity
o Vendor: Siemens
o Equipment: Nucleus
o Vulnerability: Use of Insufficiently Random Values
2. UPDATE INFORMATION
This updated advisory is a follow-up to the original advisory titled
ICSA-21-103-14 Siemens Nucleus DNS that was published April 13, 2021, to the
ICS webpage on www.cisa.gov/uscert.
3. RISK EVALUATION
Successful exploitation of this vulnerability could allow an attacker to poison
the DNS cache or spoof DNS resolving.
4. TECHNICAL DETAILS
4.1 AFFECTED PRODUCTS
The following Nucleus products and versions are affected:
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o Nucleus NET, All versions
o Nucleus RTOS, All versions that include affected DNS modules
o Nucleus ReadyStart, All versions prior to v2013.08
o Nucleus Source Code, All versions that include the affected DNS modules
o [S: VSTAR, All versions that include the affected DNS modules :S]
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4.2 VULNERABILITY OVERVIEW
4.2.1 USE OF INSUFFICIENTLY RANDOM VALUES CWE-330
The DNS client does not properly randomize UDP port numbers of DNS requests.
This could allow an attacker to poison the DNS cache or spoof DNS resolving.
CVE-2021-27393 has been assigned to this vulnerability. A CVSS v3 base score of
5.3 has been calculated; the CVSS vector string is ( AV:N/AC:L/PR:N/UI:N/S:U/
o CRITICAL INFRASTRUCTURE SECTORS: Critical Manufacturing
o COUNTRIES/AREAS DEPLOYED: Worldwide
o COMPANY HEADQUARTERS LOCATION: Germany
Siemens reported this vulnerability to CISA.
Siemens has identified the following specific workarounds and mitigations users
can apply to reduce risk:
o Nucleus NET users should upgrade to the latest versions of Nucleus
ReadyStart or Nucleus 4.
o Nucleus ReadyStart users should update to v 2013.08 or later versions .
o Avoid using DNS client of affected versions.
o For additional mitigation advice please contact customer support or the a
Nucleus sales team.
For additional information, please refer to Siemens Security Advisory
For further inquiries on security vulnerabilities in Siemens' products and
solutions, please contact Siemens ProductCERT .
CISA recommends users take defensive measures to minimize the risk of
exploitation of this vulnerability. Specifically, users should:
o Minimize network exposure for all control system devices and/or systems,
and ensure they are not accessible from the Internet .
o Locate control system networks and remote devices behind firewalls and
isolate them from the business network.
o When remote access is required, use secure methods, such as Virtual Private
Networks (VPNs), recognizing VPNs may have vulnerabilities and should be
updated to the most current version available. Also recognize VPN is only
as secure as its connected devices.
CISA reminds organizations to perform proper impact analysis and risk
assessment prior to deploying defensive measures.
CISA also provides a section for control systems security recommended practices
on the ICS webpage on cisa.gov . Several recommended practices are available
for reading and download, including Improving Industrial Control Systems
Cybersecurity with Defense-in-Depth Strategies .
Additional mitigation guidance and recommended practices are publicly available
on the ICS webpage on cisa.gov in the Technical Information Paper,
ICS-TIP-12-146-01B--Targeted Cyber Intrusion Detection and Mitigation
Organizations observing any suspected malicious activity should follow their
established internal procedures and report their findings to CISA for tracking
and correlation against other incidents.
No known public exploits specifically target this vulnerability.
For any questions related to this report, please contact the CISA at:
Toll Free: 1-888-282-0870
CISA continuously strives to improve its products and services. You can help by
choosing one of the links below to provide feedback about this product.
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