ICSA-21-313-03: Siemens Nucleus RTOS TCP/IP Stack

CISA's summary

Summary: The TCP/IP stack and related services (FTP, TFTP) of the networking component (Nucleus NET) in Nucleus Real-Time Operating System (RTOS) contain several vulnerabilities, also known as "NUCLEUS:13" and as documented below. Siemens has released updates for several affected products and recommends to update to the latest versions. Siemens recommends countermeasures for products where updates are not available.

Products, as the advisory lists them

DeSpy has not checked any unit, hardware revision or firmware.

Vulnerabilities in this advisory

• CVE-2021-31344 — CVE-2021-31344 · CWE-843

  ICMP echo packets with fake IP options allow sending ICMP echo reply messages to arbitrary hosts on the network. (FSMD-2021-0004)

• CVE-2021-31345 — CVE-2021-31345 · CWE-1284

  The total length of an UDP payload (set in the IP header) is unchecked. This may lead to various side effects, including Information Leak and Denial-of-Service conditions, depending on a user-defined applications that runs on top of the UDP protocol. (FSMD-2021-0006)

• CVE-2021-31346 — CVE-2021-31346 · CWE-1284

  The total length of an ICMP payload (set in the IP header) is unchecked. This may lead to various side effects, including Information Leak and Denial-of-Service conditions, depending on the network buffer organization in memory. (FSMD-2021-0007)

• CVE-2021-31881 — CVE-2021-31881 · CWE-125

  When processing a DHCP OFFER message, the DHCP client application does not validate the length of the Vendor option(s), leading to Denial-of-Service conditions. (FSMD-2021-0008)

• CVE-2021-31882 — CVE-2021-31882 · CWE-119

  The DHCP client application does not validate the length of the Domain Name Server IP option(s) (0x06) when processing DHCP ACK packets. This may lead to Denial-of-Service conditions. (FSMD-2021-0011)

• CVE-2021-31883 — CVE-2021-31883 · CWE-119

  When processing a DHCP ACK message, the DHCP client application does not validate the length of the Vendor option(s), leading to Denial-of-Service conditions. (FSMD-2021-0013)

• CVE-2021-31884 — CVE-2021-31884 · CWE-170

  The DHCP client application assumes that the data supplied with the “Hostname” DHCP option is NULL terminated. In cases when global hostname variable is not defined, this may lead to Out-of-bound reads, writes, and Denial-of-service conditions. (FSMD-2021-0014)

• CVE-2021-31885 — CVE-2021-31885 · CWE-805

  TFTP server application allows for reading the contents of the TFTP memory buffer via sending malformed TFTP commands. (FSMD-2021-0009)

• CVE-2021-31886 — CVE-2021-31886 · CWE-170

  FTP server does not properly validate the length of the “USER” command, leading to stack-based buffer overflows. This may result in Denial-of-Service conditions and Remote Code Execution. (FSMD-2021-0010)

• CVE-2021-31887 — CVE-2021-31887 · CWE-170

  FTP server does not properly validate the length of the “PWD/XPWD” command, leading to stack-based buffer overflows. This may result in Denial-of-Service conditions and Remote Code Execution. (FSMD-2021-0016)

• CVE-2021-31888 — CVE-2021-31888 · CWE-170

  FTP server does not properly validate the length of the “MKD/XMKD” command, leading to stack-based buffer overflows. This may result in Denial-of-Service conditions and Remote Code Execution. (FSMD-2021-0018)

• CVE-2021-31889 — CVE-2021-31889 · CWE-191

  Malformed TCP packets with a corrupted SACK option leads to Information Leaks and Denial-of-Service conditions. (FSMD-2021-0015)

• CVE-2021-31890 — CVE-2021-31890 · CWE-240

  The total length of an TCP payload (set in the IP header) is unchecked. This may lead to various side effects, including Information Leak and Denial-of-Service conditions, depending on the network buffer organization in memory. (FSMD-2021-0017)

Acknowledgments, as the advisory lists them

• Siemens ProductCERT: reporting this vulnerability to CISA.
• Yuval Halaban, Uriel Malin, Tal Zohar, Medigate: coordinated disclosure
• Daniel dos Santos, Amine Amri, Stanislav Dashevskyi, Forescout Technologies: coordinated disclosure

The advisory's legal notice

Siemens Security Advisories are subject to the terms and conditions contained in Siemens' underlying license terms or other applicable agreements previously agreed to with Siemens (hereinafter "License Terms"). To the extent applicable to information, software or documentation made available in or through a Siemens Security Advisory, the Terms of Use of Siemens' Global Website (https://www.siemens.com/terms_of_use, hereinafter "Terms of Use"), in particular Sections 8-10 of the Terms of Use, shall apply additionally. In case of conflicts, the License Terms shall prevail over the Terms of Use.

Sources

• The advisory at cisa.gov
• The CSAF document (JSON)

DeSpy's copy of this version is dated 2026-09-28.