CVE-2021-31344 Overview
A Type Confusion vulnerability has been identified in multiple Siemens products utilizing the Nucleus TCP/IP stack. The vulnerability exists in the ICMP packet handling mechanism, where ICMP echo packets with fake IP options allow an attacker to send ICMP echo reply messages to arbitrary hosts on the network. This flaw, tracked as FSMD-2021-0004, affects a wide range of Siemens industrial control systems including building automation controllers and embedded networking components.
Critical Impact
This vulnerability enables network-based attackers to redirect ICMP echo replies to arbitrary hosts without authentication, potentially facilitating network reconnaissance, denial of service amplification attacks, or network traffic manipulation in industrial control system environments.
Affected Products
- Siemens Capital Embedded AR Classic 431-422 (All versions)
- Siemens Capital Embedded AR Classic R20-11 (All versions < V2303)
- Siemens PLUSCONTROL 1st Gen (All versions)
- Siemens SIMOTICS CONNECT 400 (All versions < V0.5.0.0 and < V1.0.0.0)
- Siemens Nucleus NET, Nucleus ReadyStart V3/V4, and Nucleus Source Code
- Siemens Apogee Modular Building Controller and Equipment Controller Firmware
- Siemens Apogee PXC Compact and Modular Firmware
- Siemens Talon TC Compact and Modular Firmware
Discovery Timeline
- November 9, 2021 - CVE-2021-31344 published to NVD
- November 21, 2024 - Last updated in NVD database
Technical Details for CVE-2021-31344
Vulnerability Analysis
This vulnerability is classified as CWE-843 (Access of Resource Using Incompatible Type, also known as Type Confusion). The flaw resides in how the Nucleus TCP/IP stack processes ICMP echo request packets that contain manipulated IP options. When the stack encounters these malformed packets, it fails to properly validate the IP options field, leading to incorrect processing of the packet's source and destination addresses.
The vulnerability can be exploited remotely over the network without requiring authentication or user interaction. An attacker can craft specially designed ICMP echo packets with spoofed IP options that cause the vulnerable system to send ICMP echo reply messages to arbitrary third-party hosts rather than the actual requesting host.
Root Cause
The root cause of this vulnerability is improper type handling in the ICMP packet processing logic within the Nucleus TCP/IP stack. Specifically, the code fails to properly validate and handle IP options fields in incoming ICMP echo request packets. When fake or malformed IP options are present, the stack incorrectly interprets the routing information, causing the echo reply to be sent to an unintended destination address specified in the manipulated options rather than the legitimate source.
Attack Vector
The attack vector is network-based, allowing remote exploitation without authentication. An attacker with network access to a vulnerable device can exploit this vulnerability by:
- Crafting ICMP echo request packets with specially constructed fake IP options
- Sending these packets to a vulnerable Siemens device on the network
- The vulnerable device processes the malformed packet and sends ICMP echo replies to arbitrary hosts specified in the fake options
This attack pattern could be used for network reconnaissance, to conduct denial of service amplification attacks against third-party targets, or to interfere with network operations in industrial environments.
The vulnerability affects critical industrial control systems used in building automation and motor control applications, making proper remediation essential for operational technology (OT) security.
Detection Methods for CVE-2021-31344
Indicators of Compromise
- Unexpected ICMP echo reply traffic originating from Siemens devices to hosts that did not send corresponding echo requests
- ICMP packets with unusual or malformed IP options targeting vulnerable Siemens controllers
- Network traffic anomalies showing ICMP amplification patterns from industrial control systems
- Log entries indicating ICMP processing errors on affected devices
Detection Strategies
- Implement network intrusion detection rules to identify ICMP packets with suspicious IP options fields targeting Siemens devices
- Deploy deep packet inspection to analyze ICMP traffic for malformed IP options in industrial network segments
- Monitor for unusual ICMP echo reply patterns where replies are directed to hosts different from the original requester
- Configure SIEM correlation rules to detect ICMP amplification attack patterns involving affected products
Monitoring Recommendations
- Establish baseline ICMP traffic patterns for affected Siemens devices and alert on deviations
- Implement network traffic analysis on OT/ICS network segments to identify abnormal ICMP behavior
- Monitor affected devices for unusual network activity using Siemens diagnostic tools
- Deploy network flow monitoring to track ICMP traffic volumes and destinations from vulnerable systems
How to Mitigate CVE-2021-31344
Immediate Actions Required
- Review the Siemens Security Advisory SSA-044112 for product-specific remediation guidance
- Identify all affected Siemens products in your environment using asset inventory systems
- Apply network segmentation to isolate affected devices from untrusted networks
- Implement firewall rules to restrict ICMP traffic to and from affected systems where operationally feasible
- Plan and schedule firmware updates during maintenance windows
Patch Information
Siemens has released security updates to address this vulnerability across multiple product lines. Organizations should consult the following security advisories for specific patch versions:
- Siemens Security Advisory SSA-044112 - Primary advisory for Nucleus TCP/IP stack
- Siemens Security Advisory SSA-114589 - Additional affected products
- Siemens Security Advisory SSA-223353 - Extended product coverage
- Siemens Security Advisory SSA-620288 - Building automation controllers
- Siemens Security Advisory SSA-845392 - Additional guidance
For Capital Embedded AR Classic R20-11, upgrade to version V2303 or later. For SIMOTICS CONNECT 400, upgrade to version V0.5.0.0 or V1.0.0.0 or later depending on your product variant.
Workarounds
- Block ICMP traffic at network perimeters where it is not operationally required
- Implement strict network segmentation to isolate ICS/OT networks from untrusted network zones
- Deploy firewall rules to filter ICMP packets with IP options at network boundaries
- Consider deploying intrusion prevention systems (IPS) to detect and block exploitation attempts
# Example firewall rule to restrict ICMP with IP options (iptables)
# Note: Adapt to your specific firewall platform and operational requirements
iptables -A INPUT -p icmp --icmp-type echo-request -m u32 --u32 "0>>22&0x3C@0>>24=0x44:0x97" -j DROP
iptables -A INPUT -p icmp --icmp-type echo-request -d <SIEMENS_DEVICE_IP> -j LOG --log-prefix "ICMP-TO-ICS: "
Disclaimer: This content was generated using AI. While we strive for accuracy, please verify critical information with official sources.
