CVE-2025-39742: Multiple Memory Corruption Flaws in Siemens SIMATIC CN 4100 Communication Node

Executive Summary

CVE-2025-39742 covers a cluster of memory corruption defects in the Siemens SIMATIC CN 4100 communication node running versions below 5.0, including NULL pointer dereference, reachable assertion, use after free, and out-of-bounds write conditions that a network-reachable attacker can trigger. Because the CN 4100 functions as a communication and connectivity node bridging industrial segments, exploitation can degrade or halt the data path between control zones, which translates directly into loss of process visibility and coordinated control across a plant.

Technical Exposure Breakdown

The vulnerable component is the SIMATIC CN 4100 firmware stack in all versions prior to 5.0. The advisory groups several distinct memory safety weaknesses under one identifier. A NULL pointer dereference and a reachable assertion both provide a clean denial of service primitive, forcing the device into a fault or restart state on receipt of malformed input. The use after free and out-of-bounds write conditions are more consequential because they can corrupt memory the attacker does not directly own, which is the class of defect that moves from denial of service toward code execution or integrity manipulation of transiting traffic.

The attack vector is network reachability to the CN 4100 service interfaces. No physical access is required. The vendor scored the equipment vulnerability set at 9.6 while the aggregate CVSS on this record sits at 7.5, and the gap between those numbers reflects how much the impact depends on where the node sits and what it is bridging. In a flat OT network with no segmentation between the CN 4100 and engineering or corporate zones, the higher end of that range is the realistic assumption.

Operators should note that confirming exposure through active probing is dangerous here. Sending crafted or even malformed packets to a device with a known reachable assertion and out-of-bounds write is functionally an exploitation attempt. Active scanning can brick industrial components, and this advisory names the exact fault classes that make scanning risky. Inventory and version identification should come from passive traffic analysis and asset management records, not from live port and protocol sweeps.

OT Impact and Compliance Risk

The physical consequence is loss of the communication path. If the CN 4100 crashes or restarts under attack, control and telemetry crossing that node stop. Operators lose visibility into downstream process state and may lose the ability to issue coordinated commands until the node recovers. The memory write conditions raise a second concern: integrity of the traffic the node handles, which undermines the trust an operator places in the values displayed at the HMI.

For NERC CIP entities this touches CIP-007 patch management and CIP-005 electronic security perimeter posture, since an unsegmented communication node inside the perimeter widens the reachable attack surface. Under IEC 62443 this is a zone and conduit failure: the CN 4100 is a conduit device, and a vulnerable conduit collapses the segmentation model that 62443-3-3 depends on. Pipeline operators under TSA SD-02C should treat this within their network segmentation and continuous monitoring obligations. Water and wastewater utilities carrying communication nodes of this type should fold the finding into their AWIA 2018 risk and resilience assessment cycle.

Compensating Controls

Update to SIMATIC CN 4100 version 5.0 where a maintenance window allows, but do not rely on that alone in an OT environment where patch windows run in months. Immediate compensating controls should include strict conduit filtering so that only enumerated engineering hosts can reach the CN 4100 management and service interfaces, enforced at a firewall boundary rather than on the device itself. Place the node in a dedicated VLAN and deny lateral reachability from general OT and corporate address space.

A virtual patch approach fits well here. Deploy IDS coverage on the conduit and write detection for anomalous or malformed traffic to the CN 4100 service ports. A Suricata rule concept would alert on oversized or malformed protocol fields directed at the node, and on repeated session resets consistent with the assertion or NULL dereference being triggered, which serves as early warning of exploitation attempts before the device faults.

BreachSpider Intel

BreachSpider tracks CVE-2025-39742 and the wider SIMATIC exposure surface across the OT vulnerability corpus, and operators can monitor affected assets and mapped compensating controls through the BreachSpider platform.