Executive Summary
SIMATIC CN 4100 firmware below version 5.0 contains a cluster of memory corruption defects including NULL pointer dereference, reachable assertion, use-after-free, and out-of-bounds write conditions that can be triggered through network-reachable interfaces. Because the CN 4100 functions as a communications node bridging control traffic, a successful trigger degrades or halts the data path between controllers and the wider network, which translates directly into loss of visibility and loss of control over the connected process.
Technical Exposure Breakdown
The vulnerable component is the SIMATIC CN 4100 communication node running firmware prior to version 5.0. The defect classes listed against CVE-2025-39713 span several distinct memory safety failures. A NULL pointer dereference and a reachable assertion both produce clean denial of service outcomes: the process faults and the device stops forwarding traffic. The use-after-free and out-of-bounds write conditions are the higher concern because they open the door to memory state manipulation, which can lead to code execution or altered device behavior rather than a simple crash.
The attack vector is network based. An adversary who can reach the device management or communication services with malformed input can drive the parser into one of these fault states. No physical access is required. The vendor scored this at 7.5, and the underlying equipment vulnerability rating sits at 9.6, reflecting the difference between a base availability impact and the full integrity and confidentiality exposure when the write primitives are weaponized. The practical severity in a given site depends on how exposed the CN 4100 management plane is and whether the affected services are reachable from anything other than a tightly controlled engineering segment.
Preconditions That Matter
- Network reachability to the CN 4100 communication and management services.
- Firmware below version 5.0.
- Flat or poorly segmented cell networks that place the node in reach of general operations traffic.
OT Impact and Compliance Risk
The CN 4100 sits on the communication path. When it faults, the controllers behind it lose their link to SCADA, historians, and remote engineering. Operators are left blind, and depending on architecture, automatic control loops may fail over, freeze, or drop to a safe state that itself carries process consequences. In a use-after-free exploitation scenario the worse outcome is undetected traffic manipulation, where the node continues to pass data but the integrity of that data can no longer be trusted.
For NERC CIP entities, an internet or corporate reachable CN 4100 in this state is a CIP-007 patch management and CIP-005 electronic security perimeter finding. Under IEC 62443, the exposure maps to failures in zone and conduit isolation and to secure development lifecycle expectations for the component. Pipeline operators under TSA SD-02C should treat this as a segmentation and patch cadence obligation against their approved implementation plan. Water and wastewater utilities operating under AWIA 2018 risk assessments should record the CN 4100 as a single point of communication failure.
Compensating Controls
Updating to version 5.0 is the vendor path, but firmware updates on a live communications node require a maintenance window and carry outage risk, so most sites will run compensating controls first. Do not perform active vulnerability scanning against the CN 4100 to confirm the flaw. The same malformed input classes that trigger these defects are exactly what a scanner emits, and you can fault or brick the node during the scan.
- Restrict the CN 4100 management and communication services to a dedicated engineering VLAN with explicit allow-list access control lists. Deny everything else.
- Place the node behind an OT firewall and block all inbound sessions that do not originate from known controller and engineering station addresses.
- Deploy a virtual patch at the network boundary. A Suricata rule concept here inspects traffic to the CN 4100 service ports for oversized or malformed protocol fields and drops sessions that exceed known-good length boundaries before they reach the parser. This shields the memory corruption path without touching the device.
- Monitor for repeated connection resets or device reboots, which are the observable signature of the assertion and NULL dereference conditions being probed.
Validate any rule in passive or copy mode against captured baseline traffic before enforcing, since false drops on a communication node cause the same availability loss you are trying to prevent.
BreachSpider Intel
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