Executive Summary
CVE-2025-39841 covers a cluster of memory corruption defects in the Siemens SIMATIC CN 4100 communication node running firmware below version 5.0, including NULL pointer dereference, reachable assertion, use-after-free, and out-of-bounds write conditions that an attacker can trigger to degrade or corrupt the device. Because the CN 4100 sits in the communication path for distributed SIMATIC deployments, exploitation can sever the data plane between controllers and supervisory systems, stalling process visibility and control at the exact moment operators need it.
Technical Exposure Breakdown
The vulnerable component is the network stack and service handling logic within the SIMATIC CN 4100 firmware prior to 5.0. The advisory groups multiple weakness classes under a single identifier, and each has a distinct exploitation profile.
- NULL pointer dereference and reachable assertion: These map to denial of service. A malformed packet or unexpected protocol state drives the device into a crash or forced reset. On a communication node, a reset is not a cosmetic event. It drops every session traversing that node.
- Use-after-free and out-of-bounds write: These are the higher-severity primitives. A use-after-free can be shaped into controlled memory reuse, and an out-of-bounds write gives an attacker the ability to corrupt adjacent structures. In combination, these move the exposure from availability loss toward code execution or integrity compromise, which is why the vendor-scored equipment vulnerability rating reaches 9.6 while the base CVSS sits at 7.5.
The attack vector is network-reachable. There is no requirement for physical access. Any adversary with a path to the CN 4100 management or communication interfaces can attempt these triggers. In flat OT networks where the communication node shares broadcast domains with engineering workstations and controllers, the reachable surface is far larger than a segmented reference architecture would suggest.
A critical operational note. Do not attempt to confirm this exposure through active scanning or fuzzing on a production node. The very defect classes described here, reachable assertions and NULL dereferences, mean that an aggressive scan can crash the device and take down live communication. Fingerprint firmware versions passively or through the engineering toolchain, not with a network scanner pointed at the running node.
OT Impact and Compliance Risk
Physically, a compromised or crashed CN 4100 breaks the communication layer that carries process data and control commands. Loss of the node produces loss of view and, depending on architecture, loss of control. For continuous processes this can force a protective trip or leave operators blind during a transient. The integrity path is worse. An attacker who achieves memory corruption on a communication node is positioned to manipulate traffic in transit, which undermines every downstream trust assumption in the control system.
Compliance exposure is direct. Under IEC 62443-3-3, this defeats SR 3.1 communication integrity and SR 7.1 denial of service protection, and it stresses zone and conduit boundary requirements. For NERC CIP regulated entities, an internet or corporate reachable CN 4100 running affected firmware is a CIP-007 patch management and CIP-005 electronic security perimeter finding. Pipeline operators under TSA SD-02C must account for this in their network segmentation and critical cyber system inventory, and water utilities inside AWIA 2018 risk and resilience assessments should treat a communication node that carries SCADA traffic as an assessed asset.
Compensating Controls
The vendor fix is version 5.0 or later, but firmware updates on a communication node require an outage window and validation, so most sites need bridging controls first.
- Segmentation: Confine the CN 4100 to a dedicated conduit. Restrict management and communication access to an explicit allowlist of engineering hosts through a stateful firewall. Deny everything else.
- Virtual patching: Deploy inline detection at the conduit boundary. A Suricata rule concept here targets malformed protocol states and oversized fields destined for the node, alerting on packets whose length parameters exceed expected bounds for the CN 4100 service ports and dropping traffic that matches known corruption triggers once signatures stabilize.
- Monitoring: Alert on unexpected node resets and session resets. A crash-loop pattern is an early exploitation indicator for the assertion and dereference classes.
- Access reduction: Remove any exposure of the node to corporate or remote access paths until firmware is validated and deployed.
BreachSpider Intel
BreachSpider tracks CVE-2025-39841 and Siemens SIMATIC exposure across our monitored OT product catalog so your team sees affected node inventory and control status without touching the running device.