Defending Against an Active Threat to Siemens S7 Series PLCs

The National Security Agency (NSA), Cybersecurity and Infrastructure Security Agency (CISA), Federal Bureau of Investigation (FBI), Department of Energy (DOE), and Environmental Protection Agency (EPA)—hereafter referred to as the authoring agencies—are releasing this Cybersecurity Advisory to warn owners and operators of industrial control systems (ICSs) of an active cyber threat to Siemens S7 Series PLCs and provide relevant mitigations to protect and defend them.

The threat actors are conducting reconnaissance and capability development against U.S.-based Siemens PLC installations using AI-generated exploitation scripts disguised as legitimate monitoring tools. The actors leverage Internet scanning services to find Internet-exposed PLCs running outdated software or that are otherwise poorly protected. The U.S. critical infrastructure sectors most targeted by this threat activity include Critical Manufacturing, Energy, Water and Wastewater, Chemical, Food and Agriculture, and Commercial Facilities. This is not a theoretical risk—it is an active threat. Depending on the specific circumstances, exploitation of poorly protected PLCs could lead to disruption of critical industrial processes, safety incidents, downtime or equipment damage, compromise of sensitive data, compliance violations, and cascading impacts across interconnected systems.

The authoring agencies urge all owners and operators of operational technology (OT) systems using Siemens S7 Series and other PLC devices to proactively check their systems:

- are properly protected with all applicable security patches and updates,

- are isolated from the Internet wherever possible,

- have strong access controls, and

- employ security tooling to monitor ICS environments for anomalous or malicious activity.

These mitigations are particularly important for owners and operators who work with third-party service providers or system integrators who may have remote access to PLCs, as the asset owners may not realize that their systems are exposed and at risk.

Note: This advisory uses the MITRE ATT&CK® Matrix for ICS1 framework, version 19, and the MITRE ATT&CK Matrix for Enterprise framework, version 19. This advisory also uses MITRE D3FENDTM, version 1.5.0. See Appendix A and Appendix B for tables of the activity mapped to MITRE ATT&CK and MITRE D3FEND tactics, techniques, and countermeasures.

Threat actors are actively targeting the following Siemens PLC models:

- S7-200 Series (all CPU variants)

- S7-300 Series (all CPU variants including 314, 315, 317 models)

- S7-400 Series (all CPU variants)

- S7-1200 Series (CPU 1211C, 1212C, 1214C, 1215C, 1217C variants)

- S7-1500 Series (all CPU variants, including F-series safety controllers)

Threat actors are using AI assistance to generate exploitation scripts using publicly available information on these Siemens S7 Series PLCs for initial access, credential access, denial of service, and other objectives. If these PLCs are exposed to the Internet or insufficiently segmented, then threat actors can exploit various critical and high severity known vulnerabilities in these PLCs.

Note: Using AI to generate exploitation scripts represents an evolution in threat actor capabilities, dramatically reducing the technical expertise and time required to develop working ICS exploitation scripts and malicious tools. In addition, AI enables adversaries to rapidly leverage additional attack vectors and adapt to defensive measures. Threat actors can easily collect public information about vulnerabilities and weaknesses, find exposed and exploitable PLCs, and use AI-generated scripts to act on that information. If PLCs are exposed to the Internet, they are at high risk for exploitation.

Threat actors are leveraging open source industrial automation libraries—specifically snap7.dll/python-snap7—combined with AI-assisted scripting to create custom tools that mimic legitimate OT monitoring solutions. These tools provide read/write access to Siemens S7 Series PLC memory, configuration data, and ladder logic programs via the S7comm protocol.

Threat actors are:

- Using Internet scanning services (e.g., Censys, ZoomEye) to identify Internet-exposed or insufficiently segmented Siemens S7 Series PLCs [T1596.005]

- Rapidly iterating exploit code through AI-assisted development, lowering technical barriers to ICS attacks [T1587.004, T1588.007]

- Taking advantage of insecure credentials to access exposed devices that have unconfigured (default) or minimally configured authentication [T1694]

- Deploying AI-generated Python scripts that incorporate the snap7.dll library from public repositories [T0834] to gain read/write access to the PLC and mimic legitimate tools

- Masquerading malicious scripts as legitimate monitoring tools to evade detection by security teams [T0849]

- Conducting read/write operations on data blocks, potentially for reconnaissance, capability testing, or pre-positioning for effects operations [T0893, T0821]

The authoring agencies assess this activity pattern is likely intended as persistent reconnaissance in targeted sectors and facilities to develop capabilities and prepare to cause operational effects against critical infrastructure. For capability development, actors are testing and refining their exploitation techniques against specific PLC models to improve their ability to compromise the PLCs. To prepare for operational effects, actors are leveraging read access to understand target environments, enabling preparation and positioning for future write operations to cause disruption or other operational impacts.

The U.S. critical infrastructure sectors most targeted by this threat activity include Critical Manufacturing, Energy, Water and Wastewater, Chemical, Food and Agriculture, and Commercial Facilities. Additionally, Siemens S7 Series PLCs are used in other sectors, including the Defense Industrial Base (DIB), and could be targeted there as well. Unauthorized access to PLCs could result in:

- Disruption of critical industrial processes affecting production throughput, product quality, and public services

- Safety incidents affecting personnel through manipulation of safety interlocks, emergency shutdown systems, or process parameters

- Equipment damage and extended operational downtime from process upsets, improper sequencing, or forced equipment operation outside design parameters

- Compromise of sensitive operational data, including proprietary process recipes, control strategies, and facility configurations

- Cascading impacts across interconnected systems affecting supply chains, dependent facilities, and integrated business operations

- Regulatory compliance violations and potential liability from process safety management failures

Since threat actors are developing capabilities using AI to compromise PLCs using known vulnerabilities, misconfigurations, and other weaknesses and then may use compromised PLCs to interfere with normal operations, the authoring agencies urge organizations to implement comprehensive defense-in-depth strategies, in addition to Common Vulnerabilities and Exposures (CVE) remediation, to protect and defend their PLCs.

Organizations should implement detection strategies and hunt for anomalies that may indicate a compromise, focusing on [D3-PM]:

- Anomalous S7comm behavior: Connections from non-engineering workstations, unusual data block access patterns, or write operations outside change windows

- Reconnaissance indicators: Sequential IP scanning on port 102, repeated connection attempts with varying parameters, or enumeration of CPU properties

- Tool artifacts: Snap7.dll library usage outside approved engineering workstations, Python scripts with S7comm functionality, or unauthorized monitoring software installations

- Temporal anomalies: S7comm activity during off-hours, unexpected connection patterns consistent with automated scripting rather than human operators, or configuration changes without corresponding work orders or change tickets

- Geographic anomalies: Connections originating from unexpected countries or IP ranges not associated with vendors or integrators

To counter threats to PLCs, the authoring agencies recommend all PLC owners and operators follow the mitigations in joint guidance Primary Mitigations to Reduce Cyber Threats to Operational Technology.