Securing Programmable Logic Controllers (PLCs): Key Threats and Mitigations

Introduction

Programmable Logic Controllers (PLCs) are essential components in industrial automation systems. Originally developed to replace mechanical relays, PLCs have evolved into sophisticated devices for controlling complex manufacturing processes, robotic devices, and other activities that require high reliability, ease of programming, and fault diagnosis. They are used across various industries, including automotive, electronics, chemical manufacturing, and more. PLCs monitor inputs, make decisions based on programmed logic, and control outputs to automate machinery and processes.

However, the increasing connectivity and integration of PLCs into broader industrial networks expose them to numerous security threats. As PLCs become more interconnected through industrial Internet of Things (IoT) devices and cloud services, the risk of cyberattacks grows. Therefore, understanding and mitigating these threats is critical to maintaining the integrity, availability, and confidentiality of industrial control systems.

Top Threats & Mitigations to PLCs

1. Unauthorized Access

Threat: Attackers gain unauthorized access to the PLC, disrupting control and potentially causing significant operational damage. (ATT&CK Enterprise - T1078 - Valid Accounts)

CWE: CWE-287: Improper Authentication

STRIDE Category: Elevation of Privilege

Mitigations:

1. Implement strong, multi-factor authentication mechanisms for all access points to the PLC.
2. Regularly update and change passwords, avoiding default or easily guessable passwords.
3. Use role-based access control to ensure only authorized personnel have access to critical functions of the PLC. (ATT&CK Enterprise - M1026 - Privileged Account Management)
4. Monitor and log all access attempts and review logs regularly for any unauthorized attempts.
5. Implement network segmentation to isolate the PLC from other network segments, reducing the risk of lateral movement. (ATT&CK Enterprise - T1078 - Valid Accounts)

2. Malware Infection

Threat: Malicious software infiltrates the PLC, altering its programming or data, leading to incorrect operations or shutdowns.

CWE: CWE-506: Embedded Malicious Code

STRIDE Category: Tampering

Mitigations:

1. Install and regularly update anti-malware software specifically designed for industrial control systems. (ATT&CK Enterprise - M1049 - Antivirus/Antimalware)
2. Conduct regular scans of all devices connected to the PLC to detect and remove malware.
3. Use whitelisting to ensure only approved software and firmware can be executed on the PLC.
4. Train personnel on the safe handling of removable media and the dangers of malware.
5. Regularly update the PLC's firmware and software to the latest secure versions. (ATT&CK Enterprise - M1031 - Software Update)
6. Monitor and report on changes to PLC software and ladder diagrams

3. Network-Based Attacks

Threat: Attackers target the PLC through the network, causing denial of service or unauthorized control. (ATT&CK Enterprise - T1499 - Endpoint Denial of Service)

CWE: CWE-400: Uncontrolled Resource Consumption

STRIDE Category: Denial of Service

Mitigations:

1. Segment the network to isolate the PLC from other less secure network segments. (ATT&CK Enterprise - M1030 - Network Segmentation) Although air gapping has been shown to not be foolproof in protecting from lateral movement of malware or network-based attacks, it can certainly provide a layer of protection. Additional considerations should be taken for monitoring for additional types of communication such as NFC or Radio Frequency communications on some basis. See this article for more information about circumventing air gapping - New PLC Hack Jumps the Air Gap
2. Use firewalls and intrusion detection/prevention systems (IDS/IPS) to monitor and control traffic to and from the PLC. (ATT&CK Enterprise - M1030 - Network Segmentation)
3. Implement rate limiting and traffic shaping to prevent denial-of-service attacks.
4. Monitor network traffic for unexpected communication between endpoints
5. Regularly update network devices and firmware to patch known vulnerabilities.
6. Conduct regular network security assessments to identify and address potential vulnerabilities.

4. Physical Tampering

Threat: Physical access to the PLC allows attackers to manipulate components or connections, leading to disruptions.(ATT&CK Enterprise - T1076 - Physical Access)

STRIDE Category: Tampering

CWE: CWE-284: Improper Access Control

Mitigations:

1. Install physical security controls such as locks, security cameras, and access control systems to restrict unauthorized access to the PLC. (ATT&CK Enterprise - M1035 - Physical Access Control)
2. Use tamper-evident seals and enclosures for PLC units to detect any unauthorized physical access.
3. Implement security patrols and monitoring to detect and respond to physical security incidents promptly. (ATT&CK Enterprise - M1049 - Monitor Physical Access)
4. Conduct regular physical security audits to identify and address potential vulnerabilities.
5. Train personnel on the importance of physical security and the procedures for reporting suspicious activities.

5. Data Interception and Manipulation

Threat: Intercepted data between the PLC and other systems can be altered, leading to unauthorized control or disclosure of sensitive information. (ATT&CK Enterprise - T1040 - Network Sniffing)

STRIDE Category: Information Disclosure & Tampering

CWE CWE-319: Cleartext Transmission of Sensitive Information

Mitigations:

1. Use encryption for all data transmissions to and from the PLC to prevent interception and manipulation. (ATT&CK Enterprise - M1041 - Encrypt Sensitive Information)
2. Implement secure communication protocols such as TLS or IPSec for network communication.
3. Regularly update and patch communication protocol software to protect against known vulnerabilities.
4. Use network security tools to detect and block unauthorized interception attempts.
5. Conduct regular security assessments to identify and mitigate potential communication vulnerabilities.

6. Insider Threats

Threat: Authorized individuals misuse their access to cause harm or steal data.

STRIDE Category: Information Disclosure

CWE: CWE-284: Improper Access Control

Mitigations:

1. Establish strict access controls and limit access to the PLC based on the principle of least privilege. (ATT&CK Enterprise - M1026 - Privileged Account Management)
2. Implement monitoring and logging mechanisms to detect and investigate suspicious activities by insiders.
3. Conduct regular audits and reviews of access logs to identify potential misuse.
4. Provide security training and awareness programs to employees to help them recognize and report insider threats.
5. Implement a robust incident response plan to address insider threats promptly and effectively.
6. Restrict the egress of data from sensitive assets across the organization (ATT&CK Enterprise - M1026 - Privileged Account Management)

7. Exploitation of Vulnerabilities

Threat: Attackers exploit vulnerabilities in the PLC's firmware or software.

STRIDE Category: Tampering

CWE: CWE-1329: Reliance on Component That is Not Updateable

Mitigations:

1. Establish a regular schedule for updating the PLC's firmware and software to the latest secure versions provided by the manufacturer. (ATT&CK Enterprise - M1031 - Software Update && ATT&CK Enterprise - M1051 - Update Software)
2. Subscribe to security bulletins from the PLC manufacturer to stay informed about newly discovered vulnerabilities and patches.
3. Implement automated update mechanisms where possible to ensure timely deployment of patches.
4. Test all updates in a controlled environment before applying them to production systems to ensure compatibility and stability.
5. Maintain an inventory of all PLCs and their firmware versions to track and manage updates efficiently.

8. Supply Chain Attacks

Threat: Compromised components introduced via the supply chain. (ATT&CK Enterprise - T1195 - Supply Chain Compromise)

STRIDE Category: Tampering

CWE: CWE-494: Download of Code Without Integrity Check

Mitigation:

1. Establish strict criteria for selecting and vetting suppliers to ensure they meet security standards. (ATT&CK Enterprise - M1036 - Supply Chain Management)
2. Require suppliers to provide security certifications and conduct regular security audits of their processes.
3. Implement integrity checks for all components received from suppliers, such as cryptographic signing and verification.
4. Maintain visibility into the entire supply chain and track the provenance of all components.
5. Conduct regular security assessments and penetration testing of PLC components to identify and mitigate vulnerabilities.

Conclusion

Securing PLCs requires a comprehensive approach that addresses both technical and operational aspects. By understanding these key threats and implementing robust mitigations, organizations can protect their industrial control systems from potential attacks, ensuring the continuity and safety of their operations.

Threat modeling during the design and integration phases of PLCs can significantly reduce risks by identifying potential vulnerabilities early and applying appropriate security measures proactively. By integrating threat modeling into the development lifecycle, organizations can build more resilient systems, minimize the impact of security incidents, and maintain operational integrity.

For more detailed guidance, refer to industry standards and best practices from sources like NIST, CIS, and ISO/IEC. Stay informed, stay secure.