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By Heather MacKenzie
Industrial wireless applications are seeing more and more action by leading manufacturers and operators to improve availability and reduce costs. That sounds great, but before considering it for your facility, you might be wondering how difficult it is to make sure industrial wireless networks are secure.

The good news is the best practices, technologies and products available now make it straightforward for engineering teams to implement wireless applications securely.

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This two-part series explains how a Defense in Depth approach, combined with WLAN protection and detection strategies, work together for secure wireless network design.

Wireless applications are no different that wired applications when it comes to an essential ICS security best practice – Defense in Depth (DiD). DiD is a holistic approach built on three core concepts:

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1. Multiple layers of defense: A variety of security solutions end up used so if an attacker bypasses one area, another can provide the needed defense.
2. Differentiated layers of defense: Each security layer is slightly different so an attacker can′t automatically get through all layers of defense.
3. Threat-specific layers of defense: Each defense is for the specific context and threat, allowing protection based on the behavior and context of the systems using these protocols.

Whether a threat is an accidental internal incident or a deliberate external attack, a DiD approach will detect, isolate and control it. The wireless defense strategies outlined in the rest of this series work together to provide the layers of protection needed to make sure your WLAN is secure.

Protection Technique #1
A challenge with WLAN transmission paths is they can broadcast outside a company’s property boundaries. Thus attackers don′t need direct, physical access to an industrial network in order to interfere with its operation and capture critical and confidential information.

That sounds scary, but in fact industry cooperation has led to standards, IEEE 802.11i/WPA2, that protect the confidentiality and integrity of wireless data. All current products on the market must comply with these standards, ensuring control system communications are authentic and attackers cannot extract sensitive data.

In regard to WPA2, be sure to implement its Enterprise mode for strong device authentication. Unlike personal networks, WPA2 (Enterprise mode) provides different keys for different devices, with the keys managed in a central database such as RADIUS. Lost or stolen devices can be disconnected from the network simply by removing their information from the database.

Furthermore, with WPA2 (Enterprise mode), individual devices can be assigned to different virtual LANs (VLANs) so devices with different roles can be clearly differentiated. This segmentation makes it difficult for an attacker to gain further access to a network should a single device be compromised – an example of DiD.

Protection Technique #2
Another aspect of wireless communications you want to protect is management frames. These are network packets that organize the internal operation of the network and they are exceptionally vulnerable to forgery and wiretapping.

Devices use management frames to log on and off the network, initiate new key exchanges and report when they roam from one access point to another. Information can be captured from wiretapped management frames, and forged management frames can be sent out with a wrong sender identity. An attacker can disrupt the operation of the network by disconnecting a victim device from the network.

To combat such attacks, Protected Management Frames (PMF) encrypt and protect management frames against forgery. They extend the mechanism for authentication and encryption present in WPA2 to management frames. By using products with the PMF capability you make it impossible for misused management functions to attack a network.

Protection Technique #3
Even the most effective WLAN encryption doesn’t offer protection when a security incident originates inside the network. But, by selectively limiting communication to only what is required to run the industrial application, you establish additional barriers that limit the impact of internal attacks.

This type of limitation is another Defense in Depth mechanism that considerably increases the all-around security of a network. Tried and true strategies for limiting communication within the network are:
• Protect WLAN data by implementing a configurable Layer 2 firewall at the Ethernet level. To do this you need to make sure you are using Access Points with a built-in Layer 2 firewall. The best ones can filter routed and bridged traffic as well as packet filter traffic between WLAN clients.
• Apply stateful Deep Packet Inspection (DPI) to secure protocols. After the Layer 2 firewall rules are applied, the DPI firewall inspects the content of the contained messages and applies more detailed rules. For example, a Modbus DPI firewall can determine if the Modbus message is a read or a write message and then drop all write messages. Good DPI firewalls can also “sanity check” traffic for strangely formatted messages or unusual behaviors.

Be aware that Deep Packet Inspection is sometimes known by other terms, such as content inspection or protocol whitelisting. It is not a widely available capability.

DPI firewalls are often used to protect zones of equipment with similar security requirements as per ISA IEC 62443 or to protect equipment critical to the process.

Key Strategy
Industrial wireless benefits such as improved uptime, reduced costs, faster and more accurate data collection are readily achievable today. Protecting data, devices and the control network is possible using the techniques described in this article.

To recap:
• All new wireless products comply with IEEE 802.11i and WPA2 for data confidentiality and integrity. Check any wireless currently in use to make sure they comply with these two standards. If you have products that support the predecessor to WPA2, which is WEP, your network is at risk.
• Use WPA-enterprise mode for authentication of individual wireless devices.
• Implement VLANs to segment the network, restricting the impact of device compromise to a subsection of the control network
• Use products with Protected Management Frames (PMF) capability and make sure it is enabled
• Limit communication between WLAN clients to specific peers or protocols using a Layer 2 firewall built-in to Access Points
• Further protect key production cells or critical equipment using a DPI firewall

Our next installment in the series will examine the second pillar of secure wireless network design: Anomaly detection strategies.
Heather MacKenzie is with Tofino Security, a Belden company. Mark Cooksley is a product manager with Hirschmann Automation and Control and an expert on industrial cyber security. Click here to view Heather’s blog. This article is from the White Paper “A Construction Kit for Secure Wireless” written by Dr. Tobias Heer. Dr. Heer is the manager of embedded software development and functions for our Hirschmann industrial networking group.

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