This is an updated version of this article, which was first appeared on Belden Blogs on March 3, 2012 and on on June 30, 2011

Recently I gave a talk focused on air gaps as a security strategy in control systems. The talk was at the AusCERT 2012 conference and to my amazement, it generated a large amount of discussion in the media both inside and outside Australia. Here are a few examples:

While all this interest is very heartening, a number of the people commenting seem to have misunderstood my message. Today I am writing to make my views on air gaps a bit clearer.

Eric Presenting at AusCERT 2012

Eric Byres presenting "Unicorns and Air Gaps" at AusCERT 2012

Supporters of Air Gaps Do Exist

The theory of the air gap sounds great; by creating a physical gap between the control network and the business network, bad things like hackers and worms can never get into critical control systems. But as you can probably guess from the title of my blog, I don’t believe that true air gaps actually exist in the ICS and SCADA world.

Certainly, there are people that disagree with me outright. For example, Paul Ferguson, an Internet Security Intelligence blogger at Trend Micro wrote:

I've written about SCADA issues in the past, but one issue that I’ve consistently tried to emphasize is that critical control systems should never, ever interact nor interconnect with Internet systems in any way, shape, or form. There’s a good reason for this, and it’s always been referred to as the “Air Gap” Principle.” 1

(ENote from Eric Byres July 12, 2012: Paul has informed me that this is an old opinion (circa 2008) and he has since changed his views on the feasibility of air gaps. Sorry Paul! For more on this, see my subsequent blog article.)

Similarly, last year there was a flood of SCADA and ICS vulnerability notices with advice on addressing the issue by using an air gap. One example I gave in the past came from the original Siemens Security Advisory addressing the vulnerabilities in Siemens SIMATIC S7-1200 PLC line:

"In addition, it is important to ensure your automation network is protected from unauthorized access using the strategies suggested in this document or isolate the automation network from all other networks using an air gap.” 2

Vendors are Moving Away from the “Air Gap Principle”

Now the interesting thing (and a real credit to Siemens) is that they removed this recommendation from this advisory (and all other advisories) a few months later.

I suspect that Stefan Woronka, Siemens Director of Industrial Security Services, had something to do with this when he publically stated:

“Forget the myth of the air gap – the control system that is completely isolated is history.”

Similarly, all the Schneider Electric and Rockwell security advisories make no mention of air gaps. Rockwell’s mitigation guidance is very clear:

“Block all traffic to the EtherNet/IP or other CIP protocol based devices from outside the Manufacturing Zone by restricting or blocking access to TCP and UDP Port# 2222 and Port# 44818 using appropriate security technology (e.g. a firewall, UTM devices, or other security appliance)." Source: KB Article 470154 - EtherNet/IP™ Product Vulnerabilities

I think that all of the PLC and DCS vendors have come to realize that air gaps conflict with their architectures. For example, check out the diagram of a high security architecture taken directly from the Siemens’ Security Concept manual (pg 42). (Note: you can click on the image to enlarge it.)


Can you spot the air gap in the drawing? I can’t! Or try another vendor - download the security manual from Rockwell, search for the term “Air Gap”. You won’t find it. Search the diagrams for an air gap. You won’t find it. Check out all the major vendors engineering guides and you won’t find the air gap mentioned anywhere (if you do find an example of a PLC or DCS vendor recommending air gaps, please send it to me).

Air Gaps Don’t Work in the Real World

There is a good reason why you won’t find the air gap mentioned in vendor engineering manuals and why it is disappearing from security advisories. As a theory, the air gap is wonderful. In real life, it doesn’t work.

Sure you can simply unplug the connection between the control system and the business network and presto, you have an "air gap”. Then one day you get new logic from your engineering consultant – perhaps it addresses a design flaw that has been causing your company considerable downtime. A little while later Adobe sends you a software update – perhaps it is for a critical vulnerability in the PDF Reader your staff uses to view operational manuals. Next your lab group sends a process recipe that will improve product quality. The list keeps growing – patches for your computer operating systems, anti-virus signatures, remote support and system software – you can’t ignore them all.

So what do you do? Maybe you load some files onto a USB drive and carry that onto the plant floor. But isn’t that how Stuxnet spread? Or maybe putting everything onto a laptop is the solution, but what if the laptop is infected? A serial line and a modem – sorry, the Slammer worm got into a number of control systems that way. Even the trusty CD can be turned into the carrier of evil bits.

As much as we want to pretend otherwise, modern control systems need a steady diet of electronic information from the outside world. Severing the network connection with an air gap simply spawns new pathways – pathways like the mobile laptop and the USB key, which are more difficult to manage and just as easy to infect.

New call-to-actionAnyone Who Has Ever Seen an Air Gap, Please Raise Your Hand

So are there air gaps in any control systems? Sure – in trivial systems. For example, the digital thermostat controlling the heat pump in my home probably has a true air gap. And maybe in very very high risk systems – for example, I am led to believe that reactor control systems in nuclear plants are truly air gapped.

But do air gaps exist for all the control systems that manage our power grid, our transportation systems, our water and our factories? I will let Mr. Sean McGurk, the Director, National Cybersecurity and Communications Integration Center (NCCIC) at the Department of Homeland Security answer that:

"In our experience in conducting hundreds of vulnerability assessments in the private sector, in no case have we ever found the operations network, the SCADA system or energy management system separated from the enterprise network. On average, we see 11 direct connections between those networks. In some extreme cases, we have identified up to 250 connections between the actual producing network and the enterprise network.3

A control system protected by a real air gap

Time to Grow Up and End the Fairy Tale

The control system vendors have accepted that the dream of the air gap as a security strategy is finished. The government agencies like ICS-CERT have also accepted that a true air gap is impossible. Now it is time for the consultants and end-users to give up on the air gap myth. Believing that your plant’s security is under control because the control systems are “isolated” is just a dangerous illusion:

"None of the vulnerabilities [uncovered at the NESCOR summit] pose as great a risk as the belief that your system is isolated." - Chris Blask, CEO, ICS Cybersecurity Inc.

For effective ICS and SCADA security, the entire industry needs to move past the myth of air gaps and learn to deal with the reality:

All control systems are connected to the outside world in some fashion. It might be a network connection, a serial line or USB “sneakernet”, but it is a pathway that can be exploited by modern malware like Stuxnet and Flame. Cyber security countermeasures must face up to this fact.

1 Source: Paul Ferguson, Internet Security Intelligence, Advanced Threats Research, Trend Micro, Apr 8, 2008 Ed. Note:  This source reference was updated from 2012 to 2008 on July 6, 2012 as per Paul Ferguson's comment below.
2 Source: SIEMENS-SSA-625789: Security Vulnerabilities in Siemens SIMATIC S7-1200 CPU
3 Source: The Subcommittee on National Security, Homeland Defense, and Foreign Operations May 25, 2011 hearing. 58:30 -- 59:00

Related Links

•Blog: Air Gaps won’t Stop Stuxnet’s Children
•Blog: Cyber Attacks on U.S. Critical Infrastructure will Intensify
•Blog: Flame Malware and SCADA Security: What are the Impacts?