How Stuxnet Spreads - A Study of Infection Paths in Best Practice Systems

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The Stuxnet worm is a sophisticated piece of computer malware designed to sabotage industrial processes controlled by Siemens SIMATIC WinCC, S7 and PCS 7 control systems. The worm used both known and previously unknown vulnerabilities to spread, and was powerful enough to evade state-of-the-practice security technologies and procedures.

Since the discovery of the Stuxnet worm in July 2010, there has been extensive analysis by Symantec, ESET, Langner and others of the worm’s internal workings and the various vulnerabilities it exploits. From the antivirus point of view, this makes perfect sense. Understanding how the worm was designed helps antivirus product vendors make better malware detection software.

What has not been discussed in any depth is how the worm might have migrated from the outside world to a supposedly isolated and secure industrial control system (ICS). To the owners and operators of industrial control systems, this matters. Other worms will follow in Stuxnet's footsteps and understanding the routes that a directed worm takes as it targets an ICS is critical if these vulnerable pathways are to be closed. Only by understanding the full array of threats and pathways into a SCADA or control network can critical processes be made truly secure.

It is easy to imagine a trivial scenario and a corresponding trivial solution:
Scenario: Joe finds a USB flash drive in the parking lot and brings it into the control room where he plugs it into the PLC programming station.
Solution: Ban all USB flash drives in the control room.

While this may be a possibility, it is far more likely that Stuxnet travelled a circuitous path to its final victim. Certainly, the designers of the worm expected it to - they designed at least seven different propagation techniques for Stuxnet to use. Thus, a more realistic analysis of penetration and infection pathways is needed.

This White Paper is intended to address this gap by analyzing a range of potential "infection pathways" in a typical ICS system. Some of these are obvious, but others less so. By shedding light on the multitude of infection pathways, we hope that the designers and operators of industrial facilities can take the appropriate steps to make control systems much more secure from all threats.

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