Thanks to some tips from a Dutch Profibus expert who responded to our call for help, we’ve connected a critical piece of the puzzle.

Since our discovery that Stuxnet actually modifies code on PLCs in a potential act of sabotage, we have been unable to determine what the exact purpose of Stuxnet is and what its target was.

However, we can now confirm that Stuxnet requires the industrial control system to have frequency converter drives from at least one of two specific vendors, one headquartered in Finland and the other in Tehran, Iran. This is in addition to the previous requirements we discussed of a S7-300 CPU and a CP-342-5 Profibus communications module.

The target system would potentially look something like the diagram below:

A frequency converter drive is a power supply that can change the frequency of the output, which controls the speed of a motor. The higher the frequency, the higher the speed of the motor.

The new key findings are:

We are now able to describe the purpose of all of Stuxnet’s code.

Stuxnet requires particular frequency converter drives from specific vendors, some of which may not be procurable in certain countries.

Stuxnet requires the frequency converter drives to be operating at very high speeds, between 807 Hz and 1210 Hz. While frequency converter drives are used in many industrial control applications, these speeds are used only in a limited number of applications.

Stuxnet changes the output frequencies and thus the speed of the motors for short intervals over periods of months. Interfering with the speed of the motors sabotages the normal operation of the industrial control process.

Stuxnet’s requirement for particular frequency converter drives and operating characteristics focuses the number of possible speculated targets to a limited set of possibilities.

Stuxnet monitors the current operating frequency of these motors, which must be between 807 Hz and 1210 Hz, before Stuxnet modifies their behavior. Relative to the typical uses of frequency converter drives, these frequencies are considered very high-speed and now limit the potential speculated targets of Stuxnet. We are not experts in industrial control systems and do not know all the possible applications at these speeds, but for example, a conveyor belt in a retail packaging facility is unlikely to be the target. Also, efficient low-harmonic frequency converter drives that output over 600Hz are regulated for export in the United States by the Nuclear Regulatory Commission as they can be used for uranium enrichment. We would be interested in hearing what other applications use frequency converter drives at these frequencies.

Once operation at those frequencies occurs for a period of time, Stuxnet then hijacks the PLC code and begins modifying the behavior of the frequency converter drives. In addition to other parameters, over a period of months, Stuxnet changes the output frequency for short periods of time to 1410Hz and then to 2Hz and then to 1064Hz. Modification of the output frequency essentially sabotages the automation system from operating properly. Other parameter changes may also cause unexpected effects.

With this discovery, we now understand the purpose of all of Stuxnet’s code. We’ve modified our paper, in particular multiple subsections of the Modifying PLCs section, to include the finer details. Since we are far from experts in industrial control systems, we appreciate any feedback or further tips or explanation of some of the data. You can click on my name at the top of the blog post to get in touch.

We’d like to sincerely thank the Dutch Profibus expert who got in touch, serving as the catalyst to this breakthrough in understanding the purpose and potential targets of Stuxnet.

Here is the link to the updated paper.

Also, last month we presented how Stuxnet hijacks PLCs at the Virus Bulletin conference. As part of that presentation, we performed a live demonstration. Because we couldn’t afford to purchase a gas refinery or waste management system, we had to settle for balloons. We’ve created a video of the demonstration, which you can watch below.