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How the Worm Turned

Stuxnet versus the Iranian nuclear program.

Dec 13, 2010, Vol. 16, No. 13 • By JONATHAN V. LAST
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Last week Mahmoud Ahmadinejad acknowledged that Iran’s uranium enrichment program had suffered a setback: “They were able to disable on a limited basis some of our centrifuges by software installed in electronic equipment,” the Iranian president told reporters. This was something of an understatement. Iran’s uranium enrichment program appears to have been hobbled for the better part of a year, its technical resources drained and its human resources cast into disarray. The “software” in question was a computer worm called Stuxnet, which is already being viewed as the greatest triumph in the short history of cyberwarfare.

How the Worm Turned

Photo Credit: Gary Locke

Stuxnet first surfaced on June 17 of this year when a digital security company in Minsk, VirusBlokAda, discovered it on a computer belonging to one of its Iranian clients. It quickly became clear that Stuxnet was not an ordinary piece of malware.

Stuxnet is not a virus, but a worm. Viruses piggyback on programs already resident in a computer. Worms are programs in their own right, which hide within a computer and stealthily propagate themselves onto other machines. After nearly a month of study, cybersecurity engineers determined that Stuxnet was designed to tamper with industrial systems built by the German firm Siemens by overriding their supervisory control and data acquisition (SCADA) protocols. Which is to say that, unlike most malware, which exists to manipulate merely virtual operations, Stuxnet would have real-world consequences: It wanted to commandeer the workings of a large, industrial facility, like a power plant, or a dam, or a factory. Exactly what kind of facility was still a mystery.

From the beginning, everything about Stuxnet was anomalous. Worms that tampered with SCADA are not unheard of, but are exceptionally rare. And as a physical piece of code, Stuxnet was enormous—weighing in at half a megabyte, it dwarfed the average piece of malware by many multiples. Finally, there was its infection radius. Stuxnet found its way onto roughly 100,000 computers worldwide; 60 percent of these were in Iran.

As a work of engineering, Stuxnet’s power and elegance made it even more intriguing. Most industrial systems are run on computers which use Microsoft’s Windows operating system. Hackers constantly probe software for what are known as “zero day” vulnerabilities, weak points in the code never foreseen by the original programmers. On a sophisticated and ubiquitous piece of software such as Windows, discovering even a single zero day vulnerability is extremely uncommon. The makers of Stuxnet found, and utilized, four of them. No one in cybersecurity had ever seen anything like it.

The worm gained initial access to a system through an ordinary USB drive. Picture what happens when you plug a flash drive into your computer. The machine performs a number of tasks automatically; one of them is pulling up icons to be displayed on your screen, representing the data on the drive. On an infected USB drive, Stuxnet exploited this routine to pull the worm onto the computer.

The challenge is that once on the machine, the worm becomes visible to security protocols, which constantly query files looking for malware. To disguise itself, Stuxnet installed what’s called a “rootkit”—a piece of code that intercepts security queries and sends back false “safe” messages, indicating that the worm is innocuous.

But installing a rootkit requires using drivers, of which Windows machines are well trained to be suspicious. Windows requires that all drivers provide verification that they’re on the up-and-up through presentation of a secure digital signature. These digital keys are closely guarded secrets. Stuxnet’s malicious drivers presented genuine signatures from two genuine computer companies, Realtek Semiconductor and JMichron Technologies. Both firms have offices in the same facility, Hsinchu Science Park, in Taiwan. Either by electronic trickery or a brick-and-mortar heist job, the creators of Stuxnet stole these keys​—and in a sophisticated enough manner that no one knew they had been compromised.

So to recap: The security keys enable the drivers, which allow the installation of the rootkit, which hides the worm that was delivered by the corrupt USB drive. Stuxnet’s next job was to propagate itself efficiently but quietly. Whenever another USB drive was inserted into an infected computer, it became infected, too. But in order to reduce traceability, Stuxnet allowed each infected USB drive to pass the worm onto only three computers.

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