Back to Stuxnet: the missing link

Two weeks ago, when we announced the discovery of the Flame malware we said that we saw no strong similarity between its code and programming style with that of the Tilded platform which Stuxnet and Duqu are based on.

Flame and Tilded are completely different projects based on different architectures and each with their own distinct characteristics. For instance, Flame never uses system drivers, while Stuxnet and Duqu’s main method of loading modules for execution is via a kernel driver.

But it turns out we were wrong. Wrong, in that we believed Flame and Stuxnet were two unrelated projects.

Our research unearthed some previously unknown facts that completely transform the current view of how Stuxnet was created and its link with Flame.

The Flame inside Stuxnet

First of all, let’s recap the Stuxnet story. We managed to recover just three different variants of the worm, created in June 2009, and in March and April 2010.

The March 2010 variant was responsible for the greatest number of infections and was detected in June 2010 by specialists from the company VirusBlokAda in Belarus. This particular version was subjected to the most detailed analysis by anti-malware companies.

Shortly afterwards, when news of Stuxnet had already become widespread, files related to its June 2009 incarnation were detected. This version, the so-called Stuxnet.A (1.0), differed considerably from the 2010 variants.

The main differences were:

• The 2009 variant didn’t use the MS10-046 LNK file vulnerability
• In 2009, Stuxnet only had one driver file; in 2010 there were two (the second was added specifically to work with the LNK vulnerability)
• In 2009, Stuxnet used a special trick with the “autorun.inf” file to infect USB drives.

All the other differences involve minor modifications to Stuxnet’s internal structure – some modules were deleted and their functions transferred to other modules.

The most significant of those changes involved “resource 207”.

Resource “207” is 520,192 bytes in size and can be found in the 2009 version of Stuxnet. It was later dropped altogether in the 2010 version, its code merged into other modules.

List of resources in the March 2010 variant of Stuxnet

List of resources in the 2009 variant of Stuxnet

Despite the fact that Stuxnet has been the subject of in-depth analysis by numerous companies and experts and lots has been written about its structure, for some reason, the mysterious “resource 207” from 2009 has gone largely unnoticed. But it turns out that this is the missing link between Flame and Stuxnet, two seemingly completely unrelated projects.

The Tocy story

In October 2010, our automatic system received a sample from the wild. It analyzed the file thoroughly and classified it as a new Stuxnet variant, Worm.Win32.Stuxnet.s.

With Stuxnet being such a big thing, we looked at the sample to see what it was! Sadly, it didn’t look like Stuxnet at all, it was quite different. So we decided to rename it to Tocy.a and thought “silly automatic systems!”.

When Flame was discovered in 2012, we started looking for older samples that we might have received. Between samples that looked almost identical to Flame, we found Tocy.a.

Going through the sample processing system logs, we noticed it was originally classified as Stuxnet. We thought, how was it possible? Why did the system think that this Flame sample was related to Stuxnet? Checking the logs, we discovered that the Tocy.a, an early module of Flame, was actually similar to “resource 207” from Stuxnet. It was actually so similar, that it made our automatic system classify it as Stuxnet. Practically, Tocy.a was similar to Stuxnet alone and to no other sample from our collection.

Going back to the story, this is how we discovered the incredible link between Flame and Stuxnet.

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