The Stuxnet Sting

For the past week or so, we’ve been closely tracking a new family of threats called Stuxnet (a name derived from some of the filename/strings in the malware – mrxcls.sys, mrxnet.sys).  In the past few days, it has become a  popular topic of discussion amongst security researchers and in the media. First and foremost, we have recently released one additional signature for this threat, and urge our readers to be sure that you’ve got the latest anti-malware definition updates installed.

Prevalence and distribution

In terms of numbers of attacks, the most reports are coming from the US, Indonesia, India, and Iran.  When you factor in the number of MMPC monitored machines along with the number that are reporting attacks, the US falls further down the list, giving way to Iran and Indonesia with attack attempts far higher than the global average.

Figure 1: Geographic saturation of Stuxnet infection attempts

Although the number of new machines reporting an infection attempt has remained constant at around a thousand per day, the number of attempts (tries per machine) has increased over the past few days:

Figure 2: Threat prevalence

Hacker exchange

In addition to these attack attempts, about 13% of the detections we’ve witnessed appear to be email exchange or downloads of sample files from hacker sites.  Some of these detections have been picked up in packages that supposedly contain game cheats (judging by the name of the file).

Threat details

What is unique about Stuxnet is that it utilizes a new method of propagation. Specifically, it takes advantage of specially-crafted shortcut files (also known as .lnk files) placed on USB drives to automatically execute malware as soon as the .lnk file is read by the operating system. In other words, simply browsing to the removable media drive using an application that displays shortcut icons (like Windows Explorer) runs the malware without any additional user interaction. We anticipate other malware authors taking advantage of this technique. Stuxnet will infect any usb drive that is attached to the system, and for this reason we’ve classified the malware as a worm.  This classification for the malware should not be confused with another vector used by this worm, the newly disclosed vulnerability (CVE-2010-2568) covered in today’s advisory.  The vulnerability itself is not wormable.

Stuxnet uses the aforementioned .lnk technique to install additional malware components.  It first injects a backdoor (Worm:Win32/Stuxnet.A) onto the compromised system, and then drops two drivers:

  • Trojan:WinNT/Stuxnet.A – hides the presence of the .lnk files
  • Trojan:WinNT/Stuxnet.B – injects (formerly) encrypted data blobs (.tmp files) into memory, each of which appear to serve different purposes as the Stuxnet deployment system infrastructure (drivers, .lnk files, propagation, etc.).

These drivers are signed with a digital certificate belonging to a well-known hardware manufacturer called Realtek Semiconductor Corp., which is unusual because it would imply that the malware authors somehow had access to Realtek’s private key.  Microsoft MMPC has been working with Verisign to revoke this certificate, and did so at 08:05:42 PM UTC with the agreement and support of Realtek.

Threat prevention

We have multiple signatures that detect this threat for customers using Microsoft Security Essentials, Microsoft Forefront Client Security, Windows Live OneCare, the Forefront Threat Management Gateway, and the Windows Live Safety Platform. In addition to using antimalware technology, MSRC has released an advisory with work-around details.

Initial malware (the dropper):

Malware the dropper attempts to drop onto the system:
Trojan:WinNT/Stuxnet.B (initially called VirTool:WinNT/Rootkitdrv.HK)

Attack vector (.lnk files):
Exploit:Win32/CplLnk.A (added recently – versions

We suspect that Stuxnet has been active for at least a month, possibly longer… we have detection for it and its various components and will keep you posted with developments as our talented researchers (like Matt McCormack, Holly Stewart, Peter Ferrie, Patrick Nolan, Andrei Florin Saygo and Francis Allan Tan Seng) continue tracking this threat.

–Tareq Saade

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