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7/16/2010
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Malware Spreading Via USB Drives

The Stuxnet rootkit launches even with AutoRun and AutoPlay disabled and is known to affect Windows 7 Enterprise Edition x86 operating systems.

Security experts are warning of never-before-seen malware, dubbed Stuxnet, that spreads via USB drives, infecting PCs via an unknown -- aka zero-day -- Windows vulnerability. Unfortunately, the attack works even with AutoRun and AutoPlay disabled, and affects at least Windows 7 Enterprise Edition x86 operating systems.

The malware was first reported by VirusBlokAda, an antivirus company based in Belarus. According to an analysis released by the firm, "You just have to open [an] infected USB storage device using Microsoft Explorer or any other file manager that can display icons, to infect your operating system and allow execution of [the] malware program." Once a PC is infected, the rootkit takes additional steps to then camouflage the .lnk files and its system process threads.

Reportedly, the malware's purpose is to gather any information relating to Siemens SCADA (supervisory control and data acquisition) system software.

But exactly how Stuxnet works remains somewhat unclear. "Although analysis is not complete, it would appear that the flaw is in how Windows Explorer loads the image to display when showing a shortcut," according to Chester Wisniewski, a security expert at antivirus vendor Sophos. "This feature is being used to exploit a vulnerability and execute a DLL to load the malware on the system."

Here's what else is known: The malware uses .lnk files to "to launch files from USB storage devices, a method which hasn't been used before," said Wisniewski. To infect further USB drives, the malware also dynamically generates a new .lnk file for each new USB device. "At this time it is unclear whether this is necessary for the exploit to work, or whether it is a control mechanism for the perpetrators of this attack," he said.

Interestingly, the DLL is disguised as a device driver, which is what allows it to auto-load, thanks to the malware having a valid digital signature from Realtek Semiconductor, a legitimate company. Security researchers are anxious to learn how attackers got their hands on the digital signature, since such signatures are critical for differentiating good software from bad.

As that suggests, "digitally signed malware is a nightmare for antivirus developers," said Aleks Gostev, a security expert at antivirus vendor Kaspersky Lab, in a blog post.

Patching the vulnerability or vulnerabilities exploited by Stuxnet will likely require an operating system fix from Microsoft, rather than simply recalling Realtek's digital signature. "Recalling a certificate from a company like this simply isn't feasible -- it would cause an enormous amount of the software which they've released to become unusable," said Gostev.

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