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That revelation came after researchers at CrySyS Lab at the Budapest University of Technology and Economics managed to recover a dropper file, aka installer, for Duqu. Droppers are typically the first malicious files to infect a computer, and then they download and install further malicious files onto the computer.

"The installer file is a Microsoft Word document (.doc) that exploits a previously unknown kernel vulnerability that allows code execution," said Vikram Thakur, principal security response manager at Symantec, which has been working with CrySyS to unravel Duqu's inner workings. "We contacted Microsoft regarding the vulnerability and they're working diligently towards issuing a patch and advisory."

Simply opening the malicious Word file enables it to execute malicious code, which exploits the zero-day (aka previously unknown) vulnerability, and then installs multiple Duqu binary files. Interestingly, the recovered dropper was designed to target just a single organization, and would only have operated for an eight-day window in August. Thakur, however, cautioned that this is the only Duqu installer to have been recovered, meaning that others may operate in different ways, and still be targeting other businesses.

[Fortune 100 companies have been targeted by malware seeking to steal proprietary information. Learn more: Nitro Malware Targeted Chemical Companies.]

Until Microsoft patches the zero-day vulnerability, there's no surefire safeguard against this type of attack. "Unfortunately, no robust workarounds exist at this time other than following best practices, such as avoiding documents from unknown parties and utilizing alternative software," said Thakur. "Fortunately, most security vendors already detect and block the main Duqu files, thereby preventing the attack."

Researchers have also found that Duqu also has the alarming ability to infect and control computers that aren't connected to the Internet. "In one organization, evidence was found that showed the attackers commanding Duqu to spread across SMB shares," said Thakur. "Interestingly though, some of the newly infected computers did not have the ability to connect to the Internet and thereby the command-and-control (C&C) server. The Duqu configuration files on these computers were instead configured not to communicate directly with the C&C server, but to use a file-sharing C&C protocol with another compromised computer that had the ability to connect to the C&C server."

In other words, Duqu can use Internet-connected PCs as proxies for infecting PCs that may be operating in a designated "secure zone" that's network-connected, but lacks Internet connectivity. Such computers would be one location where businesses might store confidential or propriety information, to help safeguard it against online attackers.

That, of course, gels with what researchers already know about Duqu. While apparently related to Stuxnet, Duqu appears to have been designed for cyber espionage purposes, and in particular to steal design documents relating to industrial control facilities in a number of different countries.

In other Duqu news, on Monday, Symantec said it had discovered what's now the second known Duqu C&C server, this one operating from Belgium (the first was in India). Symantec said that the Belgian service provider with control of the server took it down rapidly after being contacted.

But those servers may just be the tip of the iceberg. "As well as a unique set of Duqu files for each victim, there may well be a unique command server for each entity that was attacked," said Alexander Gostev, who heads the global research and analysis team at Kaspersky Lab, in a blog post.

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