The Duqu trojan infects systems by exploiting a previously unknown Windows kernel vulnerability that is remotely executable, security vendor Symantec said today.

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Symantec said in a blog post that CrySys, the Hungarian research firm that discovered the Duqu Trojan earlier this month, has identified a dropper file that was used to infect systems with the malware.

The installer file is a malicious Microsoft Word document designed to exploit a zero-day code execution vulnerability in the Windows kernel.

“When the file is opened, malicious code executes and installs the main Duqu binaries” on the compromised system, Symantec said.

According to Symantec, the malicious Word document in the recovered installer appears to have been specifically crafted for the targeted organization. The file was designed to ensure that Duqu would only be installed during a specific eight-day window in August, Symantec noted.

No known workarounds exist for the zero-day vulnerability that Duqu exploits. The installer that was recovered is one of several that may have been used to spread the Trojan.

It is possible that other methods of infection are also being used to spread Duqu, Symantec noted.

Jerry Bryant, Microsoft’s Trustworthy Computing group manager, said that the company is working “diligently” to address the issue.

“Microsoft is collaborating with our partners to provide protections for a vulnerability used in targeted attempts to infect computers with the Duqu malware,” Bryant said in an email.

The company will issue a security update to address the vulnerability “through our security bulletin process,” Bryant said.

The Duqu trojan was discovered earlier this month by CrySys and has garnered considerable attention because of its supposed link to last year’s Stuxnet worm that was used to disrupt industrial control equipment at Iran’s Natanz nuclear facility.

Symantec, one of the first researchers to release a detailed analysis of the Duqu malware, has labeled it a precursor to the next Stuxnet because of what it said are similarities in code and function.

Symantec said that its researchers determined that Duqu was likely created by Stuxnet’s authors, and was designed specifically to steal information from vendors of industrial control systems.

The company said it believes the information gathered from the systems will be used to craft another Stuxnet-like worm.

In today’s update, Symantec noted that once Duqu gains a foothold in an organization, it can be remotely commanded to infect other systems.

In one of the six organizations that are confirmed to have been infected by the malware, attackers remotely ordered Duqu to spread by using the Server Message Block protocol used for file and printer sharing functions, Symantec said.

In some cases, computers infected with Duqu did not have the ability to communicate with a central command and control server, so the malware was configured to use a file-sharing protocol to communicate with another compromised computer on the same network that could to connect to a server.

“Consequently, Duqu creates a bridge between the network’s internal servers and the C&C (control and command) server. This allowed the attackers to access Duqu infections in secure zones with the help of computers outside the secure zone being used as proxies,” Symantec said.

Syamantec said it confirmed that systems in six organizations in eight countries — France, India, Iran, Netherlands, Switzerland, Sudan, Ukraine and Vietnam — have been infected with Duqu. Unconfirmed infections have also been reported in Hungary, Indonesia and the United Kingdom, Symantec said.

Meanwhile, researchers discovered two command and control servers used to communicate with computers infected with Duqu.

The first one was found in India and taken down last week, while the second one, located in Belgium, was also shut down.

Don Jackson, a security researcher at Dell SecureWorks, said today that it’ss not clear from Symantec’s description whether the zero-day flaw exists in the Windows kernel, in Word, or in both.

Finding and exploiting a Windows kernel level zero-day vulnerability suggests that those behind Duqu likely has “pretty high level of technical capability” and/or is very well funded, Jackson said.

Zero-day flaws in the Windows kernel can easily cost upwards of $10,000 in the underground market, Jackson noted.