Microsoft said hackers exploited a bug in its Windows operating system to infect computers with the Duqu virus, which some security experts say could be the next big cyber threat.

Microsoft plans to release a ecurity update for the issue.

According to Symantec, the attackers behind Duqu used a Microsoft Windows zero-day as part of their attack campaign. Researchers at the Laboratory of Cryptography and System Security (CrySyS) - the group that initially discovered the original Duqu binaries - has located an installer for the malware. The installer file is a malicious Microsoft Word document that exploits a previously-unknown kernel vulnerability that allows code execution.

"When the file is opened, malicious code executes and installs the main Duqu binaries," blogged Vikram Thakur, principal security response manager at Symantec.

"The Word document was crafted in such a way as to definitively target the intended receiving organization," he added. "This installer is the only installer to have been recovered at the time of writing?the attackers may have used other methods of infection in different organizations."

Unfortunately, no robust workarounds exist at this time other than following best practices, such as avoiding documents from unknown parties and utilizing alternative software. Fortunately, most security vendors already detect and block the main Duqu files, thereby preventing the attack.

Once Duqu is able to get a foothold in an organization through the zero-day exploit, the attackers can command it to spread to other computers. According to Symantec, evidence was found that showed the attackers commanding Duqu to spread across SMB shares. 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. Consequently, Duqu creates a bridge between the network's internal servers and the C&C 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.

So far, the damage done by Duqu remains limited. According to Symantec, six organizations are believed to have been infected with the malware. Some of the organizations are only traceable back to an ISP, so it is possible all six are not separate organizations. The targets identified so far by Symantec have a presence in eight countries: France, India, Iran, Vietnam, Sudan, Netherlands, Ukraine and Switzerland. Other security vendors however have reported infections in other countries as well, including Hungary, Austria and the U.K.