Attacks exploiting a previously unknown and currently unpatched vulnerability in Microsoft's Internet Explorer browser have spread to at least nine other websites, including those run by a big European company operating in the aerospace, defense, and security industries as well as non-profit groups and institutes, security researchers said.

The revelation, from a blog post published Sunday by security firm AlienVault, means an attack campaign that surreptitiously installed malware on the computers of federal government workers involved in nuclear weapons research was broader and more ambitious than previously thought. Earlier reports identified only a website belonging to the US Department of Labor as redirecting to servers that exploited the zero-day remote-code vulnerability in IE version 8.

A separate blog post from security firm CrowdStrike said its researchers unearthed evidence suggesting that the campaign began in mid-March. Their analysis of logs from the malicious infrastructure used in the attacks revealed the IP addresses of visitors to the compromised sites. The logs showed addresses from 37 different countries, with 71 percent of them in the US, 11 percent in South/Southeast Asia, and 10 percent in Europe. CrowdStrike's data showed IP addresses before exploit code was run against the visitors' machines. Not all those visitors were likely compromised since the exploit code worked only against people using IE8.

CrowdStrike researchers seemed to concur with their counterparts from Invincea, who—as Ars reported on Friday—said the attacks at least in part targeted people working on sensitive government programs. Malicious links embedded in the Department of Labor website focused on webpages that dealt with illnesses suffered by employees and contractors developing atomic weapons for the Department of Energy. But they went on to say the campaign could be much broader.

"The specific Department of Labor website that was compromised provides information on a compensation program for energy workers who were exposed to uranium," CrowdStrike said. "Likely targets of interest for this site include energy-related US government entities, energy companies, and possibly companies in the extractive sector. Based on the other compromised sites other targeted entities are likely to include those interested in labor, international health and political issues, as well as entities in the defense sector."

Such "watering hole" attacks—which plant malware exploits on websites that are frequented by specific groups or people—have become a common technique in targeted attacks. Once compromised by the IE zero-day, computers are infected with a version of Poison Ivy, a backdoor tool that has been widely used in past espionage campaigns. The command-and-control servers used to communicate with infected machines show signs that they were set up by a Chinese hacking crew known as DeepPanda.

Microsoft confirmed the remote code-execution vulnerability on Friday night. Versions 6, 7, 9, and 10 of the browser are immune to these attacks, so anyone who can upgrade to one of the latest two versions should do so immediately or switch to a different browser. For anyone who absolutely can not move away from IE 8, company researchers recommend the following precautions:

Set Internet and local intranet security zone settings to "High" to block ActiveX Controls and Active Scripting in these zones

This will help prevent exploitation but may affect usability ; therefore, trusted sites should be added to the Internet Explorer Trusted Sites zone to minimize disruption.

This will help but may ; therefore, trusted sites should be added to the Internet Explorer Trusted Sites zone to minimize disruption. Configure Internet Explorer to prompt before running Active Scripting or to disable Active Scripting in the Internet and local intranet security zones

This will help prevent exploitation but can affect usability, so trusted sites should be added to the Internet Explorer Trusted Sites zone to minimize disruption.

Users can also install EMET—short for Enhanced Mitigation Experience Toolkit—which adds a variety of exploit mitigations and security defenses and is especially useful for users of older versions of Windows, such as XP.

Technical details about the "use after free" bug are available here from Rapid7. The security firm has already folded attack code exploiting the vulnerability into the Metasploit framework used by security professionals and hackers. Researchers at FireEye have also delved into the exploit circulating online. They found it uses "return oriented programming," a technique used to defeat data-execution prevention and other exploit mitigations. The FireEye researchers said they also verified the exploit works against IE8 on Windows 7.

Microsoft's advisory on Friday said researchers were still investigating the vulnerability. When the inquiry concludes, they will decide whether to release an unscheduled update or provide a fix as part of the company's regular patching cycle.

Story updated to add details from FireEye in second-to-last paragraph.