Microsoft has received reports of "a small number of targeted attacks" based on a vulnerability in Internet Explorer 9 or earlier versions, and the company has issued a security advisory to help mitigate the risk of compromise.

Security researcher Eric Romang identified the exploit code on a server used by the "Nitro" hacking group, believed to have exploited the Java zero-day vulnerability reported last month.

Yunsun Wee, director of Microsoft's Trustworthy Computing Group, says that a security patch is being developed and that Microsoft customers should deploy the company's Enhanced Mitigation Experience Toolkit (EMET). He also advises blocking ActiveX Controls and Active Scripting by setting Internet and local intranet security zone settings to "High" and configuring IE to either prompt before running Active Scripting or to disable Active Scripting entirely. However, he notes, this may affect website usability.

[ AlienVault researcher believes he has identified creator of PlugX malware. Read Is 'Virus Expert' Tied To PlugX RAT Malware? ]

Security firm Rapid7 advises that Internet users try a different Web browser. "Since Microsoft has not released a patch for this vulnerability yet, Internet users are strongly advised to switch to other browsers, such as Chrome or Firefox, until a security update becomes available," the company said in a blog post on Monday.

Rapid7 develops the Metasploit penetration testing tool and has updated the software to include the new exploit, which means security administrators need to take steps to defend against the exploit immediately.

AlienVault, another security firm, says that "whg," the Chinese hacker believed to be behind the PlugX RAT malware, also appears to be involved in the creation of this latest exploit.

"We know that the group actively using the PlugX malware, also called Flowershow, had access to the Internet Explorer ZeroDay days before it was uncovered," Jaime Blasco, labs manager at AlienVault, said in a blog post. "Due to the similarities of the new discovered exploit code and the one discovered some days ago, it is very likely that the same group is behind both instances."

The group in question is the known as the Network Crack Program Hacker (NCPH), a Chinese hacker group that operates out of Zigong in China's Sichuan Province. According to Blasco, the similarities in the attack techniques suggest that "whg" is involved.

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