A new security vulnerability in Microsoft's Internet Explorer has been discovered, and attackers have already used it to create a sophisticated exploit, according to researchers at FireEye.

In a blog posted Monday, FireEye researchers disclosed a memory access vulnerability that works on Windows XP with IE 7 and 8, as well as Windows 7 with IE 9. The vulnerability enables attackers to compromise IE machines that access a malicious website.

"The exploit leverages a new information leakage vulnerability and an IE out-of-bounds memory access vulnerability to achieve code execution," the blog states. The attack infects cracks in a Windows machine at the DLL level, enabling it to infect the machine with a large, multistage shellcode payload, the researchers say.

In a separate blog, the researchers describe a sophisticated exploit found in the wild that uses the newly discovered vulnerability.

"Specifically, the attackers inserted this zero-day exploit into a strategically important website known to draw visitors that are likely interested in national and international security policy. We have identified relationships between the infrastructure used in this attack and that used in Operation DeputyDog," an attack that infected a number of organizations in Japan in September, the researchers say.

"Furthermore, the attackers loaded the payload used in this attack directly into memory without first writing to disk -- a technique not typically used by advanced persistent threat [APT] actors," the blog says. In-memory attacks generally cannot be detected by traditional anti-malware tools, FireEye notes.

"This technique will further complicate network defenders' ability to triage compromised systems using traditional forensics methods," the blog states.

John Prisco, CEO of security vendor Triumfant, agreed. "In-memory attacks steal your intellectual property faster than the response time of a manual technique relying on signatures and prior knowledge," he said.

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