FireEye today said it had discovered that attackers are actively exploiting a new, unpatched vulnerability in Internet Explorer 10 (IE10).
Microsoft confirmed the Milpitas, Calif. security company's report.
"Microsoft is aware of targeted attacks against Internet Explorer, currently targeting customers using Internet Explorer 10," a Microsoft spokesperson said via email. "We are investigating and we will take appropriate actions to help protect customers."
FireEye's disclosure came just two days after Microsoft patched every edition of IE with a large update that fixed 24 flaws, 15 of which applied to IE10. The IE update, which was not originally on this week's Patch Tuesday slate, was added at the last minute by Microsoft, which said it had completed testing of the repairs in time to make the cut.
The attack code, said FireEye, was hosted on a compromised website based in the U.S. The company called the exploit a "classic drive-by download attack," a term reserved for the most dangerous kind of browser-based assaults, one that only need entice people to a malware-infected site.
According to FireEye, the exploit sidesteps ASLR (address space layout randomization) using Flash ActionScript, an Adobe-owned scripting language most often used on sites that rely on Flash Player to execute content. ASLR is one of Windows' most important anti-exploit technologies.
"Upon successful exploitation, this zero-day attack will download a XOR-encoded payload from a remote server, decode and execute it," FireEye added.
FireEye said that it is "currently collaborating" with Microsoft's security engineers on researching the IE10 vulnerability and the related exploit.
IE10 has been on the downturn for the last four months. Starting in October 2013, Microsoft has been replacing IE10 with the newer IE11 on Windows 8 devices, including PCs and the tablets -- such as Microsoft's own Surface Pro 2 -- that run the full-featured Windows 8 and 8.1 rather than the subset-of-an-OS Windows RT.
Microsoft released IE11 for Windows 7 in November, at which point it was automatically pushed to those machines as a substitute for IE10.
Web metrics firm Net Applications estimated IE10 user share, a rough calculation of the percentage of all Internet users running a specific browser, at 9.3% in January. Most people running IE remain tied to 2009's IE8.
Gregg Keizer covers Microsoft, security issues, Apple, Web browsers and general technology breaking news for Computerworld. Follow Gregg on Twitter at @gkeizer, on Google+ or subscribe to Gregg's RSS feed . His email address is email@example.com.