Security researchers are reporting in-the-wild attacks targeting a previously unknown vulnerability in fully patched versions of Microsoft's Internet Explorer browser.

Internet users located in China report infections that result when using IE 7 to browse booby-trapped websites. Researchers from McAfee investigated the matter and found the exploits successfully target the Microsoft browser on both Windows XP Service Pack 3 and Vista SP 1.

The exploits contain shellcode that installs the Downloader-AZN, a well-known trojan that hijacks a PC's configuration settings and downloads additional pieces of malware. Anti-virus software from McAfee, and presumably other companies, detects the trojan - though at the time of writing, it appeared they didn't yet detect the zero-day exploit itself.

The attacks target a flaw in the way IE handles certain types of data that use the extensible markup language, or XML, format. The bug references already freed memory in the mshtml.dll file. According to IDG News, exploits work about one in three times, and only after a victim has visited a website that serves a malicious piece of javascript.

Microsoft researchers are looking in to the reports, a company spokesman said.

The reports came just hours ahead of Patch Tuesday, Microsoft's monthly release of security updates. The patches include a cumulative update for IE.