Microsoft today said it's investigating reports of a new unpatched vulnerability in Internet Explorer (IE) that did not get patched in the massive update on Dec. 9.

Other researchers, meanwhile, said that the timing of the attacks, which have already started, was not coincidental.

"The updates Microsoft released yesterday do not address this possible vulnerability," a Microsoft spokesman said today in an e-mail reply to questions, "but I can tell you that Microsoft is investigating these new public claims of a possible vulnerability in Internet Explorer."

Exploit code, which first surfaced in China, is actively seeking out victims, according to security researchers there and in the U.S. Those researchers have found attack code on multiple malicious domains and servers. Elsewhere today, an exploit was posted to the milw0rm.com site, a popular destination for public posting.

Symantec Corp. echoed Microsoft today, confirming that the flaw was not fixed by Tuesday's record-setting update, which included four patches, all judged "critical," for IE.

"The attack works successfully against a fully patched Windows XP SP3 with Internet Explorer 7, including all recent Microsoft Tuesday patches," said Symantec researcher Elia Florio in an entry to the company's vulnerability blog. "Also, Internet Explorer 6 could potentially be affected by the same problem and is therefore only temporarily immune to this initial exploit, which seems to target Internet Explorer 7 on Windows XP and 2003 systems."

There is some minor disagreement among researchers about the underlying bug. HD Moore, a noted vulnerability researcher and the labs director at BreakingPoint Systems, a Texas-based network test company, said his analysis points to a flaw in how IE handles the HTML "span" tag.

Others, however, said that the vulnerability is broader than that. "It's a problem in the .dll that handles the rendering of multiple types of HTML content in IE," said Ben Greenbaum, a senior manager in Symantec's security response group. "But the bug is triggered by the span tag, so it would be accurate to say it's a combination of both of those sources."

Greenbaum said Symantec has monitored attacks, but downplayed the threat for now. "Even in those regions [China and Asia], we're not seeing very high amounts of attacks," he said. "And in our own lab tests, the exploit is not successful against every machine. It's not all that reliable."

He guessed that the current attack code works, at best, a third of the time, but is most likely even less reliable than that. "Only a small portion of these attacks will be successful."