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Computerworld - Microsoft on Thursday said its Internet Explorer 9 (IE9) does not contain the bug exploited this week by an Irish researcher at the Pwn2Own hacking contest.

But while IE9 is not vulnerable to attacks using the same Pwn2Own exploit, up to 99% of IE's users may be at risk.

On Wednesday, researcher Stephen Fewer of Harmony Security chained three exploits to hack the older IE8, receiving $15,000 and a Sony laptop from HP TippingPoint for his work.

Shortly after Pwn2Own organizer Aaron Portnoy announced Fewer's success, Microsoft said it had the vulnerabilities in hand and had started investigating.

IE9, however, will not need a patch.

"The vulnerability was addressed in the RC [release candidate] and RTM [release to the Web] versions of Internet Explorer 9," said Jerry Bryant, a group manager with the Microsoft Security Response Center, in an e-mail reply to questions. "This update is already in the pipe for down level-versions of Internet Explorer."

"Down-level" refers to older editions of IE, such as IE8.

Microsoft launched IE9 RC a month ago, and plans to make IE9 RTW -- the browser's final code -- available for download on Monday, March 14, at 9 a.m. PT.

But because IE9 has such a small share of the browser market -- less than six-tenths of 1% -- the overwhelming majority of IE users will have to wait for a patch aimed at the older versions.

According to the latest statistics from Web metrics company Net Applications, IE6, IE7 and IE8 make up 99% of all versions of Internet Explorer in use.

Although Bryant said that a fix is "already in the pipe," Microsoft declined to specify a patching timetable. If Microsoft keeps to its usual practice of updating IE only on even-numbered months, the first opportunity would be April 12, that month's Patch Tuesday.

Microsoft is typically reticent to push out emergency, or "out-of-band" updates unless attacks are ongoing, and then only when the volume of those attacks spikes.

Last year, Microsoft took three months to address the vulnerability used to hack IE at Pwn2Own, matching its patching speed of 2009 but lagging behind both Mozilla and Apple, whose browsers also fell to researchers' exploits in the March 2010 contest.

Microsoft may see little need for speed, since the bugs exploited at Pwn2Own are handed to TippingPoint's Zero Day Initiative (ZDI) bug bounty program, which requires researchers to keep quiet about the vulnerabilities and their exploits. ZDI turns bugs over to vendors, who have six months to fix the flaws before TippingPoint goes public with any information.

Earlier this week, for example, Bryant downplayed the need to patch IE before Pwn2Own started, noting that with TippingPoint's practices, there was no danger of a vulnerability escaping into the wild. "Pwn2Own bugs are reported to vendors in a coordinated way," Bryant said on Tuesday.

Gregg Keizer covers Microsoft, security issues, Apple, Web browsers and general technology breaking news for Computerworld. Follow Gregg on Twitter at @gkeizer or subscribe to Gregg's RSS feed . His e-mail address is gkeizer@computerworld.com.

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