Microsoft today shipped 2010's second rush update for Internet Explorer (IE), patching 10 vulnerabilities -- including one hackers have been exploiting for weeks.

That bug had been reported to Microsoft by a Beijing security company before news broke that it was being used by attackers. In fact, Microsoft wrapped up work on the fix for IE6 by Feb. 26, according to date stamps on the affected file.

The update, tagged MS10-018 , was released two weeks early because Microsoft had tracked a growing number of attacks against IE6 and IE7. The bug has been used by malicious sites to launch drive-by attacks for much of the month.

The last emergency IE update was issued January 21 to fix eight flaws, including one that had been exploited to attack Google , Adobe and scores of other companies. Google blamed China for the attacks, a move that led to its decision to relocate its Chinese-language search engine to Hong Kong.

All 10 vulnerabilities patched in today's update -- which was originally slated for release April 13, the next regularly-scheduled Patch Tuesday -- were rated "critical," the highest level threat in Microsoft's four-step scoring system. But there were clear differences in the risk profiles of different versions of IE.

IE6, the 2001 browser that many want to see dead and buried , was affected by eight of the 10 bugs, with seven of those eight marked critical. IE7, which debuted in 2006 prior to the release of Windows Vista, contained seven out of the possible 10, with five vulnerabilities tagged critical. IE8, on the other hand, was touched by just three of the 10, with only two critical.

"The message today should be to get onto IE8," said Andrew Storms, director of security operations at nCircle Network Security. "Not just ditch IE6, but dump IE6 and IE7."

For the most part, Storms saw the 10 vulnerabilities as "pretty typical IE bugs. Except for [CVE-2010-0806], none of them are particularly troublesome, or no more than we've come to expect." CVE-2010-0806 is the Common Vulnerabilities & Exposure ID for the vulnerability that prompted the rush, or "out-of-band," update.

And that vulnerability received most of the attention today from Storms and other researchers, including HD Moore, the creator of the Metasploit framework and chief security officer at security company Rapid7, which manages the open-source Metasploit project.

According to Moore, Microsoft's out-of-band hand was forced when a Taiwanese researcher nicknamed "Nanika" revamped public exploit code so that it worked reliably against not only IE6, but also the newer IE7. "Before, Microsoft said, 'Not that big a deal,' but then the facts changed and they say, 'Sorry, this does affect IE7 reliably.' They changed their mind."

When Microsoft first warned customers of the memory corruption flaw in the "iepeers.dll" file, it said that attacks were aimed only at IE6 users, and that Protected Mode in IE7 would help protect users of that version. Protected Mode is a pseudo-sandbox that tries to keep attack code from escaping the browser to modify, add or delete data elsewhere on the PC.