Microsoft will issue an emergency security update for Internet Explorer (IE) today to patch a zero-day vulnerability that has been used to launch drive-by attacks for at least several weeks.

The update will be the second update outside of Microsoft's normal once-a-month Patch Tuesday in the last three months. Microsoft last shipped a rush IE update to customers in late January, to fix eight flaws, including one that had been used to attack several companies' networks, including Google's and Adobe's.

"The bulletin is being released to address attacks against customers of Internet Explorer 6 and Internet Explorer 7," said Microsoft in an updated advisory.

As it did in January, today's 'out-of-band' update will patch more than just the zero-day. It will, in fact, plug critical holes in every edition of IE, including the newest, IE8. "The out-of-band security bulletin is a cumulative security update for Internet Explorer and will also contain fixes for privately reported vulnerabilities rated Critical on all versions of Internet Explorer that are not related to this attack," Microsoft said.

Microsoft first warned users of the vulnerability in IE6 and IE7 on March 9, saying at the time that the bug didn't affect the browser's oldest and newest editions, IE 5.01 and IE8, respectively. At the time, Microsoft called the attacks "targeted", a term it uses to describe small-scale exploitations.

Within two days, hackers were spotted using the vulnerability to conduct drive-by attacks from malicious sites, and an Israeli researcher had published exploit code on the internet.

The update for IE will apply to all versions of the browser - IE 5.01, IE6, IE7 and IE8 - and affect all supported editions of Windows, including Windows 2000, XP, Vista, Server 2003, Server 2008, Windows 7 and Server 2008 R2.

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