Microsoft Corp. is taking the unusual step of issuing an emergency fix for a security hole in its Internet Explorer software that has exposed millions of users to having their computers taken over by hackers.

The "zero-day" vulnerability, which came to light last week, allows criminals to take over victims' machines simply by steering them to infected Web sites; users don't have to download anything for their computers to get infected, which makes the flaw in Internet Explorer's programming code so dangerous. Internet Explorer is the world's most widely used Web browser.

Microsoft said it plans to ship a security update, rated "critical," for the browser on Wednesday. People with the Windows Update feature activated on their computers will get the patch automatically.
Thousands of Web sites already have been compromised by criminals looking to exploit the flaw. The bad guys have loaded malicious code onto those sites that automatically infect visitors' machines if they're using Internet Explorer and haven't employed a complicated series of workarounds that Microsoft has suggested.

Microsoft said it has seen attacks targeting the flaw only in Internet Explorer 7, the most widely used version, but has cautioned that all other current editions of the browser are vulnerable.

Microsoft rarely issues security fixes for its software outside of its regular monthly updates. The company last did it in October, and a year and half before that.

微软正采取前所未有的针对IE安全漏洞的紧急措施，因为数百万使用IE浏览器的用户的计算机被黑客控制。

这个“Zero-day”脆弱性上周刚出现就被不法分子利用，并通过引诱用户点击已经感染的Web站点来控制用户的计算机。用户不用下载任何文件就会被感染，这样使得IE漏洞的编程代码很危险。目前，IE浏览器是世界上使用最广泛的浏览器。

微软曾计划在周三发布一个浏览器安全更新，定级为“危急”。而设置了Windows自动更新的用户的计算机将会自动获得该补丁。

上千个Web站点已经因为这一违法的利用这个缺陷而受连累。一些不法分子已经在一些网站植入病毒并传播给访问的用户，如果这些用户使用IE并且没有遵照微软承认的一系列复杂的系统环境。

微软声明攻击的目标只是广泛使用的IE7，同时警告目前所有其他浏览器的版本也是脆弱的。

微软在定期的每月更新之外，几乎很少没有发布过安全策略，距离此次，最近的也是一年半前的十月。