Rival browsers, including Mozilla's Firefox, 's Chrome and Opera Software's Opera, are also safe from the in-the-wild attacks aimed at IE6 and IE7.

The newest zero-day is the second this year that Microsoft has admitted hackers have exploited before a patch was ready. In mid-January, Microsoft said that a flaw in IE had been used to , including Google's and Adobe's. Microsoft patched the vulnerability on Jan. 21 in an out-of-band update.

Microsoft's next scheduled Patch Tuesday is April 13, more than four weeks away.

Gregg Keizer covers Microsoft, security issues, Apple, Web browsers and general technology breaking news for Computerworld. Follow Gregg on Twitter at or subscribe to . His e-mail address is .

in Computerworld's Security Knowledge Center.