Microsoft on Tuesday warned that hackers are targeting a freshly-uncovered weakness in some earlier versions of its Internet Explorer (IE) Web browser software.
Microsoft said it is investigating a hole that cyber attackers are taking advantage of in IE 6 and IE 7.
"At this time, we are aware of targeted attacks attempting to use this vulnerability," Microsoft said in an advisory posted along with a routine release of patches for Windows and Office software.
"We will continue to monitor the threat environment and update this advisory if this situation changes."
Hackers could use the flaw to remotely seize control of computers. The new IE 8 Web browser and an old IE 5 version are not affected, according to the US software colossus.
The warning came less than two months after Microsoft released a patch for an IE 6 software hole through which China-based cyber spies attacked Google and other firms.
No matter which Web browser people use, upgrading to the most current version promises to increase protection against hackers.