Microsoft said onFriday a small number of its computers, including some in itsMac software business unit, were infected with malware, butthere was no evidence of customer data being affected and it iscontinuing its investigation.
The world's largest software company said the securityintrusion was "similar" to recent ones reported by Apple and Facebook.
The incident, reported on one of the company's public blogshappened "recently", but Microsoft said it chose not to make anystatement publicly while it gathered information about theattack.
"This type of cyberattack is no surprise to Microsoft andother companies that must grapple with determined and persistentadversaries," said Matt Thomlinson, general manager ofTrustworthy Computing Security at Microsoft, in the company'sblog post.
Over the past week or so, both Apple and Facebook saidcomputers used by employees were attacked after visiting asoftware developer website infected with malicious software.
The attacks come at a time of broader concern about computersecurity.
Newspaper websites, including those of The New York Times, The Washington Post and The Wall Street Journal, havebeen infiltrated recently. Earlier this month U.S. PresidentBarack Obama issued an executive order seeking better protectionof the country's critical infrastructure from cyber attacks.