Apple, Facebook, and Twitter recently reported security breaches, apparently linked to a developer website that infected programmers' computers after they visited it.
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Thomlinson said Microsoft was being targeted by "determined and persistent adversaries."
He didn't specify who those were, but many have linked the recent attacks on Internet companies with intrustions by Chinese hackers into the systems of publishers like the New York Times and Bloomberg.
It's not clear, however, that the recent attacks originated from China.
Mike Isaac at AllThingsD recently identified the website in question as iPhoneDevSDK, a site popular with mobile-app developers.
iPhoneDevSDK recently told users that it discovered that an administrative account on its site had been hacked, which allowed hackers to inject infectious code into its Web pages. The site says it believes its software is otherwise secure.