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Computerworld - Apple today released an update to Mac OS X that blocks Safari users from reaching sites secured with certificates stolen from a Dutch company last summer.

The update follows others by Microsoft, Google, Mozilla and Opera Software, which have already blocked or permanently barred the use of all certificates issued by DigiNotar, a certificate authority, or CA, that acknowledged its servers were breached and unauthorized SSL (secure socket layer) certificates obtained by one or more attackers.

Apple's update came just days after a security researcher criticized the company for "dragging its feet." In March, Apple took a month to block nine certificates stolen from U.S.-based Comodo, three weeks longer than Microsoft.

DigiNotar first admitted the certificate theft 10 days ago on Aug. 30, although the company's servers were actually compromised as early as mid-June, according to a preliminary report released by the Dutch government.

A hacker calling himself "Comodohacker" has claimed responsibility for the DigiNotar intrusion, and the theft of 531 certificates that could be used to verify the identities of some of the Web's biggest digital properties, including Microsoft's, Google's, Twitter's and Facebook's.

Google has said one certificate was widely used in Iran to spy on Gmail users.

None of the fraudulently-obtained certificates were for an Apple website.

Apple had to issue a Mac OS X update because Safari, unlike Chrome and Firefox, relies on the operating system to tell it which certificates have been revoked or banned. The browser then either blocks access to sites that don't have a matching certificate in Mac OS X's Keychain, or warns users before they continue to a site with a revoked certificate.

"An attacker with a privileged network position may intercept user credentials or other sensitive information," Apple said in the advisory accompanying the update.

The small update removes DigiNotar from the list of trusted root certificates in Mac OS X, and reconfigures settings to not trust any certificate linked to the company.

Users running Mac OS X 10.7, aka Lion, and 10.6, known as Snow Leopard, can retrieve the update by selecting Software Update from the Mac menu.

Mac OS X 10.5, or Leopard, will not be updated.

According to Web metrics company Net Applications, about one-in-four Macs run Leopard or the even older Tiger.

Gregg Keizer covers Microsoft, security issues, Apple, Web browsers and general technology breaking news for Computerworld. Follow Gregg on Twitter at @gkeizer, on Google+ or subscribe to Gregg's RSS feed . His e-mail address is gkeizer@computerworld.com.

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