A file-sharing site that offers pirated copies of Apple's iWork '09 application suite is infected with malicious software that leaves Macs open to attack.

Security firm Intego said the 'iServices.a' Trojan horse hitchhikes on iWork '09's installer.

"The installer for the Trojan horse is launched as soon as a user begins the installation of iWork, following the installer's request of an administrator password," Intego warned.

Once installed, the Trojan 'phones home' to a malicious server to notify the hacker that the Mac has been compromised, and to await instructions. Intego did not spell out what second-stage actions the iServices.a Trojan takes, but noted that they could include delivering additional malware to the hijacked machine.

Intego said that iWork '09 download traffic on file-sharing sites has been brisk, claiming that as of early Wednesday, 20,000 copies had been downloaded.

"The risk of infection is serious, and users may face extremely serious consequences if their Macs are accessible to malicious users."

Users of Pirate Bay, a popular BitTorrent tracking site, confirmed that copies of iWork '09 harbored the Trojan.

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"I can confirm that this contains a iServices trojan," said a user identified as 'aklacat' in a comment appended to one iWork '09 listing on Pirate Bay.

"Little Snitch also confirms this," said aklacat, referring to a Mac-only personal firewall.

According to the dates assigned to Pirate Bay's iWork '09 BitTorrents, most copies were posted before Apple announced it had dropped a form of copy protection from retail copies of the suite. Apple said that it was not including serial numbers with iWork '09, an anti-piracy measure it had used to keep customers from copying earlier versions of the bundle.

Apple unveiled iWork '09 at Macworld Conference & Expo on January 6 when it touted changes and additions to Pages, the suite's word processor, and Numbers, its spreadsheet application.

iWork '09 retails for $79 (£58). Apple also offers a free 30-day trial version that does require a serial number - delivered via email at the time of payment - to run as a fully-functional version.

Mac-only malware is such a relative rarity that Apple has publicly mocked Microsoft www.microsoft.co.uk about the number of worms, viruses and Trojans that take aim at the Windows operating system. However, late last year, Apple's revision to an online recommendation that Mac users consider running antivirus software, drew lots of attention.

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