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Computer owners installing Apple's new operating system, Mac OS X Leopard, may be making their machines less secure, according to experts.

Features of the new system designed to protect a user's computer from hostile attack do not work as efficiently as they should, and could lead to hackers being able to install malicious software, according to the security firm Heise.

The default setting for Leopard's firewall - which is supposed to block unauthorised access to the machine - is to "allow all incoming connections," a report by Heise said. If a user has upgraded from an existing Mac system which had a firewall activated, the protection is de-activated, the report added.

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The weaknesses mean that "system services representing potential access points for malware" are accessible via the internet, and that unauthorised connections to such services could be established "even under the most restrictive setting," according to Jurgen Schmidt, a researcher who analysed the system.

Mr Schmidt stressed that the "peculiarities" of Leopard were "not security vulnerabilities in the sense that they can be exploited to break into a Mac," but said Apple would be "well advised to sort them out pronto."

Steve Moyle, chief technology officer with Secerno, a security firm, said that system services like the ability to access files were much more vulernable if firewalls were not in place.

"It's a bit like the burglar knowing that the door they might break through is there in the first place. They mightn't be able to get in, but with a firewall, they wouldn't even know the door existed."

He said that the Heise report seemed "plausible", though it had not been verified, and that firewalls were generally "relatively dumb devices."

In a separate development, Apple users were warned about a new 'trojan horse' - a type of malware - which masqueraded as a download necessary to view certain video content.

Symantec, the security firm, said that the trojan - which was found a number of pornography sites - was a sign that "the Mac is becoming popular enough that the 'bad guys' think it is worth spending time and effort in developing malware for the Mac OS."

Apple was not immediately available for comment.

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