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Apple has released a software update, designed to protect users of its Leopard operating system from malware attacks.

The firm had already been forced to rush out security updates for machines working on its newer Snow Leopard and Lion operating systems.

It follows the Flashback trojan attack which security experts believe infected up to half a million Macs.

Apple has faced criticism for the time it has taken to react to the threat.

The Flashback malware targeted a vulnerability in the Java software that is used in Windows machines, Apple computers and many others.

Macs were the biggest victims because Apple did not patch the loophole in its version of Java for several weeks after the vulnerability became known.

Of its latest patch Apple said: "The update removes the most common variants of the Flashback malware. If the Flashback malware is found, a dialog will notify you that malware was removed. In some cases, the update may need to restart your computer in order to completely remove the Flashback malware."

For added security, the patch also disables versions of Adobe Flash Player that do not include the latest security updates, and encourages users to get the latest version directly from Adobe's website.

"This additional level of protection when it comes to Safari users running Flash is good to see - as Adobe's software is so frequently exploited by malware authors and malicious hackers to infect web surfers," said Graham Cluley, senior consultant at security firm Sophos <link> <caption>in his blog</caption> <url href="http://nakedsecurity.sophos.com/2012/05/14/apple-lion-security-updates-malware/" platform="highweb"/> </link> .

It shows, he added, that the firm is keen to cover all bases.

"It's encouraging to see Apple has not left users of this older version of the Mac OS X operating system completely out in the cold when it comes to protecting against the latest threats. Clearly they realise that it's not good for the Apple Mac's image if older computers connected to the internet are harbouring malware," said Mr Cluley.

But he added that more may still need to be done.

"Of course, there are still users of Mac OS X 10.4 Tiger out there - they don't have the benefit of these security updates and are effectively playing a dangerous game with their systems as the malware threat on the Mac platform increases."

Apple is due to release its latest OS - Mountain Lion - in late summer 2012.