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Security researchers have confirmed what may be the worst-yet security flaw for Apple's Mac OS X, following the appearance of two Mac worms in a single week.

The newly disclosed bug allows attackers to disguise malicious shell scripts as harmless files stored in ZIP archives. The bug is considered particularly dangerous because it can be used to execute malicious code on a system automatically via the Safari web browser, which is set by default to open ZIP archives.

Apple has not yet released a patch, according to security vendors. The flaw's discovery was attributed to Michael Lehn and was confirmed by several security vendors.

Mac users should disable Safari's automatic processing of ZIP archives and other "safe" files, according to Symantec. "While there is no known exploit at this time, users are encouraged to turn off the ‘Open safe files after downloading option’ in their Safari browsers and watch for further information from Apple," said Alfred Huger, senior director of engineering at Symantec Security Response, in an advisory.

The company gave the flaw a "high severity" rating. Secunia and FrSIRT, which run independent vulnerabilities databases, both gave the bug their highest severity ratings.

The bug is caused by an error in processing metadata in the ".__" file contained in ZIP archives and extracted to the "__macosx" directory, according to researchers. It can be exploited to run malicious code on a user's system when the Safari browser visits a malicious web page.

FrSIRT warned that the flaw also affects OS X's built-in e-mail client, Mail. An attacker can exploit the flaw by sending a malicious script with a harmless-looking extension such as .jpg; if sent as an attachment using the AppleDouble compression format, the script can be made to execute when the user clicks on it, according to FrSIRT.

Here again, the problem is that the attacker is allowed to specify that an application such as Terminal is used to open the contents of an archive, researchers said.

Secunia confirmed that Mail is also affected. "The underlying issue appears to be the same. There are just different attack vectors," said Secunia chief technology officer Thomas Kristensen. "Mac OS X users should be really careful these days."

Several security researchers have predicted that 2006 will be the year Mac OS X loses its image as a "safe" operating system, and as if on cue, two low-level worms targeting the platform appeared last week. One targeted Bluetooth and another spreading via instant messaging.

Contrary to most reports, the worms weren't the first to appear on Mac OS X - others have surfaced in recent years, such as 2004's Renepo worm, which also posed little risk.

Security researchers fear that the Mac's growing success and the Unix roots of Mac OS X are leading attackers and virus writers to take more interest in the platform.

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