Apple reintroduced a critical vulnerability in Leopard, the newest version of Mac OS X, that it had patched more than 20 months ago in Tiger, security experts said this week.

A bug in Apple Mail -- the e-mail client included in Mac OS X -- that Apple fixed in Tiger on March 1, 2006, has cropped up in Leopard, according to security researchers from Symantec Corp. and Intego Inc. Attackers can use the new-old vulnerability to hide malicious code in seemingly harmless file attachments and get Apple Mail to run the malware without warning the user as it is designed to do, said Symantec in a warning to customers of its DeepSight threat management service.

The Heise Security Web site first noted the returned flaw and said attackers could disguise their code as a benign file -- a JPG image file, for example -- then exploit the file's "resource fork" to launch Terminal, the Mac OS X command line interface, which in turn would run a camouflaged code. The site even produced a harmless proof-of-concept e-mail attachment that demonstrated an exploit of the bug.

"On a current installation of the Tiger OS, Apple Mail issues a warning that the supposed image file is a[n executable] program and is to be opened with Terminal," Heise reported. "Apple apparently either did not incorporate this update into Leopard or did not do it correctly."

Intego, the antivirus software vendor that earlier this month warned of an in-the-wild Mac Trojan horse, noted that Leopard's security system does not notify users as does Tiger's did. "The bug causing this has to do with the way Leopard manages [download] quarantines," Intego said in an alert. "The first time a user opens an attachment, Mail opens the file directly without passing through the quarantine system."

Apple patched Tiger to ensure that its e-mail program would warn users of unsafe files as part of its Security Update 2006-001, which was released last year.

Until Apple fixes Leopard, Symantec told users to use caution when handling e-mail attachments received in Mail. Intego gave similar advice: "It is important that users do not open attachments from unknown senders, especially those that come with spam messages," its alert said.

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