The security problem is the third to surface for the operating system in the past week. It exposes Mac users to risks that are more familiar to Windows users: Visiting a malicious Web site using Apple's Safari Web browser could result in a rootkit, a backdoor or other malicious software being installed on the computer without the user noticing anything, experts said.
"This could be really bad," the SANS Internet Storm Center, which tracks network threats, said Tuesday. "Attackers can run shell scripts on your computer remotely just by visiting a malicious Web site."
Apple is developing a patch for the flaw, a company representative told CNET News.com. "We're working on a fix so that this doesn't become something that could affect customers," the representative said, but could not give a delivery date for the update.
Word of the new vulnerability comes after the recent discovery ofand a . The operating system had not been in the security crosshairs previously.
The new problem, discovered by Michael Lehn and first reported by Heise Online, lies in the way Mac OS X processes archive files. An attacker could embed malicious code in a ZIP file and host that on a Web site. The file and the embedded code would run when a Mac user visits the site using the Safari browser, experts said.
"Essentially, the operating system is executing commands that come in the metadata for ZIP files," said Alfred Huger, senior director of engineering at Symantec. "That is exacerbated by the problem that Safari will automatically open the file when you encounter it on the Web."
The issue may go beyond archive files, SANS said in updated notes on its Web site. "The attacker doesn't need to send a ZIP archive; the shell script itself can be disguised to practically anything," the note said.
The culprit appears to be the Mac OS Finder, the component of the operating system used to view and organize files, according to the SANS posting. A malicious file can be masked to look innocent--for example, like a JPEG image--yet it will run and execute when opened, SANS said.
This occurs because the operating system assigns an identifying image for the file based on the file extension, but decides which application will handle the file based on file permissions, SANS said. If the file has any executable bits set, it will be run using Terminal, the Unix command line prompt used in Mac OS X, SANS said.
There are no known attacks that take advantage of the flaw, experts said. However, proof-of-concept code that demonstrates the security vulnerability is publicly available online and could be tweaked for use in cyberattacks. "The skill level required to exploit it is very low. Pretty much anyone can do it," Huger said.
In the Windows world, such flaws are often exploited to install spyware or ad-serving software on vulnerable PCs. While such insidious software may be rare for the Mac, there are back doors and rootkits for the operating system, Huger said. "I think you'd likely see those installed with this type of vulnerability," he said.
The vulnerability is rated "extremely critical" by security monitoring company Secunia. Symantec also rates it "fairly high risk," Huger said. "If you have a Mac and use Safari, it is something you should remediate immediately," he said.
Mac OS X users can protect themselves by disabling the "Open safe files after downloading" option in Safari. In addition, users should be cautious when surfing the Web, the Apple representative said. "Apple always advises Mac users to only accept files from vendors and Web sites that they know and trust."
Users of alternative browsers such as Firefox and Camino on the Mac are not exposed to the Web-based attack vector, experts said.