The security issue could allow an attacker to transfer and then run a malicious program on a Mac, if the Mac's user can be enticed to go to a fake Web page on which the program has been placed.
"This, in my mind, is the first critical vulnerability on OS X," said Richard Forno, a security researcher and the former chief of security for domain registrar Network Solutions. "Downloading the patch and seeing that there were some things that were fixed and some things that weren't, tells me that there is more work to be done."
Two other software companies have confirmed the issue. Security information company Secunia raised its rating of the potential risk to "extremely critical" after determining that the vulnerability is more widespread than Apple apparently first thought. Independent software maker Unsanity released a tool this week to work around the problem and put out a white paper describing the issue.
Apple would not comment. The company released the original patch on Friday after news of the vulnerability appeared on the Internet.
The vulnerability actually involves two flaws. One allows a Web site to place a file on the Mac's hard drive when a user clicks on a uniform resource locator, or URL, specifically designed to bypass Mac OS X's security. The other gives an attacker the ability to run a file on another user's computer, provided the location of the file is known. Used together, the flaws constitute a major security hole that could result in a potential instant-messaging or email virus.
Perhaps the biggest problem is that there seems to be no easy solution, Jason Harris, a programmer for Unsanity, wrote in the company's white paper.
"There's lots of overlap between useful applications of this functionality and malicious ones, meaning that Apple can't easily fix this without removing useful features from its operating system and from existing apps," he wrote.
The issue is the first major security problem for Mac OS X that has not been caused by the operating system's underlying Unix roots. Previously, Mac OS X has mainly had to patch problems that affected FreeBSD, the Unix-like operating system on which it is based. However, the current issue is in the code that the company built on top of that software.
Forno maintains that the Mac is more secure than Windows but stressed that this problem should have been caught in testing before the operating system had shipped. Moreover, in light of the problematic patch and previous issues with Apple downplaying security problems, he said the company needs to start being more proactive about security.
"Apple is coming to terms with dealing with these types of issues, "Forno said.
