Mozilla is scrambling to fix a vulnerability related to the Firefox browser that poses a serious danger for users who also have Apple's QuickTime installed.

According to Petko Petkov, a UK-based web application penetration tester, the current version of QuickTime contains a flaw in its Media Link (.qtl file formats) function. Any file with a QuickTime-supported extension - there are more than 60 - will be parsed by Apple's media player. However, because it fails to sanitise the XML content, an attack can sneak links to malicious JavaScript into the file, and get QuickTime to run it.

"In practice I can do anything with the browser, like installing browser back-doors, and the operating system if the victim is running with administrative privileges," Petkov said in his write-up. He said he first disclosed the vulnerability, as well as a second one in QuickTime, in September 2006. When he didn't hear from Apple, he did so again in December.

Although some security companies, including eEye Digital Security, said the open-source Firefox browser is required for a successful exploit, Petkov noted that the bug is in QuickTime and affects users of other browsers, including Microsoft's Internet Explorer. "It is not Firefox specific," he wrote on his blog. "It works for IE as well, although the impact is less critical. This is due to the tightened security policies IE implements for local zone scripts."

Others who left comments on Petkov's post, however, made a wide range of claims. Some, for instance, reported that the proof-of-concept samples that Petkov offered up failed on Windows XP SP2 when running Opera, while others said Firefox on Mac OS X is also invulnerable to the hacks.

Mozilla's security chief, Window Snyder, said her team is on the case. "Mozilla is working with Apple to keep our users safe and we are also investigating ways to mitigate this more broadly in Firefox," she said yesterday on the company's security blog.

She did not downplay the danger, calling it a "very serious issue" and warning users that Petkov's proof code "may be easily converted into an exploit."

Not surprisingly, though, various Mozilla developers found fault with QuickTime in their ongoing Bugzilla dialogue about a patch strategy. "I don't see what on our side would need to be fixed, if QuickTime didn't have this flaw," said Gavin Sharp.

"There's conflicting information [about] whether this is cross-platform," said Ben Greenbaum, a senior manager with Symantec's security response, "but it is cross browser. If a user has Firefox installed, even if he is running IE, he'd vulnerable."

The problem, said Greenbaum, at least with the current exploit path that Petkov laid out, is actually in Firefox - its "chrome" privileges, specifically. "If the exploit goes through chrome, it lets you bypass Firefox's security features," said Greenbaum.

Who to blame? "Is it a QuickTime problem, or a Firefox problem. Or a combination? Both vendors could take steps to mitigate this," he added.

The question of bug culpability has been aggressively debated by researchers, users and even analysts this summer, sparked by the July disclosure of protocol handling problems in Firefox and IE.

Whoever needs to fix code, however, should get going, intimated Greenbaum. Seconding yesterday's take by the DeepSight alert - which said in-the-wild exploitation is likely, and soon - he said: "Mpack, and other hostile drive-by attack kits are always looking for new exploits."

Until fixes are in place for QuickTime, Firefox, and other affected applications, Symantec recommends that users update anti-virus software and browse with the fewest user privileges possible.

eEye Digital Security, however, went much further in its advice. "The best form of mitigation is to disable the QuickTime plug-ins for each affected browser: IE7, Firefox and Opera," the California security company said.

Apple's response was a variation on its standard reply to security queries: "Apple takes security very seriously and has a great track record of addressing potential vulnerabilities before they can affect users," said spokesman Anuj Nayar in an email.

So far this year, Apple has issued four QuickTime security updates.