Mozilla has produced a patch for yet another critical flaw in Firefox, the latest embarrassment in a lengthening list this month for the open-source browser.

Tuesday, security researchers Billy "BK" Rios of VeriSign and Nathan McFeters of Ernst & Young, posted proof-of-concept code that exploited another URL protocol-handling bug in the Firefox and Netscape browsers.

By Thursday morning, Firefox developers had wrapped up a fix for the newest vulnerability. An automatic update to users, however, has not yet been scheduled.

Like a bug that was patched in Firefox 2.0.0.5 last week and a second that Mozilla acknowledged Monday, this vulnerability is in Firefox's handing of URLs passed to it by other applications.

The brouhaha began more than two weeks ago, when Danish researcher Thor Larholm spotted what he said was a critical bug in Microsoft's Internet Explorer. Larholm argued that an IE input validation bug let the browser pass potentially malicious URLs to other programs, including Firefox. He staked out the position that IE was to blame, while other security experts said it was Firefox's fault.

Monday, Mozilla's head of security admitted that Firefox was just as guilty as IE. "We thought this was just a problem with IE," said Window Snyder. "It turns out, it is a problem with Firefox as well."

The next day, Rios and McFeters spelled out their take. "IE isn't the only browser that has issues dealing with registered URI handlers," Rios said in a warning posted to his blog. "In fact, some of the behavior exhibited by URI handling issues by other browsers can lead to remote command execution." They provided exploit code that forced Firefox to launch other local programs when a malformed URL was passed to it from Internet Explorer 7 on Windows XP SP2. According to the two, only users whose machines have IE 7 as well as Firefox are at risk.

"Just to be clear, this vulnerability is delivered through the Firefox browser, not IE. You simply have to have IE7 installed somewhere on your system for this to work, which is basically most Windows XP SP2 systems," Rios said.

Synder, who responded Wednesday on her blog, said her team was investigating the vulnerability, and added: "The impact to users is unknown at this point in time. In the meantime, advise users to be cautious when browsing unknown sites."

For more enterprise computing news, visit Computerworld. Story copyright Computerworld, Inc.

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