Mozilla admits Firefox is flawed just like IE

Mozilla's chief security officer acknowledged that Firefox has a "critical vulnerability"

In a public mea culpa, Mozilla's chief security officer acknowledged Monday that Firefox includes the same flaw that the company called a "critical vulnerability" in Internet Explorer during a two-week ruckus over responsibility for a Windows zero-day bug.

"Over the weekend, we learned about a new scenario that identifies ways that Firefox could also be used as the entry point," said Window Snyder of Mozilla. "While browsing with Firefox, a specially crafted URL could potentially be used to send bad data to another application.

"We thought this was just a problem with IE," Snyder continued. "It turns out, it is a problem with Firefox as well."

The argument over responsibility for a flaw that involved both IE and Firefox began two weeks ago, when Danish researcher Thor Larholm argued that IE contained an input validation bug that passes potentially malicious URLs to other applications. Larholm called out Firefox's "firefoxurl://" protocol as one that IE mishandled. He staked out the position that IE was to blame, while other security experts said it was Firefox's fault.

As fingers pointed, Mozilla patched the IE-Firefox interaction bug by releasing an update, Version Even so, Snyder and others continued to argue that IE was the problem. "Microsoft needs to patch Internet Explorer," Snyder said last Wednesday. That same day, Asa Dotzler, director of community development, contrasted what he said were the differences between Microsoft and Mozilla on the bug. "We think it's Firefox's job to ensure that users are protected from malicious Web sites when they're surfing the Web in Firefox. Apparently, Microsoft doesn't think the same for IE," Dotzler said then.

Friday, Jesper Johansson, a former Microsoft security strategist but now a security program manager at, spelled out how Firefox was as guilty as IE of failing to validate input. In a post that leaned on the metaphor of "glass houses," Johansson showed how Firefox passes potentially malicious URLs to other applications, including the multiple-service instant messaging client Trillian. "Firefox is subject to the exact same flaw that they blame on IE. Firefox also does not escape quotes in URLs before it passes them on to protocol handlers," he said.

Snyder did not credit Johansson by name for alerting Mozilla to the Firefox bug, but she admitted that the flaw should have been spotted. "We should have caught this scenario when we fixed the related problem in," she said.

She did not specify when a patch would be issued, but one is in the works, according to an entry in Bugzilla.

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Gregg Keizer

Gregg Keizer

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