Firefox 2.0.0.6 fixes yet another protocol handling bug

Mozilla late Monday updated the Firefox browser for the second time this month to patch a pair of vulnerabilities, including yet another flaw in how it parses malformed URLs that can call up other applications.

Firefox 2.0.0.6 includes a fix for a bug disclosed by a pair of researchers a week ago, said Mozilla head of security Window Snyder. This reduces the risk of malicious data being passed through Firefox to another application that may then trigger unexpected and potentially dangerous behavior," said Synder on a Mozilla blog.

The update was the latest move in a skirmish between Firefox and Microsoft Corp.'s Internet Explorer (IE) users that began three weeks ago Tuesday.

Then, Danish researcher Thor Larholm spotted what he said was a critical input-validation bug in IE. He argued that it 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. Last Monday, Snyder acknowledged that Firefox was just as guilty as IE. "We thought this was just a problem with IE," she said. "It turns out, it is a problem with Firefox as well."

But the blame may ultimately rest not on Firefox or IE, but on Windows itself, according to an advisory posted by US-CERT. In a warning issued last week, the federally supported vulnerability tracking center pinned responsibility on Windows, saying that the operating system "fails to properly handle protocols specified in a URI, which could allow a remote, unauthenticated attacker to execute arbitrary commands on a vulnerable system."

Changes in how Windows parses uniform resource identifiers (URI) were made by IE 7, US-CERT continued, portioning some blame to the Microsoft browser. However, the root of the problem is in Windows' ShellExecute application programming interface (API), according to US-CERT.

Danish bug tracker Secunia ApS also pegged Windows as the culprit.

Mozilla's own "2007-027" advisory spelled out its position on the part Microsoft Windows plays in the vulnerability. "Since this handling is a property of the Windows Shell API, this variant appears to affect other Internet-enabled applications that pass these URIs to the Windows Shell," it read.

But because the API remains, cautioned the advisory, it's impossible to know whether the fix in Firefox 2.0.0.6 will protect in every situation. "A way to exploit a common handler with a single unexpected URI as an argument may yet be found," the warning said.

Microsoft did not immediately reply to questions about the ShellExecute API, and whether it would be fixed, if it was in fact flawed.

The second vulnerability patched by 2.0.0.6 was a bug rated "moderate" that involved certain Firefox add-ons that create about:blank Web pages.

The Firefox update can be downloaded from the Mozilla site; current Firefox 2.x users can also use the browser's built-in automatic update feature to patch their browsers.