MozillaZine

Security Exploit Uses Internet Explorer to Attack Mozilla Firefox

Wednesday July 11th, 2007

Firefox_User sent us a link to a CNET News.com article about a security threat to Windows users with both Mozilla Firefox and Microsoft Internet Explorer installed. The issue can allow an attacker to remotely trick Firefox into executing potentially malicious code. However, a user has to be running Internet Explorer to actually get exploited.

Security researcher Thor Larholm has published a description of how the security flaw works, including a proof-of-concept (though some have reported that they cannot get this to work). When installed on Windows, Firefox registers a URL protocol handler to handle firefoxurl:// URLs (this works much like a http:// or ftp:// URL protocol handler). If an IE user visits a webpage that tries to call a firefoxurl:// URL (for example, using an iframe), IE will launch Firefox with no further prompting, passing it the URL. Neither IE nor Firefox escape or sanitise the URL, which allows an attacker to inject additional parameters into the command line used to invoke Firefox. Used in combination with the -chrome parameter, the attacker can make Firefox execute dangerous JavaScript code.

There's some debate as to where the blame lies — is it IE for passing untrusted data to another application or Firefox for not validating input properly? SecurityFocus refers to the problem as a Microsoft Internet Explorer FirefoxURL Protocol Handler Command Injection Vulnerability, placing the blame with Redmond, while Secunia calls it a Firefox "firefoxurl" URI Handler Registration Vulnerability, pointing the finger at Mozilla. News.com quotes Oliver Friedrichs of Symantec's Security Response Center, who says, "It's a little bit of both."

On the official Mozilla Security Blog, the Mozilla Corporation's Window Synder (who used to work for Microsoft) says that a fix will be included in the forthcoming Firefox 2.0.0.5. That said, she seems to suggest that she considers this to be mostly a problem with IE, noting that Apple fixed a similar issue with Safari recently. However, according to the ZDNet Zero Day security weblog, Microsoft claims the firefoxurl:// bug "is not a vulnerability in a Microsoft product".

On his weblog, Jesper Johansson (who also used to work for Microsoft), says the firefoxurl:// flaw is a Mozilla problem. He also provides instructions for unregistering the URL protocol handlers.

Thanks to roseman for some of the links used in this report.


#5 Does Firefoxurl actually work properly for anyone?

by GAThrawn

Wednesday July 11th, 2007 8:51 AM

You are replying to this message

Never heard of this firefoxurl before, so Ithouoght I'd give it a try.

So I tried browsing to a few web pages in IE, replaced the "http" with "firefoxurl" and whilst it appears to get passed to Firefox I then get a dialog from Firefox saying:

External Protocol Request An external application must be launched to handle firefoxurl: links. Requested link: firefoxurl://news.bbc.co.uk Application Firefox ...blah blah security stuff...

If I click Launch Application, I then get a new blank tab in my current firefox window, and the dialog reappears, and so on in a loop every time I click the Launch Application.

(Firefox 2.0.0.4, Win XP SP2, IE7)