Security Exploit Uses Internet Explorer to Attack Mozilla Firefox

Wednesday July 11th, 2007

Firefox_User sent us a link to a CNET 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. 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 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.

#17 Sanity Checks re: Security Exploit

by DERoss

Thursday July 12th, 2007 3:48 PM

You are replying to this message

Two key issues:

1. Merely having both IE and Firefox on the same computer is not a problem. The vulnerability occurs only when browsing with IE and then only if the user selects a link that begins firefoxurl:// instead of http://, https://, etc. Thus, if you browse only with Firefox (or another Gecko application), the problem does not occur. (I use IE only to get the too frequent Windows patches and occasionally to check how my own Web pages appear with a non-Gecko browser.)

2. It appears that SeaMonkey (the browser I use) does not register a firefoxurl protocol. The string "firefox" does not appear in my WindowsXP registry.

What we have not yet seen here is an explanation of the purpose of the firefoxurl protocol. If it has no purpose (thus answering why there is no explanation) or it was never completely implemented, it should be removed.