Monday, July 21st, 2008

A Look at MFSA 2008-35

As promised… a quick look at MFSA2008-35


When FireFox is installed, it registers the following protocol handlers:

  • Gopher://
  • FirefoxURL://

gopher is cool!

Note, Firefox3 no longer registers the Gopher protocol handler, which is a great security decision.


Both of these protocol handlers point to Firefox.exe in the following manner:

  • “C:\Program Files\Mozilla Firefox\firefox.exe” -requestPending -osint -url “%1″

When Gopher:// or FirefoxURL:// are called, the arguments are passed to the “%1” portion in the string shown above.  For example, gopher://test will result in the following: 

  • “C:\Program Files\Mozilla Firefox\firefox.exe” -requestPending -osint -url “gopher://test

Knowing that we have absolute control over the –url argument being passed to Firefox.exe, we can use the “|” character to pass multiple, arbitrary URLs to the –url argument.  Firefox has protections against remote web pages from redirecting to file:// and chrome:// content, but in this instance we are passing the URLs via protocol handler.  When arguments are passed via protocol handler, it’s essentially as if we are passing the –url argument to firefox.exe via the command line.  So, thanks to the protocol handlers the file:// and chrome:// restrictions can be bypassed.  This is done in the following manner:

  • gopher:test|file:c:/path/filename
  • gopher:test|chrome://browser/content/browser.xul

Note – It is also possible to pass “javascript://” URIs to Firefox.exe, but javascript URIs passed via the command line will be loaded in the context of about:blank.  This is a great security decision on behalf of Mozilla and saved them from having a standalone sploit.


Now that we have the ability to load local content via the protocol handlers registered by Firefox, we must now find a way to plant the attacker controlled content to a known location.  There are a couple ways to plant attacker controlled content to a known location, but I’ll keep it simple (and responsible) and use the recently patched Safari “Carpet Bomb” attack as an example.  When Safari encountered an unknown content type, it would download the content of the file to the user’s desktop.  This gives us a semi known location, as we’ll have to guess the username.  We can send a LOT of guesses for username as demonstrated below. 

  • <html><body><iframe src=”gopher:file:c:/path/filename|file:c:/path/ filename2|file:c:/path/ filename3….>

There are other methods that don’t involve guessing the username, but I won’t go into that (remember, it’s the kinder, gentler BK!).


So, if a user is browsing the web with Safari and has Firefox installed, I could plant a HTML file with javascript (XMLHTTP) onto the user’s desktop.  Once the content is planted, I can launch the gopher:// protocol handler (gopher is launched by Safari without user consent) and point Firefox.exe to the local content.  When Firefox loads the local content, the XMLHTTP request has access to the entire user file system (as the script was loaded from the local file system). 


Firefox3 has implemented security measures to prevent arbitrary file access and limits the XMLHTTP request to the currently loaded directory (and possibly subdirs?), which is a great security decision. 


On a side note, IE warns users when active script is about to be run from the local file system.  I believe the IE warning message states, “you are about to do something REALLY stupid… do you wish to continue?” … or something like that.  This is a great security decision on behalf of IE.




The scenario presented above demonstrates how someone with Safari and FireFox installed could get their files stolen, but Mozilla understood that the behavior of their software could be abused by other software (not just Safari), just as Apple understood that dropping files to a user’s desktop (without consent) could be abused by other software as well (not just IE or Firefox).  Both vendors did what was right and adjusted the behavior of their software.  Thanks Mozilla and Apple!


These types of issues interest me because it represents the difficulties in securing real life systems, systems that have software from multiple vendors interacting with each other, depending on each other to make the right security decisions.  In isolation, these issues may not be of any concern, but together they create a situation where the security measures of one piece of software is bypassed because of the seemingly innocuous/insecure/stupid behavior of another, seemingly unrelated piece of software.  From what I understand, Mark Dowd and Alexander Sotirov plan to give some INSANE examples of this at Blackhat…  I’m looking forward to the talk!

Posted by xssniper | Filed in Security

4 Responses to “A Look at MFSA 2008-35”

  1. July 22nd, 2008 at 8:03 am

    Awesome AnDrEw said:

    Nice work, BK. It was a pretty thorough explanation. Does anyone even use Gopher any more? Last I had heard there were only a handful of sites left.

  2. July 22nd, 2008 at 10:10 am

    xssniper said:

    Yeah, Gopher needs to go… the attack surface/usability ratio has moved to favor attack surface (IMHO). I’m glad FF3 doesn’t register Gopher anymore and IE implements a neutered version of Gopher.

  3. July 22nd, 2008 at 10:22 am

    Zero Day mobile edition said:

    […] to form, Billy Rios promised a more in depth look at the MSFA2008-35 vulnerability which is another protocol handler flaw in Firefox 3.  As previously reported here, this was […]

  4. July 22nd, 2008 at 7:01 pm

    Dan Veditz said:

    I’m told Firefox registers “firefoxURL” because of some Vista default-app-setting requirement. They don’t work, however–they were disabled in Firefox after you abused them last year.

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