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Threat Modeling Again, Threat modeling and the fIrefoxurl issue.

Threat Modeling Again, Threat modeling and the fIrefoxurl issue.

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Yesterday I presented my version of the diagrams for Firefox's command line handler and the IE/URLMON's URL handler.  To refresh, here they are again:

 Here's my version of Firefox's diagram:

 And my version of IE/URLMON's URL handler diagram:


As  I mentioned yesterday, even though there's a trust boundary between the user and Firefox, my interpretation of the original design for the Firefox command line parsing says that this is an acceptable risk[1], since there is nothing that the user can specify via the chrome engine that they can't do from the command line.  In the threat model for the Firefox command line parsing, this assumption should be called out, since it's important.


Now let's think about what happens when you add in the firefoxurl URL handler to the mix?


For that, you need to go to the IE/URLMON diagram.  There's a clear trust boundary between the web page and IE/URLMON.  That trust boundary applies to all of the data passed in via the URL, and all of the data should be considered "tainted".  If your URL handler is registered using the "shell" key, then IE passes the URL to the shell, which launches the program listed in the "command" verb replacing the %1 value in the command verb with the URL specified (see this for more info)[2].  If, on the other hand, you've registered an asynchronous protocol handler, then IE/URLMON will instantiate your COM object and will give you the ability to validate the incoming URL and to change how IE/URLMON treats the URL.  Jesper discusses this in his post "Blocking the FIrefox".

The key thing to consider is that if you use the "shell" registration mechanism (which is significantly easier than using the asynchronous protocol handler mechanism), IE/URLMON is going to pass that tainted data to your application on the command line.


Since the firefoxurl URL handler used the "shell" registration mechanism, it means that the URL from the internet is going to be passed to Firefox's command line handler.  But this violates the assumption that the Firefox command line handler made - they assume that their command line was authored with the same level of trust as the user invoking firefox.  And that's a problem, because now you have a mechanism for any internet site to execute code on the browser client with the privileges of the user.


How would a complete threat model have shown that there was an issue?  The Firefox command line threat model showed that there was a potential issue, and the threat analysis of that potential issue showed that the threat was an accepted risk.

When the firefoxurl feature was added, the threat model analysis of that feature should have looked similar to the IE/URLMON threat model I called out above - IE/URLMON took the URL from the internet, passed it through the shell and handed it to Firefox (URL Handler above).  


So how would threat modeling have helped to find the bug?

There are two possible things that could have happened next.  When the firefoxurl handler team[3] analyzed their threat model, they would have realized that they were passing high risk data (all data from the internet should be treated as untrusted) to the command line of the Firefox application.  That should have immediately raised red flags because of the risk associated with the data.

At this point in their analysis, the foxurl handler team needed to confirm that their behavior was safe, which they could do either by asking someone on the Firefox command line handling team or by consulting the Firefox command line handling threat model (or both).  At that point, they would have discovered the important assumption I mentioned above, and they would have realized that they had a problem that needed to be mitigated (the actual form of the mitigation doesn't matter - I believe that the Firefox command line handling team removed their assumption, but I honestly don't know (and it doesn't matter for the purposes of this discussion)).


As I mentioned in my previous post, I love this example because it dramatically shows how threat modeling can help solve real world security issues.

I don't believe that anything in my analysis above is contrived - the issues I called out above directly follow from the threat modeling process I've outlined in the earlier posts. 

I've been involved in the threat modeling process here at Microsoft for quite some time now, and I've seen the threat model analysis process find this kind of issue again and again.  The threat model either exposes areas where a team needs to be concerned about their inputs or it forces teams to ask questions about their assumptions, which in turn exposes potential issues like this one (or confirms that in fact there is no issue that needs to be mitigated).


Next: Threat Modeling Rules of thumb.


[1] Obviously, I'm not a contributor to Firefox and as such any and all of my comments about Firefox's design and architecture are at best informed guesses.  I'd love it if someone who works on Firefox or has contributed to the security analysis of Firefox would correct any mistakes I'm making here.

[2] Apparently IE/URLMON doesn't URLEncode the string that it hands to the URL handler - I don't know why it does that (probably for compatibility reasons), but that isn't actually relevant to this discussion (especially since all versions of Firefox before, seem to have had the same behavior as IE).  Even if IE had URL encoded the URL before handing it to the handler, Firefox is still being handed untrusted input which violates a critical assumption made by the Firefox command line handler developers.

[3] Btw, I'm using the term "team" loosely.  It's entirely possible that the same one individual did both the Firefox command line handling work AND the firefoxurl protocol handler - it doesn't actually matter.

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  • Based on the enormous lack of comments, perhaps it would be better to move on to another topic.  People just aren't interested in threat modelling.  Heck, even the tetris post got more comments than most of the threat modelling ones.....

  • I would like to see the threat modeling continued.

  • Jeff, I've got about 2 or 3 more posts in the series (probably the rest of the week).

    Ben, I'm not planning on stopping 'til I'm done.

  • This blog used to be interesting, seriously.

  • For the record, it seems to me that the critical assumption made by Firefox (and many other developers) was that a registered URI handler would only be handed legal URIs.  Note that it wouldn't matter if the URIs were malicious, just so long as they met the syntax requirements.

    In fact the specification for calling URI handlers not only fails to require that the URI passed be legal, but actually requires that legal URIs be converted into illegal ones by improperly decoding them!

    However, your point stands; threat modeling would probably have called out the discrepancy between this (IMO reasonable) assumption and the actual specification.

  • Is it this blog or threat modeling?

    If it is threat modleing it is funny how we complain about security issues. Then when a solution that is not fun to play with like threat modeling we dismiss it.

    If it is about the blog I would say no. Good lite reading. Imagine if you would for a moment if FireFox did something in this nature then would the issue be there? So is the issue that not having some form of "Threat Modeling" in place. Or was it the handling of the url being passed?

  • Harry, I'm not 100% sure if that's true.  At a minimum, the firefoxurl handler team should have realized that they were going to receive non URL encoded strings (it's really easy to verify this) and then built their assumptions accordingly.

    And the fact that Firefox before FF 2.0.6 used to have exactly the same behavior as IE when it invoked registered URL handlers leads me to believe that even if IE's behavior is a mistake (and I'm not sure it is), someone on the Firefox team believed that IE's behavior was correct.

  • I don't think it was unreasonable for the firefoxurl handler team to assume that the strings being passed would be legal URIs.  Remember that encoding is mandatory in URIs; any URI with unencoded characters outside of the permitted set is illegal.  (Note that I'm not arguing that making this assumption wasn't a mistake; it was.  But it was an easy mistake to make, and a common one.)

    Neither Firefox nor IE validate the legality of the URL as it passes from the web page to the browser, across the trust boundaries in your diagram for IE.  Personally, I think they should, but I'm told this might cause compatibility issues - the argument seems to be that browsers have accepted illegal URIs for so long that doing so is a de facto standard.  Oh well. :-)

  • Me too, I like this topic. Just doesn't have anything to add so I'm best stay to be a quiet listener. :)

  • The above post should be me... I was thinking of inserting "Ben:" in the beginning... but... :P

  • I believe that back in the olden days, the idea was that you could make URL handlers out of ordinary programs. That way you could have a URL like "" or "" and the program intended to receive it wouldn't have to know about URLs. Of course nowadays anything designed to be a handler would know about URLs, but this facility was designed for programs written before browsers were popular.

    I don't understand why the FireFox people didn't just create a "-unencodedurl" command line option for untrusted input in the first place.

  • "But this violates the assumption that the Firefox command line handler made - they assume that their command line was authored with the same level of trust as the user invoking firefox.  And that's a problem, because now you have a mechanism for any internet site to execute code on the browser client with the privileges of the user."

    This isn't a problem. If you're running code at all under Windows, you fully have the authority to do anything the user can do using simulated mouse clicks and key presses. (Which I believe is entirely broken.) The problem is that you can install a URL extension that any website is able to exploit. Websites should be able to do little more than give interactive information to the user, and recieve input from the user. Allowing a website to do something the user didn't authorize is in itself a security hole.

  • Sorry for posting twice like this, but is there some way to configure it to warn the user that they are letting arbitrary code execute on their machine whenever one of these URL extensions is invoked? Because I didn't consider how useful it could be in certain circumstances to let a website be able to use it.

  • Gabe: They did - they added a "-url:'<url>'" command line option.  But they stopped parsing the -url argument when they got to the closing quote (which is fine if you trust your caller).

    _^_: I believe that both Firefox and IE warn when launching an external handler, but I'm not sure.

    Triangle: The assumption made by the command line parsing folks was just fine.  The firefoxurl logic violated that assumption because now the command line could be authored by an attacker.

    The "simulated mouse clicks and key presses" problem is an interesting one - essentially it boils down to "You can't trust the return address:" - anything that the OS can do to indicate validity of the keypresses can be spoofed.

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