Tuesday, January 23, 2007

Redeclaring Javascript Properties

Ok, so the title of this was originally "Don't we have memories for a reason?", and had a bit of a rant here, but I decided the rant part was a bit unwarranted and stupid (exceptionally stupid, really), so I've removed it so I don't subject anyone else to that crap. Anyway, on with the post:

Jeremiah Grossman (who does some great work, actually), had the idea to stop XSS Worms by denying them access to some crucial functions like XMLHttpRequest() and createElement. You can read the whole post here: http://jeremiahgrossman.blogspot.com/2007/01/preventing-csrf-when-vulnerable-to-xss.html.

Now while I don't think people have attempted to do exactly that before, there were efforts to do the same thing to deny attackers access to cookies a while back, which used the same techniques which it turned out could be easily subverted. Now this isn't my rant, things happen and people might not know about what has happened before, fair enough.

But when people who know about what happened before and that it can be overwritten start saying that Jeremiah's idea works you've really got to wonder why we have memories if we don't use them.

Oh, and here's some code to prove my point: (check your error console to see there are no errors, and the appropriate functions are being called)

javascript:window.__defineGetter__("open", function() { }); delete window.open; window.open("http://kuza55.blogspot.com/",null,"");

javascript:document.createElement = function () {}; delete document.createElement; var bold = document.createElement('b'); bold.innerHtml = 'createElement Works'; document.body.appendChild(bold);

javascript:document.__defineGetter__("write", function() { }); delete document.write; document.write('document.write works');

I also found it humorous that someone was recommending using delete to remove the window element so that people could not call the function, instead of overwriting it.

Ok, now onto some more interesting things. The idea that Jeremiah Proposed for getting rid of the XMLHttpRequest() object was quite a good one, because whenever we try to delete a function it doesn't work, we CANNOT delete functions; it seems you can only delete objects and properties.

The documentation for the delete operator can be found here, it doesn't mention why we can't (or how to) delete functions though: http://developer.mozilla.org/en/docs/Core_JavaScript_1.5_Guide:Operators:Special_Operators#delete

So essentially that definitely works, so good job on coming up with that. Well, untill someone else figures out a way past that as well.

But there are some fun things you can do like the following:


function XMLHttpRequest() { }

<iframe name='test' id='test' src='http://www.google.com/'></iframe>

var req = new window.frames.test.XMLHttpRequest();


So if we can somehow create an iframe, with a name, we can circumvent it. We could also use an iframe created by advertising code, but that is limited by the fact that we would need to use window.frames, and the only things which you can't replace with functions are window, document, and possibly some other constants I can't remember right now.

Disclaimer: I'm not saying that anyone who I've mentioned does bad work (and even when I was ranting, I wasn't saying that), but seriously, you were told about something once, do you need to be told again?

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