Saturday, September 8, 2012

Cross Domain XHR

Recently I was doing a project (REST web services) where there was an interesting implementation of a defense against CSRF attacks. You would login to the application and immediately be assigned a session ID in a cookie; along with this you would also be assigned an AntiCSRF token, in another cookie. Now this immediately means that both cookies are going to be sent across to the server each time and going to be verified each time. Hence, if this had been the only defense it would be pretty useless; as the additional cookie does..nothing.

In the application though, what was happening was..the cookie 'token' was read, its value grabbed and a new custom header called X-Security-token added in the header with its value. So if on login I get a cookie: token=abcd, the next time I an Edit Profile...I have a new HTTP header called X-Security-token: abcd as part of the request header.

Now my first thought was, Ha, spoofable...the attacker just needs to write a little Javascript, read the cookie (document.cookie) and then add a new custom header and send the request off.

The data was all in JSON format and I wanted a POC to show so I thought I'd just have a simple HTML file with some Jquery code which would make an XHR request by doing all the stuff I talked about above. The code seemed all okay when I tested it on localhost. As in..I set a cookie with the domain as localhost and value as 'arvind' and tried writing JQuery to pick it up and create a new header. That worked and the request DID read the cookie, append it to the header..and send a request to localhost.

The moment I changed the URL to the actual site URL, the request never even fired. I could understand it if the cookie didn't get sent but the request never fired either. More reading revealed that this was due to XHR's limitation of sending cross origin requests. It just refused to send anything at all; I couldn't even see anything in Firebug. I change the URL to localhost..immediately an XHR request gets fired. I tried removing the cookie and resending luck..same behavior. Posts to localhost. Okay. Anywhere else. Browser doesn't allow sending an XHR request anywhere else. Not yet totally clear what's happening.

The other strange thing though is that there is this addon called a REST client which is installed in the same browser and seems to be able to make calls happily to multiple domains other than localhost.

So the question is..What is the difference between an HTML file hosted on a local web server..making XHR calls to AND a firefox addon making calls (not sure if its XHR as I did not see an X-Requested-With header) to Why does the latter work?

So as a CSRF doesn't seem safe but I'm not able to get it working with XHR at least. There's some suggestions elsewhere that some versions of Flash allow this; but right now I'm not sure. I'll update this later if I stumble upon something.