Anti CSRF header

Recently I came across an application which was preventing crsf attacks using a unique non-traditional approach. In traditional approach the csrf is thwarted by embedding unique random tokens, called nonce, in each sensitive page. But this application, which was making ajax calls and used jQuery, was creating a header to identify the valid and invalid requests altogether. The idea is to generate a custom header, x-session-token in this case, with every request which is considered sensitive and includes any sort of transaction. For example:

xhr.setRequestHeader('x-session-token', csrf_token)
At the server level, server checks for this header if found request is fulfilled, otherwise rejected.

We need to use xhr calls for making use of this technique, not useful in regular POST and GET requests. Since, I was not aware of this kind of countermeasures, probably, since most of the applications I did were using standard requests. So, I searched a bit and found even Google also uses this approach predominately. However, it seems not be best approach, where custom headers can also be exploited, but certainly, it raises the bar and simpler approach. And a new thing for my knowledge also!


Anonymous said…
how will you exploit custom header? as u mentioned in post, the csrf_token is sent from server, embedded in page, how will the attacker come to know of this?
Nilesh Kumar said…
@ Anonymous: If you see the reference link, they have mentioned some external link, where the custom headers can be spoofed. Even I am new to this approach, but seems it's no more secure now, so we need to fall back on the old traditional methods of anti-CSRF rather than using custom headers.
Anonymous said…
getting the random value is impossible, provided they are not running vulnerable version of flash etc., in which case the random value in the post body can also be leaked. So this is as good a place as any to place the random token, i think.
Nilesh Kumar said…
@ Anonymous: Honestly, I have not read the link, but since I came to know about this technique, I wrote about it. Still, I would prefer the embedding nonce in pages approach. But thanks for your insight!

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