ViewStateUserKey: ViewStateMac Relationship

Posted by on November 26, 2013

I apologize for the delay as I recently spoke about this at the SANS Pen Test Summit in Washington D.C. but haven’t had a chance to put it into a blog. While I was doing some research for my presentation on hacking ASP.Net applications I came across something very interesting that sort of blew my mind. One of my topics was ViewStateUserKey, which is a feature of .Net to help protect forms from Cross-Site Request Forgery. I have always assumed that by setting this value (it is off by default) that it put a unique key into the view state for the specific user. Viewstate is a client-side storage mechanism that the form uses to help maintain state.

I have a previous post about ViewStateUserKey and how to set it here: http://www.jardinesoftware.net/2013/01/07/asp-net-and-csrf/

While I was doing some testing, I found that my ViewState wasn’t different between users even though I had set the ViewStateUserKey value. Of course it was late at night.. well ok, early morning so I thought maybe I wasn’t setting it right. But I triple checked and it was right. Upon closer inspections, my view state was identical between my two users. I was really confused because as I mentioned, I thought it put a unique value into the view state to make the view state unique.

My Problem… ViewStateMAC was disabled. But wait.. what does ViewStateMAC have to do with ViewStateUserKey? That is what I said. So I started digging in with Reflector to see what was going on. What did I find? The ViewStateUserKey is actually used to modify the ViewStateMac modifier. It doesn’t store a special value in the ViewState.. rather it modifies how the MAC is generated to protect thew ViewState from Parameter Tampering.

So this does work*. If the MAC is different between users, then the ViewState is ultimately different and the attacker’s value is different from the victim’s. When the ViewState is submitted, the MAC’s won’t match which is what we want.

Unfortunately, this means we are relying again on ViewStateMAC being enabled. Don’t get me wrong, I think it should be enabled and this is yet another reason why. Without it, it doesn’t appear that the ViewStateUserKey doesn’t anything. We have been saying for the longest time that to protect against CSRF set the ViewStateUserKey. No one has said it relies on ViewStateMAC though.

To Recap.. Things that rely on ViewStateMAC:

  • ViewState
  • Event Validation
  • ViewStateUserKey

It is important that we understand the framework features as disabling one item could cause a domino effect of other items. Be secure.

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