Asp.net EnableViewStateMac and malicious code [December security updates]

Question

In the latest Asp.Net December 2013 Security updates Microsoft released a patch for ASP.Net - 'Insecure ASP.NET Web Forms (.aspx) configuration could allow remote code execution'. Related KB is 2905247

As I know ViewStateMac just used to be sure that this ViewState was generated by server, and not an attacker. But in security updates article they say:

If a web developer sets EnableViewStateMac=false for any page in his site, an attacker could leverage this to upload and invoke arbitrary executable code within the context of the web service account. This is an example of a remote code execution (RCE) attack.

For example if I disable EnableViewStateMac for some aspx page, how it allows attacker to execute malicious code in context of my web application? As I understand in the worst case an attacker can spoof ViewState for some fake data\events\validation. But it will affect just this page. And I can't upload any malicious C# code via ViewState that will be executed. What are they mean by RCE attack in that case?

Answer 1

By itself, it doesn't necessarily allow an attacker to run code, but if you act upon some input expecting it to be valid -- hidden field with values or something similar, then it could be the key they need.

Answer 2

I can't find any further details of this vulnerability, so my answer only speculates to a possible attack vector.

The MAC is a signature of the ViewState value, and with EnableViewStateMac=true ASP.NET will check whether the MAC signature actually signs the ViewState value as authentic. This means that the ViewState value sent from the client in the __VIEWSTATE hidden field has been verified to come from the server.

Now imagine that the code processing the ViewState value is vulnerable to say object deserialisation. The Microsoft advisory states:

An unauthenticated attacker could send specially crafted HTTP content to the targeted server, potentially allowing the attacker to run code on the server in the context of the service account running on the ASP.NET site.

With EnableViewStateMac=true this vulnerability cannot be exploited because the MAC is validated before the ViewState value is processed. Any value that has not come from the server will be met with a Validation of viewstate MAC failed exception message.

However, with EnableViewStateMac=false the whole ViewState value could be manipulated by an attacker to exploit the buffer overflow/escape attack with privilege escalation and execute arbitrary code that is injected as a payload into the __VIEWSTATE field.

So in summary, as the ViewState value is no longer being validated it opens up the field to attack by this unpublicised attack vector.