Invalid Viewstate since ASP.NET Oracle Padding Vulnerability Security Patch
<p>Since installing the security patch for the ASP.NET Oracle Padding vunerability any user that was keeping themselves logged in to our site is getting error messages when hitting any page.</p>

<p>The errors logged on the server are</p>

<pre><code>System.Web.UI.ViewStateException: Invalid viewstate. 
Client IP: xxx.xxx.xxx.xxx
Port: 55796
User-Agent: Mozilla/5.0 (Windows; U; Windows NT 5.1; en-US; rv:1.9) Gecko/2008052906 Firefox/3.0
ViewState: l4nsXEvWcOwlDpmdbxw916bpHoPiqdBP7Syb+zCQAv44xv/r3oLtETKTL28/Gts6
Referer: 
Path: /product/4795/fender-usa-deluxe-stratocaster-mn-olympic-white-pearl
</code></pre>

<p>With custom errors switched off a user sees the following information</p>

<pre><code>Validation of viewstate MAC failed. If this application is hosted by a Web Farm or cluster, ensure that &lt;machineKey&gt; configuration specifies the same validationKey and validation algorithm. AutoGenerate cannot be used in a cluster. 
Description: An unhandled exception occurred during the execution of the current web request. Please review the stack trace for more information about the error and where it originated in the code. 

Exception Details: System.Web.HttpException: Validation of viewstate MAC failed. If this application is hosted by a Web Farm or cluster, ensure that &lt;machineKey&gt; configuration specifies the same validationKey and validation algorithm. AutoGenerate cannot be used in a cluster.

Source Error: 

An unhandled exception was generated during the execution of the current web request. Information regarding the origin and location of the exception can be identified using the exception stack trace below.  

Stack Trace: 


[ViewStateException: Invalid viewstate. 
Client IP: xxx.xxx.xxx.xxx
Port: 3588
User-Agent: Mozilla/4.0 (compatible; MSIE 8.0; Windows NT 5.1; Trident/4.0; GTB6.5; .NET CLR 2.0.50727; OfficeLiveConnector.1.3; OfficeLivePatch.0.0; .NET CLR 3.0.4506.2152; .NET CLR 3.5.30729)
ViewState: s0toPCu7bxkB7a3G+KTxawY3ILf1qunZyIqNBKg8xSoqY2BkWIUCJAHKFKo2RnJw
Referer: 
Path: /]

[HttpException (0x80004005): Validation of viewstate MAC failed. If this application is hosted by a Web Farm or cluster, ensure that &lt;machineKey&gt; configuration specifies the same validationKey and validation algorithm. AutoGenerate cannot be used in a cluster.]
System.Web.UI.ViewStateException.ThrowError(Exception inner, String persistedState, String errorPageMessage, Boolean macValidationError) +118
System.Web.UI.ViewStateException.ThrowMacValidationError(Exception inner, String persistedState) +13
System.Web.UI.ObjectStateFormatter.Deserialize(String inputString) +238
System.Web.UI.ObjectStateFormatter.System.Web.UI.IStateFormatter.Deserialize(String serializedState) +5
System.Web.Mvc.AntiForgeryDataSerializer.Deserialize(String serializedToken) +90
</code></pre>

<p>The solution to this problem is to delete all cookies and log back in, but obviously an average user, won't know to do this and I'm worried they will just think our site is broken.</p>

<p>Is there anything I can do about this like force people who were logged in to log in again?</p>

<p>Thanks for any help you can be</p>
