Validation of viewstate MAC failed after installing .NET 3.5 SP1

After installing .NET 3.5 SP1 you may get Validation of viewstate MAC failed exceptions when doing post backs on ASP.NET pages

Validation of viewstate MAC failed. If this application is hosted by a Web Farm or cluster, ensure that <machineKey> 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 <machineKey> configuration specifies the same validationKey and validation algorithm. AutoGenerate cannot be used in a cluster.

Stack Trace:

[HttpException (0x80004005): Unable to validate data.]
   System.Web.Configuration.MachineKeySection.GetDecodedData(Byte[] buf, Byte[] modifier, Int32 start, Int32 length, Int32& dataLength) +289
   System.Web.UI.ObjectStateFormatter.Deserialize(String inputString) +140

[ViewStateException: Invalid viewstate.
   Client IP: 127.0.0.1
   Port: 34562
   User-Agent: Mozilla/4.0 (compatible; MSIE 7.0; Windows NT 5.2; WOW64; .NET CLR 2.0.50727; .NET CLR 1.1.4322; InfoPath.2; .NET CLR 3.0.04506.30; .NET CLR 3.0.04506.590; .NET CLR 3.0.04506.648; .NET CLR 3.5.21022; Zune 3.0; .NET CLR 3.0.4506.2152; .NET CLR 3.5.30729; MS-RTC LM 8)
   ViewState: /wEPDwUKMTc2NzE0NzA0NmRkmWc0SFS8H55FfURfpUekG2KhS8g=
   Referer: http://localhost/MySite/Default3.aspx
   Path: /MySite/Default.aspx]

[HttpException (0x80004005): Validation of viewstate MAC failed. If this application is hosted by a Web Farm or cluster, ensure that <machineKey> 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) +106
   System.Web.UI.ViewStateException.ThrowMacValidationError(Exception inner, String persistedState) +14
   System.Web.UI.ObjectStateFormatter.Deserialize(String inputString) +242
   System.Web.UI.ObjectStateFormatter.System.Web.UI.IStateFormatter.Deserialize(String serializedState) +4
   System.Web.UI.Util.DeserializeWithAssert(IStateFormatter formatter, String serializedState) +37
   System.Web.UI.HiddenFieldPageStatePersister.Load() +207
   System.Web.UI.Page.LoadPageStateFromPersistenceMedium() +105
   System.Web.UI.Page.LoadAllState() +43
   System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint) +6785
   System.Web.UI.Page.ProcessRequest(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint) +242
   System.Web.UI.Page.ProcessRequest() +80
   System.Web.UI.Page.ProcessRequestWithNoAssert(HttpContext context) +21
   System.Web.UI.Page.ProcessRequest(HttpContext context) +49
   ASP.default_aspx.ProcessRequest(HttpContext context) +4
   System.Web.CallHandlerExecutionStep.System.Web.HttpApplication.IExecutionStep.Execute() +181
   System.Web.HttpApplication.ExecuteStep(IExecutionStep step, Boolean& completedSynchronously) +75

Cause

This will happen if you have specified an action on the forms element, and if the action is different than the page you are browsing to, i.e. in this case the page I am browsing to is Default3.aspx, but the action is set to Default.aspx (as you can see from the Path and Referrer in the error message)

<form id="form1" runat="server" action="Default.aspx">

The reason this occurs after installing SP1 for .NET 3.5 is because prior to this service pack, the action attribute was ignored. Now that it is no longer ignored, the post-back will actually post back to the action page, and of course the view state for Default3.aspx will not be valid for Default.aspx.

You’ve heard it before :) this is not a bug, it’s a feature… in this case that’s actually true…

Resolution

Remove the action attribute, or change it to post to the right page if you want to have viewstate enabled.

Laters, Tess