A Case of Invalid Viewstate

Last week I was helping a colleague of mine with a viewstate case that turned out to be pretty interesting...


The customer was getting events similar to the following in the eventlog and needed to know why they occurred

 Event Type: Information
Event Source: ASP.NET 2.0.50727.0
Event Category: Web Event 
Event ID: 1316
Date: 2007-06-11
Time: 09:48:02
User: N/A
Computer: MYMACHINE 

Event code: 4009 
Event message: Viewstate verification failed. Reason: The viewstate supplied failed integrity check. 
Event time: 2007-06-11 09:48:02 
Event time (UTC): 2007-06-11 07:48:02 
Event ID: 14cc57c05e834de98c7df506a013a706 
Event sequence: 10 
Event occurrence: 1 
Event detail code: 50203 

Application information: 

Application domain: /LM/w3svc/1/root/MyApp-3-128260216693527710 
Trust level: Full 
Application Virtual Path: /MyApp 
Application Path: c:\inetpub\wwwroot\MyApp\ 
Machine name: MYMACHINE 

Process information: 

Process ID: 3640 
Process name: w3wp.exe 
Request information: 
Request URL: http://mymachine/MyApp/MyWebForm.aspx
Request path: /TestBadViewstate/WebForm1.aspx 
User host address: 
Is authenticated: False 
Authentication Type: 

ViewStateException information: 

Exception message: Invalid viewstate. 
Client IP: 
Port: 14644 
User-Agent: Mozilla/4.0 (compatible; MSIE 7.0; Windows NT 5.2; WOW64; .NET CLR 2.0.50727; .NET CLR 1.1.4322; .NET CLR 3.0.04324.17; InfoPath.2) 
Referer: http://SomeThirdPartySite.com/SomePage.htm
Path: /MyApp/MyWebForm.aspx 

Custom event details: 
For more information, see Help and Support Center at http://go.microsoft.com/fwlink/events.asp. 


And the callstack reported was:

 [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) +119
System.Web.UI.ObjectStateFormatter.Deserialize(String inputString) +252
System.Web.UI.ObjectStateFormatter.System.Web.UI.IStateFormatter.Deserialize(String serializedState) +5
System.Web.UI.Util.DeserializeWithAssert(IStateFormatter formatter, String serializedState) +37
System.Web.UI.HiddenFieldPageStatePersister.Load() +222
System.Web.UI.Page.LoadPageStateFromPersistenceMedium() +80
System.Web.UI.Page.LoadAllState() +35
System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint) +9041
System.Web.UI.Page.ProcessRequest(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint) +217
System.Web.UI.Page.ProcessRequest() +85
System.Web.UI.Page.ProcessRequestWithNoAssert(HttpContext context) +20
System.Web.UI.Page.ProcessRequest(HttpContext context) +110
ASP.MyPage_aspx.ProcessRequest(HttpContext context) +30
System.Web.CallHandlerExecutionStep.System.Web.HttpApplication.IExecutionStep.Execute() +405
System.Web.HttpApplication.ExecuteStep(IExecutionStep step, Boolean& completedSynchronously) +65



If you get viewstate errors on 1.1. and dont get events like the above, make sure that you have SP1 installed or at least a version of 1.1 later than http://support.microsoft.com/?id=831150  since this hotfix introduced the type of logging above, which is crucial to troubleshooting most viewstate errors.

Viewstate, as most of you know, is a Base64 encoded string containing information about the state of the controls on the webform.  To avoid tampering the viewstate is validated against the machine key, pagename etc. and if the string is either corrupt in some way such that it cant be Base64 decoded or such that it doesn't pass validation you will get an error like the above.

Typically viewstate errors will occur if

  • You're on a web farm and the machine keys are not consistent on all nodes
  • You're on a web farm and the page is slightly different on one node than ot the others, for example if one node is upgraded to 2.0 and the other is still 1.1 or if some controls used on the page are built differently

For a more comprehensive list see: http://support.microsoft.com/?id=829743


In this particular case they had just upgraded to 2.0 from 1.1, they were not running on a web farm.  The error occurred intermittently and only on two specific pages, but most of the time these pages were served just fine without viewstate errors.

Since it is very unusual for errors like this to occur in a non-web farm scenario, especially intermittently, this issue was very curious.   I have seen that happen sometimes when the client was a mobile device and the viewstate was very large as some mobile devices will only send x kb of data so the issue there would be that the viewstate was corrupted because not all the viewstate was sent.   In this case the client was IE 7.0 so no such luck...

I used Fritz Onion's viewstatedecoderr to decode the viewstate to verify that it wasn't corrupted and sure enough the viewstate was just fine, the contents seemed to be fairly standard textboxes etc.

Lucky break

Lucky for me, and for them:)  I have seen enough encoded viewstate to know what viewstate is supposed to look like, so when i took another look at the eventlog entry i noticed that the persisted state started with dDw...    to most people this probably just looks like mumbojumbo but I happen to know that viewstate on 2.0 is supposed to start with /wE...  in fact when I browsed their site on the internet I could indeed see that the typical viewstate for these affected pages (from the hidden viewstate field in view source) was perfectly normal 2.0 viewstate while the dDw... that was sent to the page is typical for 1.1 viewstate.

Knowing this, the question is now, where does this 1.1. viewstate come from?   This is where the eventlog entry comes in handy again... and if you have jumped ahead a bit you have probably noticed by now that the referer is a htm page http://somethirdpartysite.com/SomePage.htm, in other words. there is nothing wrong with the site itself, the problem is that somehow this 3rd party site is posting 1.1. viewstate to the page, and of course that won't validate well against the new 2.0 page.

Browsing to the 3rd party site and looking at the source for the htm file we can see what we already suspected... the page contains a form, with a hidden viewstate field and the post action for the form is our aspx page, so this 3rd party site apparently did a bit of a hack, copying the viewstate while this site was still 1.1. and posting that viewstate to the form... 

Technically, noone but the aspx page itself should really be allowed to post to the aspx page, so anytime the referer of a post to the aspx page is someone other than itself that is bad, and again, to guard against things like this the viewstate validation exists, but of course it might prove to cause a bit of a troubleshooting headache for you.



The solution in this case was to talk to the 3rd party site, let them know about the upgrade and ask them to avoid posting to the site but rather link to the site or redirect to it since there is no way of knowing how long viewstate will be valid as it can change with any type of upgrade to the page.

If there is a benefit to having the 3rd party site being able to post data and show the results in their proprietary format, a web service would be a pretty good solution.


Until next time