Is this an attempt to break my ASP.Net site's security? - asp.net

Question

I am fairly new to ASP.NET. I recently set up automated email from my website to notify me of an unhandled exceptions. Just a few hours ago in 3 minutes there were 10 unhandled exceptions and all stack traces were similar. There is a lot in the error messages I do not understand, but I do not like the way this looks.
Here is one of the email messages:
An unhandled exception occurred:
Message: Padding is invalid and cannot be removed.
Stack Trace:
at System.Security.Cryptography.RijndaelManagedTransform.DecryptData(Byte[] inputBuffer, Int32 inputOffset, Int32 inputCount, Byte[]& outputBuffer, Int32 outputOffset, PaddingMode paddingMode, Boolean fLast)
at System.Security.Cryptography.RijndaelManagedTransform.TransformFinalBlock(Byte[] inputBuffer, Int32 inputOffset, Int32 inputCount)
at System.Security.Cryptography.CryptoStream.FlushFinalBlock()
at System.Web.Configuration.MachineKeySection.EncryptOrDecryptData(Boolean fEncrypt, Byte[] buf, Byte[] modifier, Int32 start, Int32 length, IVType ivType, Boolean useValidationSymAlgo)
at System.Web.UI.Page.DecryptStringWithIV(String s, IVType ivType)
at System.Web.Handlers.AssemblyResourceLoader.System.Web.IHttpHandler.ProcessRequest(HttpContext context)
at System.Web.HttpApplication.CallHandlerExecutionStep.System.Web.HttpApplication.IExecutionStep.Execute()
at System.Web.HttpApplication.ExecuteStep(IExecutionStep step, Boolean& completedSynchronously)
Is this an attempt to hack into my site or something else?
Many thanks go to those who provided Answers and Comments provided which pointed me in the right direction to get an answer to this exception. It's cause can be difficult to determine, especially when not occuring in a web farm.
IE 5.5 was not causing the problem as I had earlier thought.
It was not easily found, but a post on my web host forum by one of the staff mentioned reports of Viewstate errors. The cause was attributed to the asp worker process or the server recycling.
The asp worker process settings refers to the processModel Element in the machine.config file. See http://msdn.microsoft.com/en-us/library/7w2sway1(VS.80).aspx for more information.
The recommended fix was to set an encrypted machineKey in the web.config file. The Machine Key node is in the system.web element.
This was easily done and solved the problem thanks to the handy ASP.NETResources site which has a MachineKey Generator. See http://www.aspnetresources.com/tools/keycreator.aspx.

Answer 1

This exception is thrown when the assembly resource handler gets an invalid request. It is unlikely this is related to any malicious activity; it's usually caused by an incorrectly configured machine config on the server.
Some background:
ASP.NET has built-in mechanisms for exposing resources from the assembly (dll) via an HttpHandler. Resources such as JavaScripts and images can be stored as text in the assembly and requested by the browser via .axd handlers. But for security reasons, the handlers don't accept a plain-text location of the resource, which might expose clues about how your code works. Instead, it uses information in the machine.config on the server to encrypt a unique identifier to the resource. This exception gets thrown when a resource is requested, but when the server tries to decrypt the identifier provided, it fails.

Answer 2

The error is because your appdomain was recycled/restarted. When that happens the application and the machine key is set to auto, it changes. That affects the decryption of the info in the url of the resources urls (.axd). Setting up a fixed machine key will prevent it from ever happening again.
Please check this for more info on a similar case (the explanation is with an issue with viewstate validation, but the cause is the same one):
http://www.developmentnow.com/blog/InvalidViewstate+Or+Unable+To+Validate+Data+Error.aspx
Ps. this explains it on single server deployments :) - although the solution is the same for both multi-single server, the fix was usually only explained to make all servers use the same machine key.
Update 1: The padding is invalid message doesn't have relation to the css padding. If it is only happening on ie 5.5, it is likely the parameters for the webresource.axd are being messed up, just like in this question: Invalid Webresource.axd parameters being generated.