Asp.net-mvc – Potentially dangerous Request.Form value was detected from the client

asp.net-mvcexceptionhttpwebrequestsystem.web

I am running an ASP.Net MVC application and facing the following error. As I am new to ASP.Net, could someone please help me as to what does it mean and how can I resolve it?

I tried googling to understand it, but found different answers for the same error which left me more confused.

Exception caught in Global.asax:System.Web.HttpRequestValidationException (0x80004005): A potentially dangerous Request.Form value was detected from the client (ctl00$MainContent$WarningCtl1$TXTWarningText="

This is the warni…").
at System.Web.HttpRequest.ValidateString(String value, String collectionKey, RequestValidationSource requestCollection)
at System.Web.HttpRequest.ValidateNameValueCollection(NameValueCollection nvc, RequestValidationSource requestCollection)
at System.Web.HttpRequest.get_Form()
at System.Web.HttpRequest.get_HasForm()
at System.Web.UI.Page.GetCollectionBasedOnMethod(Boolean dontReturnNull)
at System.Web.UI.Page.DeterminePostBackMode()
at System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint)
at System.Web.UI.Page.ProcessRequest(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint)
at System.Web.UI.Page.ProcessRequest()
at System.Web.UI.Page.ProcessRequest(HttpContext context)
at ASP.app_config_appttypes_groupappttypes_aspx.ProcessRequest(HttpContext context)
at System.Web.HttpApplication.CallHandlerExecutionStep.System.Web.HttpApplication.IExecutionStep.Execute()
at System.Web.HttpApplication.ExecuteStep(IExecutionStep step, Boolean& completedSynchronously)

Please suggest.

Best Solution

You need to add the ValidateInputAttribute to your controller (which applies it to all of your action methods for that controller, so be careful):

[ValidateInput (false)]
public class MyController : Controller { ... }

Or your action method:

public class MyOtherController : Controller
{
    [ValidateInput (false)]
    public ActionResult MyActionMethod (MyObjectThatTakesInHtml myObject)
    { ... }
}

Edit

As @dotjoe pointed out, and I forgot to mention, you also have access to the AllowHtmlAttribute (found in System.Web.Mvc) on a property in your model.

public class MyObjectThatTakesInHtml
{
    [AllowHtml]
    public string MyHtmlProperty { get; set; }
}