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 Answer

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; }
}

Related Solutions

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

I think you are attacking it from the wrong angle by trying to encode all posted data.

Note that a "<" could also come from other outside sources, like a database field, a configuration, a file, a feed and so on.

Furthermore, "<" is not inherently dangerous. It's only dangerous in a specific context: when writing strings that haven't been encoded to HTML output (because of XSS).

In other contexts different sub-strings are dangerous, for example, if you write an user-provided URL into a link, the sub-string "javascript:" may be dangerous. The single quote character on the other hand is dangerous when interpolating strings in SQL queries, but perfectly safe if it is a part of a name submitted from a form or read from a database field.

The bottom line is: you can't filter random input for dangerous characters, because any character may be dangerous under the right circumstances. You should encode at the point where some specific characters may become dangerous because they cross into a different sub-language where they have special meaning. When you write a string to HTML, you should encode characters that have special meaning in HTML, using Server.HtmlEncode. If you pass a string to a dynamic SQL statement, you should encode different characters (or better, let the framework do it for you by using prepared statements or the like)..

When you are sure you HTML-encode everywhere you pass strings to HTML, then set ValidateRequest="false" in the <%@ Page ... %> directive in your .aspx file(s).

In .NET 4 you may need to do a little more. Sometimes it's necessary to also add <httpRuntime requestValidationMode="2.0" /> to web.config (reference).

Strange unhandled exception from asp.net application – Validation of viewstate MAC failed

I seem to recall that this error can occur if you click a button/link etc before the page has fully loaded.

If this is the case, the error is caused by an ASP.net 2.0 feature called Event Validation. This is a security feature that ensures that postback actions only come from events allowed and created by the server to help prevent spoofed postbacks. This feature is implemented by having controls register valid events when they render (as in, during their actual Render() methods). The end result is that at the bottom of your rendered form tag, you'll see something like this:

<input type="hidden" name="__EVENTVALIDATION" id="__EVENTVALIDATION"  value="AEBnx7v.........tS" />

When a postback occurs, ASP.net uses the values stored in this hidden field to ensure that the button you clicked invokes a valid event. If it's not valid, you get the exception that you've been seeing.

The problem you're seeing happens specifically when you postback before the EventValidation field has been rendered. If EventValidation is enabled (which it is, by default), but ASP.net doesn't see the hidden field when you postback, you also get the exception. If you submit a form before it has been entirely rendered, then chances are the EventValidation field has not yet been rendered, and thus ASP.net cannot validate your click.

One work around is of course to just disable event validation, but you have to be aware of the security implications. Alternatively, just never post back before the form has finished rendering. Of course, that's hard to tell your users, but perhaps you could disable the UI until the form has rendered?

from http://forums.asp.net/p/955145/1173230.aspx

Best Answer

Related Solutions

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

Strange unhandled exception from asp.net application – Validation of viewstate MAC failed

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