A potentially dangerous Request.Form value was det

Question

Every time a user posts something containing < or > in a page in my web application, I get this exception thrown.

I don't want to go into the discussion about the smartness of throwing an exception or crashing an entire web application because somebody entered a character in a text box, but I am looking for an elegant way to handle this.

Trapping the exception and showing

An error has occurred please go back and re-type your entire form again, but this time please do not use <

doesn't seem professional enough to me.

Disabling post validation (validateRequest="false") will definitely avoid this error, but it will leave the page vulnerable to a number of attacks.

Ideally: When a post back occurs containing HTML restricted characters, that posted value in the Form collection will be automatically HTML encoded. So the .Text property of my text-box will be something & lt; html & gt;

Is there a way I can do this from a handler?

Answer 1

You could also use JavaScript's escape(string) function to replace the special characters. Then server side use Server.URLDecode(string) to switch it back.

This way you don't have to turn off input validation and it will be more clear to other programmers that the string may have HTML content.

Answer 2

There's a different solution to this error if you're using ASP.NET MVC:

• ASP.NET MVC – pages validateRequest=false doesn’t work?
• Why is ValidateInput(False) not working?
• ASP.NET MVC RC1, VALIDATEINPUT, A POTENTIAL DANGEROUS REQUEST AND THE PITFALL

C# sample:

[HttpPost, ValidateInput(false)]
public ActionResult Edit(FormCollection collection)
{
    // ...
}

Visual Basic sample:

<AcceptVerbs(HttpVerbs.Post), ValidateInput(False)> _
Function Edit(ByVal collection As FormCollection) As ActionResult
    ...
End Function

Answer 3

If you are on .NET 4.0 make sure you add this in your web.config file inside the <system.web> tags:

<httpRuntime requestValidationMode="2.0" />

---

In .NET 2.0, request validation only applied to aspx requests. In .NET 4.0 this was expanded to include all requests. You can revert to only performing XSS validation when processing .aspx by specifying:

requestValidationMode="2.0"

You can disable request validate entirely by specifying:

validateRequest="false"

Answer 4

If you don't want to disable ValidateRequest you need to implement a JavaScript function in order to avoid the exception. It is not the best option, but it works.

function AlphanumericValidation(evt)
{
    var charCode = (evt.charCode) ? evt.charCode : ((evt.keyCode) ? evt.keyCode :
        ((evt.which) ? evt.which : 0));

    // User type Enter key
    if (charCode == 13)
    {
        // Do something, set controls focus or do anything
        return false;
    }

    // User can not type non alphanumeric characters
    if ( (charCode <  48)                     ||
         (charCode > 122)                     ||
         ((charCode > 57) && (charCode < 65)) ||
         ((charCode > 90) && (charCode < 97))
       )
    {
        // Show a message or do something
        return false;
    }
}

Then in code behind, on the PageLoad event, add the attribute to your control with the next code:

Me.TextBox1.Attributes.Add("OnKeyPress", "return AlphanumericValidation(event);")

Answer 5

I ended up using JavaScript before each postback to check for the characters you didn't want, such as:

<asp:Button runat="server" ID="saveButton" Text="Save" CssClass="saveButton" OnClientClick="return checkFields()" />

function checkFields() {
    var tbs = new Array();
    tbs = document.getElementsByTagName("input");
    var isValid = true;
    for (i=0; i<tbs.length; i++) {
        if (tbs(i).type == 'text') {
            if (tbs(i).value.indexOf('<') != -1 || tbs(i).value.indexOf('>') != -1) {
                alert('<> symbols not allowed.');
                isValid = false;
            }
        }
    }
    return isValid;
}

Granted my page is mostly data entry, and there are very few elements that do postbacks, but at least their data is retained.

Answer 6

In ASP.NET, you can catch the exception and do something about it, such as displaying a friendly message or redirect to another page... Also there is a possibility that you can handle the validation by yourself...

Display friendly message:

protected override void OnError(EventArgs e)
{
    base.OnError(e);
    var ex = Server.GetLastError().GetBaseException();
    if (ex is System.Web.HttpRequestValidationException)
    {
        Response.Clear();
        Response.Write("Invalid characters."); //  Response.Write(HttpUtility.HtmlEncode(ex.Message));
        Response.StatusCode = 200;
        Response.End();
    }
}