A: 

try encoding your CKEditor HTML output. Add this to your CKEditor declaration:

oCKeditor.Config['HtmlEncodeOutput'] = true;

More information about the error you're getting on ASP.NET Website

Marek Karbarz
where do I wirte this code?
mohammad reza
are you using just plain javascript version of the CKEditor or the ASP.NET control (FCKeditor.Net 2.6.3)?
Marek Karbarz
my friend I can solve this problem with your example in the Link,and thank you
mohammad reza
if you're setting your validation to false (<%@ Page validateRequest="false" %>) then it's not exactly the safest way to go; but if that works for you I guess that all that matters
Marek Karbarz
A: 

<%@ Page Language="C#" AutoEventWireup="true" CodeFile="Default.aspx.cs" Inherits="Default" ValidateRequest="false" %>

madhavi
That's a really bad suggestion, pretty much turns security off for that page.
mhenrixon