A potentially dangerous Request.Form value was detected from

Question

A potentially dangerous Request.Form value was detected from the client (ctl00$ConPlacHolderMasterPageForm$txt_desc="<p>
hi this</p>
").

Description: ASP.NET has detected data in the request that is potentially dangerous because it might include HTML markup or script. The data might represent an attempt to compromise the security of your application, such as a cross-site scripting attack. If this type of input is appropriate in your application, you can include code in a web page to explicitly allow it. For more information, see http://go.microsoft.com/fwlink/?LinkID=212874. 

Exception Details: System.Web.HttpRequestValidationException: A potentially dangerous Request.Form value was detected from the client (ctl00$ConPlacHolderMasterPageForm$txt_desc="<p>
hi this</p>
").

Note : I have allready add ValidateRequest="false" on my Page Directory but after this we are also getting above problem when upadate or insert data.