Add this in Web.config
<configuration>
<system.web>
<compilation debug="true" targetFramework="4.0"/>
<httpRuntime requestValidationMode="2.0"/>
</system.web>
</configuration>
and add ValidateRequest = "false"
in Page directive like this
<%@ Page Language="C#" AutoEventWireup="true"
CodeFile="Default.aspx.cs" Inherits="_Default" ValidateRequest="false" %>
for your reference click here