[Source] A potentially dangerous Request.Form value was detected

This problem is because there are HTML strings in the form you submit, for example, you enter html tags in the TextBox, or use the HtmlEditor component in the page, etc., the solution is to disable validateRequest.

If you are .NET 4.0 or higher, be sure to look at Method 3.

This method works in both asp.net webForms and MVC

Add this sentence to the header of the .aspx file:

Modify the web.config file:

Because validateRequest defaults to true. Just set it to false.

web.config

Because 4.0 validation is enabled before HTTP BeginRequest, the validation of the request applies to all ASP.NET resources, aspx pages, ashx pages, web services, and some HTTP handlers, etc.