RequestValidation Changes in ASP.NET 4.0

There’s been a change in the way the ValidateRequest attribute on WebForms works in ASP.NET 4.0. I noticed this today while updating a post on my WebLog all of which contain raw HTML and so all pretty much trigger request validation. I recently upgraded this app from ASP.NET 2.0 to 4.0 and it’s now failing to update posts. At first this was difficult to track down because of custom error handling in my app – the custom error handler traps the exception and logs it with only basic error information so the full detail of the error was initially hidden. After some more experimentation in development mode the error that occurs is the typical ASP.NET validate request error (‘A potentially dangerous Request.Form value was detetected…’) which looks like this in ASP.NET 4.0: