ASP.NET MVC Read Raw JSON Post Data

Your initial approach should work if you take into consideration the fact, that ASP.NET MVC model binding has already read the stream, so you should rewind it:

[HttpPost]
public ActionResult Callback(string secret)
{
    Request.InputStream.Seek(0, SeekOrigin.Begin);
    string jsonData = new StreamReader(Request.InputStream).ReadToEnd();

    // ...
}