Session is null in ashx
If the HttpContext.Current.Session is null in your .ashx handler, add System.Web.SessionState.IReadOnlySessionState to your class declaration, like this:
public class MyHandler : IHttpHandler, System.Web.SessionState.IReadOnlySessionState
Also, if you need to write to Session[], you can use IRequresSessionState instead of IReadOnlySessionState.
see also:
http://forums.asp.net/t/1773026.aspx/1
http://stackoverflow.com/questions/7705802/httpcontext-current-session-is-null-in-mvc-3-appplication