proof in concept logic for user authentication verification based on DNN cookie.

CheckAuthenticationCookie.cs

private bool CheckAuthenticationCookie()
{
    var cookie = this.Context.Request.Cookies[FormsAuthentication.FormsCookieName];
    if (!this.Context.Request.IsAuthenticated || cookie == null)
    {
        return false;
    }

    FormsAuthenticationTicket ticket = FormsAuthentication.Decrypt(cookie.Value);
    string username = ticket.Name;
    int portalId = PortalController.GetCurrentPortalSettings().PortalId;
    var user = UserController.GetCachedUser(portalId, username);
    return user != null;
}