Custom authorize filter for restricting access based on group

AuthorisedADAttribute.cs

public class AuthoriseAdAttribute : AuthorizeAttribute
    {
        protected override bool AuthorizeCore(HttpContextBase httpContext)
        {
            var groupList = GetGroupList();

            if (base.AuthorizeCore(httpContext))
            {
                if (string.IsNullOrEmpty(groupList))
                    return true;

                var groups = groupList.Split(',').ToList();

                var context = new PrincipalContext(ContextType.Domain, "YOURDOMAIN");

                var userPrincipal = UserPrincipal.FindByIdentity(
                    context,
                    IdentityType.SamAccountName,
                    httpContext.User.Identity.Name);

                if (userPrincipal == null)
                    return false;

                try
                {
                    foreach (var group in groups)
                        if (userPrincipal.IsMemberOf(context,
                             IdentityType.Name,
                             group))
                            return true;
                }
                catch
                {
                    return false;
                }
            }
            return false;
        }

        protected override void HandleUnauthorizedRequest(AuthorizationContext filterContext)
        {
            if (filterContext.HttpContext.User.Identity.IsAuthenticated)
            {
                var result = new ViewResult
                {
                    ViewName = "NotAuthorized",
                    MasterName = "_Layout"
                };
                filterContext.Result = result;
            }
            else
                base.HandleUnauthorizedRequest(filterContext);
        }

        private static string GetGroupList()
        {
                var groupList = WebConfigurationManager.AppSettings["ADGROUPS"];
                return groupList;
        }
    }