Custom authorization attribute not working in WebAPI

1. Looks like you are using an MVC filter instead of a Web API filter. It can be detected in the sample because it uses HttpContextBase. Instead use the filter from the System.Web.Http.Filters namespace.
2. You need to override OnAuthorization or OnAuthorizationAsync on the Web API filter.
3. You don't need to register a global filter and decorate your controller with it. Registering it will make it run for all controllers.

Web API filter code: https://github.com/aspnetwebstack/aspnetwebstack/blob/master/src/System.Web.Http/Filters/AuthorizationFilterAttribute.cs

YOur custom attribute should inherit from System.Web.Http.Filters.AuthorizationFilterAttribute

and it should look like this

using System.Web.Http.Controllers;
using System.Web.Http.Filters;
public class CustomAuthorizeAttribute : System.Web.Http.Filters.AuthorizationFilterAttribute
{   
    public override bool AllowMultiple
    {
        get { return false; }
    }

    public override void OnAuthorization(HttpActionContext actionContext)
    {
        //Perform your logic here
        base.OnAuthorization(actionContext);
    }
}