Custom Authorization attribute asp.net core

Using policies for something like this isn't overkill. You need a requirement:

public class WhitelistRequirement: IAuthorizationRequirement
{
}

A handler:

public class WhitelistHandler : AuthorizationHandler<WhitelistRequirement>
{

    // Implement a constructor to inject dependencies, such as your whitelist

    protected override Task HandleRequirementAsync(AuthorizationHandlerContext context,
                                                   WhitelistRequirement requirement)
    {
        if (isInWhitelist) // Your implementation here
        {
            context.Succeed(requirement);
        }

        return Task.CompletedTask;
    }
}

Register both in ConfigureServices:

services.AddAuthorization(options =>
            options.AddPolicy("WhitelistPolicy",
            b => b.AddRequirements(new WhitelistRequirement())));

services.AddSingleton<IAuthorizationHandler, WhitelistHandler>();

Then use your policy:

[Authorize(Policy = "WhitelistPolicy")]

You can apply the policy globally with a global filter:

services.AddMvc(config =>
{
    var policy = new AuthorizationPolicyBuilder()
                     .AddRequirements(new WhitelistRequirement())
                     .Build();
    config.Filters.Add(new AuthorizeFilter(policy));
})

The resulting behavior for unauthenticated or forbidden users depends on the implementation of the "challenge" and "forbid" behaviors in your app's authentication handler.

See here.