How to implement Permission Based Access Control with Asp.Net Core

I had same requirement and i have done it as below and it works fine for me. I am using .Net Core 2.0 Webapi

[AttributeUsage(AttributeTargets.Class | 
                         AttributeTargets.Method
                       , AllowMultiple = true
                       , Inherited = true)]
public class CheckAccessAttribute : AuthorizeAttribute, IAuthorizationFilter
{
  private string[] _permission;
  public CheckAccessAttribute(params string[] permission)
  {
      _permission = permission;
  }

  public void OnAuthorization(AuthorizationFilterContext context)
  {
     var user = context.HttpContext.User;

     if (!user.Identity.IsAuthenticated)
     {
        return;
     }

     IRepository service = 
     (IRepositoryWrapper)context.HttpContext.RequestServices.GetService(typeof(IRepository));
     var success = service.CheckAccess(userName, _permission.ToList());
     if (!success)
     {
        context.Result = JsonFormatter.GetErrorJsonObject(
                               CommonResource.error_unauthorized,
                               StatusCodeEnum.Forbidden);
        return;
     }
     return;
   }
}

In Controller use it like below

[HttpPost]
[CheckAccess(Permission.CreateGroup)]
public JsonResult POST([FromBody]Group group)
{
   // your code api code here.
}

Based on the comments, here an example on how to use the policy based authorization:

public class PermissionRequirement : IAuthorizationRequirement
{
    public PermissionRequirement(PermissionEnum permission)
    {
         Permission = permission;
    }

    public PermissionEnum Permission { get; }
}

public class PermissionHandler : AuthorizationHandler<PermissionRequirement>
{
    private readonly IUserPermissionsRepository permissionRepository;

    public PermissionHandler(IUserPermissionsRepository permissionRepository)
    {
        if(permissionRepository == null)
            throw new ArgumentNullException(nameof(permissionRepository));

        this.permissionRepository = permissionRepository;
    }

    protected override void Handle(AuthorizationContext context, PermissionRequirement requirement)
    {
        if(context.User == null)
        {
            // no user authorizedd. Alternatively call context.Fail() to ensure a failure 
            // as another handler for this requirement may succeed
            return null;
        }

        bool hasPermission = permissionRepository.CheckPermissionForUser(context.User, requirement.Permission);
        if (hasPermission)
        {
            context.Succeed(requirement);
        }
    }
}

And register it in your Startup class:

services.AddAuthorization(options =>
{
    UserDbContext context = ...;
    foreach(var permission in context.Permissions) 
    {
        // assuming .Permission is enum
        options.AddPolicy(permission.Permission.ToString(),
            policy => policy.Requirements.Add(new PermissionRequirement(permission.Permission)));
    }
});

// Register it as scope, because it uses Repository that probably uses dbcontext
services.AddScope<IAuthorizationHandler, PermissionHandler>();

And finally in the controller

[HttpGet]
[Authorize(Policy = PermissionEnum.PERSON_LIST.ToString())]
public ActionResult Index(PersonListQuery query)
{
    ...
}

The advantage of this solution is that you can also have multiple handlers for a requirement, i.e. if first one succeed the second handler can determine it's a fail and you can use it with resource based authorization with little extra effort.

The policy based approach is the preferred way to do it by the ASP.NET Core team.

From blowdart:

We don't want you writing custom authorize attributes. If you need to do that we've done something wrong. Instead you should be writing authorization requirements.

For a solution that doesn't require you to add a policy for each permission see my answer for another question.

It lets you decorate your Controllers and Actions with any custom attributes you wish, and access them in your AuthorizationHandler.