Custom Authorization in Asp.net WebApi - what a mess?

Ok, my suggestion is to do the following assuming that you are using OAuth bearer tokens to protect your Web API and you are setting the allowedTime as a claim for the user when you issued the token. You can read more about token based authentication here

1. Create CustomAuthorizeAttribute which derives from AuthorizationFilterAttribute

2. override method OnAuthorizationAsync and use the sample code below:

    public class CustomAuthorizeAttribute : AuthorizationFilterAttribute
   {
   
       public override Task OnAuthorizationAsync(HttpActionContext actionContext, System.Threading.CancellationToken cancellationToken)
       {
   
           var principal = actionContext.RequestContext.Principal as ClaimsPrincipal;
   
           if (!principal.Identity.IsAuthenticated)
           {
               return Task.FromResult<object>(null);
           }
   
           var userName = principal.FindFirst(ClaimTypes.Name).Value;
           var userAllowedTime = principal.FindFirst("userAllowedTime").Value;
   
           if (currentTime != userAllowedTime)
           {
               actionContext.Response = actionContext.Request.CreateResponse(HttpStatusCode.Unauthorized, "Not allowed to access...bla bla");
               return Task.FromResult<object>(null);
           }
   
           //User is Authorized, complete execution
           return Task.FromResult<object>(null);
   
       }
   }
   

3. Now in your controllers you use CustomAuthorize attribute to protect your controllers using this authorization logic.

Which methods should I use : IsAuthorized or OnAuthorization ? ( or when to use which)

You will extend AuthorizationFilterAttribute if your authorization logic is not dependent on the identity established and roles. For user related authorization, you will extend and use AuthorizeAttribute. For the former case, you will override OnAuthorization. For the latter case, you will override IsAuthorized. As you could see from the source code of these attributes, OnAuthorization is marked virtual for you to override if you derive from AuthorizationFilterAttribute. On the other hand, the IsAuthorized method is marked virtual in AuthorizeAttribute. I believe this is a good pointer to the intended usage.

when should I call base.IsAuthorized or base.OnAuthorization?

The answer to this question lies in how OO generally works. If you override a method, you can either completely provide a new implementation or piggy back on the implementation provided by parent and enhance the behavior. For example, take the case of IsAuthorized(HttpActionContext). The base class behavior is to check the user/role against what is specified in the filter vs the identity established. Say, you want to do all that but in addition, you want to check something else, may be based on a request header or something. In that case, you can provide an override like this.

protected override bool IsAuthorized(HttpActionContext actionContext)
{
    bool isAuthroized = base.IsAuthorized(actionContext);
    // Here you look at the header and do your additional stuff based on actionContext
    // and store the result in isRequestHeaderOk
    // Then, you can combine the results
    // return isAuthorized && isRequestHeaderOk;
}

I'm sorry but don't understand your Q3. BTW, Authorization filter has been around for a long time and people use it for all kinds of things and sometimes incorrectly as well.

One more thing. And finally there was this guy here who said : You shouldn't override OnAuthorization - because you would be missing [AllowAnonymous] handling.

The guy who said that is the God of access control - Dominick. Obviously it will be correct. If you look at the implementation of OnAuthorization (copied below),

public override void OnAuthorization(HttpActionContext actionContext)
{
    if (actionContext == null)
    {
        throw Error.ArgumentNull("actionContext");
    }

    if (SkipAuthorization(actionContext))
    {
        return;
    }

    if (!IsAuthorized(actionContext))
    {
        HandleUnauthorizedRequest(actionContext);
    }
}

the call to SkipAuthorization is the part that ensures AllowAnonymous filters are applied, that is, authorization is skipped. If you override this method, you loose that behavior. Actually, if you decide to base your authorization on users/roles, at that point you would have decided to derive from AuthorizeAttribute. Only correct option left for you at that point will be to override IsAuthorized and not the already overridden OnAuthorization, although it is technically possible to do either.

PS. In ASP.NET Web API, there is another filter called authentication filter. Idea is that you use that for authentication and authorization filter for authorization, as the name indicates. However, there are lots of examples where this boundary is fudged. Lots of authroization filter examples will do some kind of authentication. Anyways, if you have time and want to understand a bit more, take a look at this MSDN article. Disclaimer: It was written by me.