Identity server 4: intercept 302 and replace it with 401 - identityserver4

Question

I've got an app which is hosting simultaneously Identity Server 4 and a client app (Vue) which uses a couple of rest services defined in an area for managing the site. The idea is that users associated with a specific role can access the client app and call the rest services for performing the actions.
Currently, my problem is that when the api return 302 when the user doesn't belong to the admin role. I'd like to change this to a 401, but I'm having some problems with it.
If this was a simple aspnet core app, then I'd simply pass a lambda to the OnRedirectToLogin property of the cookie handler that takes care of the request. Unfortunately, IS4 will only allow me to set a couple of basic settings of the cookie (expiration and sliding). The same docs say that I can override the cookie handler. So, I've tried doing the following:
services.AddIdentityServer()
... // other configurations
services.AddAuthentication(sharedOptions => {
sharedOptions.DefaultAuthenticateScheme = CookieAuthenticationDefaults.AuthenticationScheme;
sharedOptions.DefaultScheme = CookieAuthenticationDefaults.AuthenticationScheme;//IdentityServerConstants.ExternalCookieAuthenticationScheme;
sharedOptions.DefaultChallengeScheme = IdentityServerConstants.SignoutScheme;
})
... //other external providers...
.AddCookie( CookieAuthenticationDefaults.AuthenticationScheme, options => {
options.Events = new CookieAuthenticationEvents {
OnRedirectToLogin = ctx => {
if (ctx.Request.Path.StartsWithSegments("/Admin", StringComparison.OrdinalIgnoreCase)) {
ctx.Response.StatusCode = (int) HttpStatusCode.Unauthorized;
}
return Task.CompletedTask;
};
});
I expected to seem my handler being called whenever a request is redirected to the login page, but it never happens. Can anyone help?
Thanks
EDIT: just to add that I'm also using aspnet identity for managing the user accounts...

Answer 1

Posting the answer here in case anyone is interested...
After some digging, I've found that using identity means that you can't customize the cookie handler by doing what I was doing. Fortunately, the ConfigureAuthenticationEvent that can be configured by the ConfigureApplicationCookie extension method already does the right thing: if it detects that the current request is an AJAX call, it will return 401; if not, it will return 302. And here was the problem: the request made from the vue client wasn't being considered an AJAX request because it wasn't setting the X-Request-With header to XMLHttpRequest.
So, all it was required was to configure axios to set the header in all the calls:
axios.defaults.headers.common['X-Requested-With'] = 'XMLHttpRequest';

Answer 2

I wrote a middleware sometime ago for this exact purpose and never looked back so if you don't find better solution, perhaps the solution can help you as well:
public class RedirectHandlingMiddleware
{
private readonly RequestDelegate _next;
private readonly ILogger<RedirectHandlingMiddleware> _logger;
public RedirectHandlingMiddleware(RequestDelegate next, ILogger<RedirectHandlingMiddleware> logger)
{
_next = next;
_logger = logger;
}
public async Task Invoke(HttpContext context)
{
await HandleRedirect(context, ex);
await _next(context);
}
private Task HandleRedirect(HttpContext context)
{
if (context.Request.Path.StartsWithSegments("/Admin", StringComparison.OrdinalIgnoreCase) && context.Response.StatusCode == 302)
{
context.Response.StatusCode = (int)HttpStatusCode.Unauthorized;
}
return Task.CompletedTask;
}
}
Just need to register in Startup.cs:
app.UseAuthentication();
app.UseMiddleware<RedirectHandlingMiddleware>();