Defaulting ASP.NET Core 3.1 To Require Authentication For A Safer Developer Experience

Typically with ASP.NET Core 3.1 when no specific authorization requirements are set all endpoints are publicly accessible. When you’re working with an authenticated required system the burden is on the developer to remember to use [Authorize] attributes everywhere appropriate. This can lead to bugs by omission — and, could result in a serious data breach.

Solution

For a safe-by-default experience, we can require authentication when no specific instructions are set.

namespace MyWebApplication
{
    public class Startup
    {
        private static readonly string PublicAuthorizationPolicy = "PublicPolicy";

        public void ConfigureServices(IServiceCollection services)
        {
            services.AddRouting();
            services.AddAuthentication();
            services.AddAuthorization(options =>
            {
                options.AddPolicy(
                    PublicAuthorizationPolicy,
                    new AuthorizationPolicyBuilder()
                        .RequireAssertion(_ => true)
                        .Build());

                options.FallbackPolicy = new AuthorizationPolicyBuilder()
                    .RequireAuthenticatedUser()
                    .Build();
            });
        }

        public void Configure(IApplicationBuilder app, IWebHostEnvironment env)
        {
            app.UseRouting();
            app.UseAuthentication();
            app.UseAuthorization();

            app.UseEndpoints(endpoints =>
            {
                // Allow public access for a health check
                endpoints.MapHealthChecks("/_health")
                    .RequireAuthorization(PublicAuthorizationPolicy);

                // Everything that doesn't explicitly allow
                // anonymous users requires authentication.
                endpoints.MapControllers();
            });
        }
    }
}

At least one assertion is required when configuring an auth policy. We can satisfy this requirement while enabling public access via RequireAssertion(_ => true) — always allow everyone.

Summary

Safe-by-default authentication and authorization is a good strategy, especially when the consequences for making a mistake are severe. We can make ASP.NET Core 3.1 safe-by-default by authenticating all requests by default, while still maintaining a way to create public endpoints as needed. With public endpoints explicitly defined mistakes can’t be made by forgetting to do something.