.NET Core Identity PasswordValidators with Umbraco CMS BackOffice Users

en_us.user.xml

<!-- Place User override lang files at /config/lang -->
<?xml version="1.0" encoding="utf-8" standalone="yes"?>
<language alias="en_us" intName="English (US)" localName="English (US)" lcid="" culture="en-US">
  <creator>
    <name>Warren Buckley</name>
    <link>https://warrenbuckley.co.uk/</link>
  </creator>
  <area alias="validation">
    <key alias="passwordTooEasy">You cannot use the word 'password' as part of your password</key>
  </area>
</language>

MyCustomIdentityErrorDescriber.cs

using Microsoft.AspNetCore.Identity;
using Umbraco.Cms.Core.Security;
using Umbraco.Cms.Core.Services;
using Umbraco.Extensions;

namespace Umbraco.Cms.Web.UI.NetCore
{
    public class MyCustomIdentityErrorDescriber : BackOfficeErrorDescriber
    {
        private ILocalizedTextService _textService;

        public MyCustomIdentityErrorDescriber(ILocalizedTextService textService) : base(textService)
        {
            _textService = textService;
        }

        public IdentityError PasswordTooEasy()
        {
            return new IdentityError
            {
                Code = "SayNoToEasyPasswords",
                Description = _textService.Localize("validation", "passwordTooEasy")
            };
        }
    }
}

SayNoToEasyPasswordValidator.cs

using System.Threading.Tasks;
using Microsoft.AspNetCore.Identity;
using Umbraco.Cms.Core.Security;

namespace Umbraco.Cms.Web.UI.NetCore
{
    public class SayNoToEasyPasswordsValidator : IPasswordValidator<BackOfficeIdentityUser>
    {
        private MyCustomIdentityErrorDescriber _errors;

        public SayNoToEasyPasswordsValidator(MyCustomIdentityErrorDescriber errors)
        {
            _errors = errors;
        }

        public Task<IdentityResult> ValidateAsync(UserManager<BackOfficeIdentityUser> manager, BackOfficeIdentityUser user, string password)
        {
            if (password.ToLowerInvariant().Contains("password"))
            {
                return Task.FromResult(IdentityResult.Failed(_errors.PasswordTooEasy()));
            }

            return Task.FromResult(IdentityResult.Success);
        }
    }
}

Startup.cs

using System;
using Microsoft.AspNetCore.Builder;
using Microsoft.AspNetCore.Hosting;
using Microsoft.Extensions.Configuration;
using Microsoft.Extensions.DependencyInjection;
using Microsoft.Extensions.Hosting;
using Umbraco.Cms.Core.DependencyInjection;
using Umbraco.Cms.Core.Security;
using Umbraco.Extensions;

namespace Umbraco.Cms.Web.UI.NetCore
{
    public class Startup
    {
        private readonly IWebHostEnvironment _env;
        private readonly IConfiguration _config;

        /// <summary>
        /// Initializes a new instance of the <see cref="Startup"/> class.
        /// </summary>
        /// <param name="webHostEnvironment">The Web Host Environment</param>
        /// <param name="config">The Configuration</param>
        /// <remarks>
        /// Only a few services are possible to be injected here https://github.com/dotnet/aspnetcore/issues/9337
        /// </remarks>
        public Startup(IWebHostEnvironment webHostEnvironment, IConfiguration config)
        {
            _env = webHostEnvironment ?? throw new ArgumentNullException(nameof(webHostEnvironment));
            _config = config ?? throw new ArgumentNullException(nameof(config));
        }



        /// <summary>
        /// Configures the services
        /// </summary>
        /// <remarks>
        /// This method gets called by the runtime. Use this method to add services to the container.
        /// For more information on how to configure your application, visit https://go.microsoft.com/fwlink/?LinkID=398940
        /// </remarks>
        public void ConfigureServices(IServiceCollection services)
        {
#pragma warning disable IDE0022 // Use expression body for methods
            services.AddUmbraco(_env, _config)
                .AddBackOffice()
                .AddWebsite()
                .AddComposers()
                .Build();
#pragma warning restore IDE0022 // Use expression body for methods

            var backofficeIdentityBuilder = new BackOfficeIdentityBuilder(services);
            backofficeIdentityBuilder.AddPasswordValidator<SayNoToEasyPasswordsValidator>();
            backofficeIdentityBuilder.AddErrorDescriber<MyCustomIdentityErrorDescriber>();

        }

        /// <summary>
        /// Configures the application
        /// </summary>
        public void Configure(IApplicationBuilder app, IWebHostEnvironment env)
        {
            if (env.IsDevelopment())
            {
                app.UseDeveloperExceptionPage();
            }

            app.UseUmbraco()
                .WithMiddleware(u =>
                {
                    u.WithBackOffice();
                    u.WithWebsite();
                })
                .WithEndpoints(u =>
                {
                    u.UseInstallerEndpoints();
                    u.UseBackOfficeEndpoints();
                    u.UseWebsiteEndpoints();
                });
        }
    }
}