Adds support for multiple custom domains

EXAMPLES.md

@@ -6,6 +6,7 @@
 - [Organizations](#organizations)
 - [Extra parameters](#extra-parameters)
 - [Roles](#roles)
+- [Multiple Custom Domain (MCD) Support](#multiple-custom-domain-mcd-support)
 - [Backchannel Logout](#backchannel-logout)
 - [Blazor Server](#blazor-server)
 
@@ -314,6 +315,162 @@ public IActionResult Admin()
 }
 ```
 
+## Multiple Custom Domain (MCD) Support
+
+Multiple Custom Domains (MCD) lets you resolve the Auth0 domain per request while keeping a single SDK instance. This is useful when one application serves multiple custom domains (for example, `brand-1.my-app.com` and `brand-2.my-app.com`), each mapped to a different `Auth0` custom domain.
+
+`MCD` is enabled by providing a `DomainResolver` function instead of a static domain string, enabling you to dynamically define the `Auth0` custom domain at run-time.
+
+Resolver mode is intended for the custom domains of a single `Auth0` tenant. It is not a supported way to connect multiple `Auth0` tenants to one application.
+
+### Dynamic Domain Resolver
+
+Provide a resolver function to select the domain at runtime. The resolver should return the `Auth0 Custom Domain` (for example, `brand-1.custom-domain.com`). Returning `null` or an empty value throws `InvalidOperationException`.
+
+### Configure with a DomainResolver
+
+Call `WithCustomDomains()` and provide a `DomainResolver` to resolve the domain dynamically based on the incoming request. The domain can be derived from a subdomain, request header, query parameter, or any other request attribute:
+
+```csharp
+services.AddAuth0WebAppAuthentication(options =>
+{
+    options.Domain = Configuration["Auth0:Domain"];
+    options.ClientId = Configuration["Auth0:ClientId"];
+})
+.WithCustomDomains(options =>
+{
+    // Example: resolve from a custom header
+    options.DomainResolver = httpContext =>
+    {
+        var tenant = httpContext.Request.Headers["X-Tenant-Domain"].FirstOrDefault();
+        return Task.FromResult(tenant ?? "default-tenant.auth0.com");
+    };
+});
+```
+
+### Resolve domain from subdomain
+
+```csharp
+services.AddAuth0WebAppAuthentication(options =>
+{
+    options.Domain = Configuration["Auth0:Domain"];
+    options.ClientId = Configuration["Auth0:ClientId"];
+})
+.WithCustomDomains(options =>
+{
+    // e.g., "acme.myapp.com" -> "acme.auth0.com"
+    options.DomainResolver = httpContext =>
+    {
+        var host = httpContext.Request.Host.Host;
+        var subdomain = host.Split('.')[0];
+        return Task.FromResult($"{subdomain}.auth0.com");
+    };
+});
+```
+
+### Redirect URI requirements
+
+When using MCD, the `redirectUri` must be an **absolute URL**. In MCD deployments, you will typically resolve the redirect URI per request so each domain uses the correct callback URL:
+
+```csharp
+var authenticationProperties = new LoginAuthenticationPropertiesBuilder()
+    // Resolve redirect URI based on the incoming request's host
+    .WithRedirectUri($"{HttpContext.Request.Scheme}://{HttpContext.Request.Host}/callback")
+    .Build();
+
+await HttpContext.ChallengeAsync(Auth0Constants.AuthenticationScheme, authenticationProperties);
+```
+
+You must validate the host and scheme safely for your deployment to prevent open redirect attacks.
+
+### Legacy sessions and migration
+
+When moving from a static domain setup to a `DomainResolver`, existing sessions can continue to work if the resolver returns the same Auth0 custom domain that was used for those legacy sessions.
+
+If the resolver returns a different domain, the SDK treats the session as missing and requires the user to sign in again. This is intentional to keep sessions isolated per domain.
+
+### Security requirements
+
+When configuring the `DomainResolver`, you are responsible for ensuring that all resolved domains are trusted. Mis-configuring the domain resolver is a critical security risk that can lead to authentication bypass on the relying party (RP) or expose the application to Server-Side Request Forgery (SSRF).
+
+**Single tenant limitation:**
+The `DomainResolver` is intended solely for multiple custom domains belonging to the same Auth0 tenant. It is not a supported mechanism for connecting multiple Auth0 tenants to a single application.
+
+**Secure proxy requirement:**
+When using MCD, your application must be deployed behind a secure edge or reverse proxy (e.g., Cloudflare, Nginx, or AWS ALB). The proxy must be configured to sanitize and overwrite `Host` and `X-Forwarded-Host` headers before they reach your application.
+
+Without a trusted proxy layer to validate these headers, an attacker can manipulate the domain resolution process. This can result in malicious redirects, where users are sent to unauthorized or fraudulent endpoints during the login and logout flows.
+
+### Configuration Manager Cache
+
+You can control how OpenID Connect configuration managers are cached per domain with `ConfigurationManagerCache`.
+
+By default, the SDK uses an in-memory cache with:
+- `maxSize: 100` entries
+- No expiration (entries remain until evicted by size pressure)
+
+The cache is keyed by the OIDC metadata endpoint URL (e.g., `https://brand-1.custom-domain.com/.well-known/openid-configuration`). Each distinct domain resolved by `DomainResolver` occupies one cache entry.
+
+Most applications can keep the defaults, but you may want to adjust them in the following cases:
+- Increase `maxSize` if one process may verify tokens for more than 100 distinct domains during its lifetime.
+- Decrease `maxSize` if memory usage matters more than avoiding repeated OIDC discovery setup.
+- Set `slidingExpiration` if you want entries that haven't been accessed within a given duration to be evicted automatically.
+- Use `NullConfigurationManagerCache` to disable caching entirely (not recommended for production).
+
+Rule of thumb: set `maxSize` to cover the number of distinct domains a single process is expected to serve, with some headroom.
+
+#### MemoryConfigurationManagerCache (Default)
+
+```csharp
+.WithCustomDomains(options =>
+{
+    options.DomainResolver = httpContext => { /* ... */ };
+
+    options.ConfigurationManagerCache = new MemoryConfigurationManagerCache(
+        maxSize: 100,                             // Maximum number of domains to cache
+        slidingExpiration: TimeSpan.FromHours(1)  // Optional: evict entries not accessed within 1 hour
+    );
+});
+```
+
+#### NullConfigurationManagerCache
+
+Disables caching entirely — a new configuration manager is created on every request (not recommended for production):
+
+```csharp
+.WithCustomDomains(options =>
+{
+    options.DomainResolver = httpContext => { /* ... */ };
+    options.ConfigurationManagerCache = new NullConfigurationManagerCache();
+});
+```
+
+#### Custom Cache Implementation
+
+Implement `IConfigurationManagerCache` for custom caching strategies (e.g., a distributed cache):
+
+```csharp
+public class MyCustomConfigurationManagerCache : IConfigurationManagerCache
+{
+    public IConfigurationManager<OpenIdConnectConfiguration> GetOrCreate(
+        string metadataAddress,
+        Func<string, IConfigurationManager<OpenIdConnectConfiguration>> factory)
+    {
+        // Return a cached instance or call factory(metadataAddress) to create one
+    }
+
+    public void Clear() { /* Evict all entries */ }
+    public void Dispose() { /* Clean up resources */ }
+}
+
+// Usage
+.WithCustomDomains(options =>
+{
+    options.DomainResolver = httpContext => { /* ... */ };
+    options.ConfigurationManagerCache = new MyCustomConfigurationManagerCache();
+});
+```
+
 ## Backchannel Logout
 
 Backchannel logout can be configured by calling `WithBackchannelLogout()` when calling `AddAuth0WebAppAuthentication`.

README.md

@@ -6,7 +6,7 @@ A library based on `Microsoft.AspNetCore.Authentication.OpenIdConnect` to make i
 ![Downloads](https://img.shields.io/nuget/dt/auth0.aspnetcore.authentication)
 [![License](https://img.shields.io/:license-MIT-blue.svg?style=flat)](https://opensource.org/licenses/MIT)
 [![Ask DeepWiki](https://deepwiki.com/badge.svg)](https://deepwiki.com/auth0/auth0-aspnetcore-authentication)
-![AzureDevOps](https://img.shields.io/azure-devops/build/Auth0SDK/Auth0.AspNetCore.Authentication/8)
+[![Build and Test](https://github.com/auth0/auth0-aspnetcore-authentication/actions/workflows/build.yml/badge.svg)](https://github.com/auth0/auth0-aspnetcore-authentication/actions/workflows/build.yml)
 
 :books: [Documentation](#documentation) - :rocket: [Getting Started](#getting-started) - :computer: [API Reference](#api-reference) - :speech_balloon: [Feedback](#feedback)
 
@@ -15,12 +15,12 @@ A library based on `Microsoft.AspNetCore.Authentication.OpenIdConnect` to make i
 - [Quickstart](https://auth0.com/docs/quickstart/webapp/aspnet-core) - our interactive guide for quickly adding login, logout and user information to an ASP.NET MVC application using Auth0.
 - [Sample App](https://github.com/auth0-samples/auth0-aspnetcore-mvc-samples/tree/master/Quickstart/Sample) - a full-fledged ASP.NET MVC application integrated with Auth0.
 - [Examples](https://github.com/auth0/auth0-aspnetcore-authentication/blob/main/EXAMPLES.md) - code samples for common ASP.NET MVC authentication scenario's.
-- [Docs site](https://www.auth0.com/docs) - explore our docs site and learn more about 
+- [Docs site](https://www.auth0.com/docs) - explore our docs site and learn more about Auth0. 
 
 ## Getting started
 ### Requirements
 
-This library supports .NET 6.0 and above.
+This library supports .NET 6.0, 7.0, 8.0, and 10.0.
 
 ### Installation
 
@@ -114,6 +114,33 @@ For more code samples on how to integrate the **auth0-aspnetcore-authentication*
 
 > This SDK also works with Blazor Server, for more info see [the Blazor Server section in our examples](https://github.com/auth0/auth0-aspnetcore-authentication/blob/main/EXAMPLES.md#blazor-server).
 
+## Multiple Custom Domain (MCD) Support
+
+Multiple Custom Domains (MCD) lets you resolve the Auth0 domain per request while keeping a single SDK instance. This is useful when one application serves multiple custom domains (for example, `brand-1.my-app.com` and `brand-2.my-app.com`), each mapped to a different `Auth0` custom domain.
+
+Resolver mode is intended for the custom domains of a single `Auth0` tenant. It is not a supported way to connect multiple `Auth0` tenants to one application.
+
+### Configuration
+
+```csharp
+services.AddAuth0WebAppAuthentication(options =>
+{
+    options.Domain = Configuration["Auth0:Domain"];
+    options.ClientId = Configuration["Auth0:ClientId"];
+})
+.WithCustomDomains(options =>
+{
+    // Example: resolve from a custom header
+    options.DomainResolver = httpContext =>
+    {
+        var tenant = httpContext.Request.Headers["X-Tenant-Domain"].FirstOrDefault();
+        return Task.FromResult(tenant ?? "default-tenant.auth0.com");
+    };
+});
+```
+
+For detailed configuration options, caching strategies, security requirements, and more examples, see the [Multiple Custom Domain (MCD) Examples](EXAMPLES.md#multiple-custom-domain-mcd-support).
+
 ## API reference
 Explore public API's available in auth0-aspnetcore-authentication.
 
@@ -152,4 +179,4 @@ Please do not report security vulnerabilities on the public GitHub issue tracker
 </p>
 <p align="center">Auth0 is an easy to implement, adaptable authentication and authorization platform. To learn more checkout <a href="https://auth0.com/why-auth0">Why Auth0?</a></p>
 <p align="center">
-This project is licensed under the MIT license. See the <a href="https://github.com/auth0/auth0-aspnetcore-authentication/blob/main/LICENSE"> LICENSE</a> file for more info.</p>
+This project is licensed under the MIT license. See the <a href="https://github.com/auth0/auth0-aspnetcore-authentication/blob/main/LICENSE">LICENSE</a> file for more info.</p>

src/Auth0.AspNetCore.Authentication/Auth0Constants.cs

@@ -14,5 +14,10 @@ public class Auth0Constants
         /// The callback path to which Auth0 should redirect back, used when configuring OpenIdConnect
         /// </summary>
         internal static string DefaultCallbackPath = "/callback";
+
+        /// <summary>
+        /// Key used to store the resolved domain in the authentication properties.
+        /// </summary>
+        internal static readonly string ResolvedDomainKey = "auth0:resolved-domain";
     }
 }

src/Auth0.AspNetCore.Authentication/Auth0WebAppAuthenticationBuilder.cs

@@ -1,10 +1,13 @@
 using Microsoft.AspNetCore.Authentication.OpenIdConnect;
 using Microsoft.Extensions.DependencyInjection;
+using Microsoft.Extensions.DependencyInjection.Extensions;
 using Microsoft.Extensions.Options;
 using Microsoft.IdentityModel.Protocols.OpenIdConnect;
 using System;
 using System.Threading.Tasks;
 using Auth0.AspNetCore.Authentication.BackchannelLogout;
+using Auth0.AspNetCore.Authentication.CustomDomains;
+using Microsoft.AspNetCore.Hosting;
 
 namespace Auth0.AspNetCore.Authentication
 {
@@ -64,6 +67,49 @@ public Auth0WebAppAuthenticationBuilder WithBackchannelLogout()
             return this;
         }
 
+        /// <summary>
+        /// Configures support for multiple Auth0 custom domains with dynamic domain resolution.
+        /// </summary>
+        /// <param name="configureOptions">A delegate used to configure the <see cref="Auth0CustomDomainsOptions"/></param>
+        /// <returns>An instance of <see cref="Auth0WebAppAuthenticationBuilder"/></returns>
+        public Auth0WebAppAuthenticationBuilder WithCustomDomains(Action<Auth0CustomDomainsOptions> configureOptions)
+        {
+            EnableCustomDomains(configureOptions);
+            return this;
+        }
+
+        private void EnableCustomDomains(Action<Auth0CustomDomainsOptions> configureOptions)
+        {
+            var customDomainsOptions = new Auth0CustomDomainsOptions();
+            configureOptions(customDomainsOptions);
+
+            // Validate that DomainResolver is configured
+            if (customDomainsOptions.DomainResolver == null)
+            {
+                throw new InvalidOperationException(
+                    $"DomainResolver must be configured when using {nameof(WithCustomDomains)}. " +
+                    $"Set the {nameof(Auth0CustomDomainsOptions.DomainResolver)} property to provide a function that resolves the Auth0 domain for each request.");
+            }
+
+            // Register the options for this authentication scheme
+            _services.Configure(_authenticationScheme, configureOptions);
+
+            // Register HttpContextAccessor - required for domain resolution
+            _services.AddHttpContextAccessor();
+
+            // Register HttpClient - required for fetching OIDC configuration per domain
+            _services.AddHttpClient();
+
+            // Register the startup filter to resolve domain early in the request pipeline
+            _services.TryAddEnumerable(
+                ServiceDescriptor.Singleton<IStartupFilter, Auth0CustomDomainStartupFilter>(
+                    _ => new Auth0CustomDomainStartupFilter(_authenticationScheme)));
+
+            // Register the post-configure options to set up custom ConfigurationManager
+            _services.TryAddEnumerable(
+                ServiceDescriptor.Singleton<IPostConfigureOptions<OpenIdConnectOptions>, Auth0CustomDomainsOpenIdConnectPostConfigureOptions>());
+        }
+
         private void EnableWithAccessToken(Action<Auth0WebAppWithAccessTokenOptions> configureOptions)
         {
             var auth0WithAccessTokensOptions = new Auth0WebAppWithAccessTokenOptions();

src/Auth0.AspNetCore.Authentication/AuthenticationBuilderExtensions.cs

@@ -156,10 +156,19 @@
                 {
                     await VerifyBackchannelLogoutSupport(context.HttpContext, oidcOptions);
 
-                    var issuer = $"https://{options.Domain}/";
+                    // Prefer issuer from the authenticated principal
+                    var resolvedIssuer = context.HttpContext.User?.FindFirst("iss")?.Value;
+
+                    // Fall back to the domain resolved by StartupFilter (cached in HttpContext.Items)
+                    if (string.IsNullOrWhiteSpace(resolvedIssuer))
+                    {
+                        resolvedIssuer = context.HttpContext.GetResolvedDomain();
+                    }
+
+                    var issuer = Utils.ToAuthority(resolvedIssuer ?? $"https://{options.Domain}/");
                     var sid = context.Principal?.FindFirst("sid")?.Value;
 
                     var isLoggedOut = await logoutTokenHandler.IsLoggedOutAsync(issuer, sid);
 
                     if (isLoggedOut)
                     {
@@ -196,7 +205,7 @@
 
                         if (isExpired && !string.IsNullOrWhiteSpace(refreshToken))
                         {
-                            var result = await RefreshTokens(options, refreshToken, oidcOptions.Backchannel);
+                            var result = await RefreshTokens(context.HttpContext, options, refreshToken, oidcOptions.Backchannel);
 
                             if (result != null)
                             {
@@ -239,17 +248,21 @@
             }
         }
 
-        private static async Task<AccessTokenResponse?> RefreshTokens(Auth0WebAppOptions options, string refreshToken, HttpClient httpClient)
+        private static async Task<AccessTokenResponse?> RefreshTokens(HttpContext httpContext, Auth0WebAppOptions options, string refreshToken, HttpClient httpClient)
         {
             var tokenClient = new TokenClient(httpClient);
-            return await tokenClient.Refresh(options, refreshToken);
+
+            // Get the resolved domain from HttpContext if available (for multiple custom domains)
+            var resolvedDomain = httpContext.GetResolvedDomain();
+
+            return await tokenClient.Refresh(options, refreshToken, resolvedDomain);
         }
 
         private static async Task VerifyBackchannelLogoutSupport(HttpContext context, OpenIdConnectOptions oidcOptions)
         {
             if (oidcOptions.Configuration == null)
             {
                 oidcOptions.Configuration = await oidcOptions.ConfigurationManager.GetConfigurationAsync(context.RequestAborted);
             }
 
             var additionalConfiguration = oidcOptions.Configuration.AdditionalData;