Allow using mTLS client authentication with the client credentials grant and support mTLS token binding for anonymous clients

src/OpenIddict.Abstractions/OpenIddictResources.resx

@@ -1860,6 +1860,12 @@ To use a custom policy relying on the system store, set 'OpenIddictServerOptions
   <data name="ID0514" xml:space="preserve">
     <value>TLS client certificates must contain a private key.</value>
   </data>
+  <data name="ID0515" xml:space="preserve">
+    <value>An existing '{0}' instance is already attached to the execution context.</value>
+  </data>
+  <data name="ID0516" xml:space="preserve">
+    <value>The '{0}' attached to the execution context could not be resolved.</value>
+  </data>
   <data name="ID2000" xml:space="preserve">
     <value>The security token is missing.</value>
   </data>
@@ -2462,16 +2468,21 @@ To use a custom policy relying on the system store, set 'OpenIddictServerOptions
   </data>
   <data name="ID2201" xml:space="preserve">
     <value>An existing '{0}' instance is already attached to the execution context.</value>
+    <comment>This resource is no longer used and will be removed in a future version.</comment>
   </data>
   <data name="ID2202" xml:space="preserve">
     <value>The '{0}' attached to the execution context could not be resolved.</value>
+    <comment>This resource is no longer used and will be removed in a future version.</comment>
   </data>
   <data name="ID2203" xml:space="preserve">
     <value>A certificate-based proof-of-possession is required to use this token.</value>
   </data>
   <data name="ID2204" xml:space="preserve">
     <value>The specified certificate-based proof-of-possession is not valid.</value>
   </data>
+  <data name="ID2205" xml:space="preserve">
+    <value>The specified TLS client certificate is not allowed or valid for this operation.</value>
+  </data>
   <data name="ID4000" xml:space="preserve">
     <value>The '{0}' parameter shouldn't be null or empty at this point.</value>
   </data>
@@ -3315,6 +3326,9 @@ This may indicate that the hashed entry is corrupted or malformed.</value>
   <data name="ID6292" xml:space="preserve">
     <value>The payload of the '{0}' reference token was used instead of its reference identifier, which may indicate that the payload stored in the database has leaked and is being used as a regular token.</value>
   </data>
+  <data name="ID6293" xml:space="preserve">
+    <value>Certificate validation failed because the token binding certificate provided by an anonymous client was not valid: {Errors}.</value>
+  </data>
   <data name="ID8000" xml:space="preserve">
     <value>https://documentation.openiddict.com/errors/{0}</value>
   </data>

src/OpenIddict.Client.SystemNetHttp/OpenIddictClientSystemNetHttpConfiguration.cs

@@ -78,7 +78,7 @@ public void Configure(string? name, HttpClientFactoryOptions options)
         // an async-local context to flow per-instance properties and uses dynamic client
         // names to ensure the inner HttpClientHandler is not reused if the context differs.
         var context = OpenIddictClientSystemNetHttpContext.Current ??
-            throw new InvalidOperationException(SR.FormatID2202(nameof(OpenIddictClientSystemNetHttpContext)));
+            throw new InvalidOperationException(SR.FormatID0516(nameof(OpenIddictClientSystemNetHttpContext)));
 
         var settings = _provider.GetRequiredService<IOptionsMonitor<OpenIddictClientSystemNetHttpOptions>>().CurrentValue;
 

src/OpenIddict.Client.SystemNetHttp/OpenIddictClientSystemNetHttpHandlers.cs

@@ -200,7 +200,7 @@ public ValueTask HandleAsync(TContext context)
 
             if (OpenIddictClientSystemNetHttpContext.Current is not null)
             {
-                throw new InvalidOperationException(SR.FormatID2201(nameof(OpenIddictClientSystemNetHttpContext)));
+                throw new InvalidOperationException(SR.FormatID0515(nameof(OpenIddictClientSystemNetHttpContext)));
             }
 
             try

src/OpenIddict.Server/OpenIddictServerBuilder.cs

@@ -116,8 +116,11 @@ public OpenIddictServerBuilder Configure(Action<OpenIddictServerOptions> configu
     /// <summary>
     /// Makes client identification optional so that token, introspection and revocation
     /// requests that don't specify a client_id are not automatically rejected.
-    /// Enabling this option is NOT recommended.
     /// </summary>
+    /// <remarks>
+    /// Enabling this option is NOT recommended and should only be used for
+    /// backward compatibility with legacy authorization server deployments.
+    /// </remarks>
     /// <returns>The <see cref="OpenIddictServerBuilder"/> instance.</returns>
     public OpenIddictServerBuilder AcceptAnonymousClients()
         => Configure(options => options.AcceptAnonymousClients = true);

src/OpenIddict.Server/OpenIddictServerHandlers.Exchange.cs

@@ -412,30 +412,18 @@ public ValueTask HandleAsync(ValidateTokenRequestContext context)
             {
                 ArgumentNullException.ThrowIfNull(context);
 
-                // Reject grant_type=authorization_code requests that don't specify a client_id or a client_assertion,
-                // as the client identifier MUST be sent by the client application in the request body if it cannot
-                // be inferred from the client authentication method (e.g the username when using basic).
-                //
-                // See https://tools.ietf.org/html/rfc6749#section-4.1.3 for more information.
-                if (context.Request.IsAuthorizationCodeGrantType() &&
-                    string.IsNullOrEmpty(context.Request.ClientId) &&
-                    string.IsNullOrEmpty(context.Request.ClientAssertion))
+                if (!context.Request.IsAuthorizationCodeGrantType() && !context.Request.IsClientCredentialsGrantType())
                 {
-                    context.Logger.LogInformation(6077, SR.GetResourceString(SR.ID6077), Parameters.ClientId);
-
-                    context.Reject(
-                        error: Errors.InvalidRequest,
-                        description: SR.FormatID2029(Parameters.ClientId),
-                        uri: SR.FormatID8000(SR.ID2029));
-
                     return ValueTask.CompletedTask;
                 }
 
-                // Reject grant_type=client_credentials requests that don't specify a client_id or a client_assertion.
+                // Reject grant_type=authorization_code and grant_type=client_credentials requests that
+                // don't specify a client_id or a client_assertion, as the client identity MUST be sent
+                // by the client application (even when using mTLS OAuth 2.0 client authentication).
                 //
-                // See https://tools.ietf.org/html/rfc6749#section-4.4.1 for more information.
-                if (context.Request.IsClientCredentialsGrantType() &&
-                    string.IsNullOrEmpty(context.Request.ClientId) &&
+                // See https://tools.ietf.org/html/rfc6749#section-4.1.3
+                // and https://tools.ietf.org/html/rfc6749#section-4.4.1 for more information.
+                if (string.IsNullOrEmpty(context.Request.ClientId) &&
                     string.IsNullOrEmpty(context.Request.ClientAssertion))
                 {
                     context.Logger.LogInformation(6077, SR.GetResourceString(SR.ID6077), Parameters.ClientId);
@@ -568,7 +556,8 @@ public ValueTask HandleAsync(ValidateTokenRequestContext context)
                 // See https://tools.ietf.org/html/rfc6749#section-4.4.1 for more information.
                 if (context.Request.IsClientCredentialsGrantType() &&
                     string.IsNullOrEmpty(context.Request.ClientAssertion) &&
-                    string.IsNullOrEmpty(context.Request.ClientSecret))
+                    string.IsNullOrEmpty(context.Request.ClientSecret) &&
+                    context.Transaction.RemoteCertificate is null)
                 {
                     context.Reject(
                         error: Errors.InvalidRequest,

src/OpenIddict.Server/OpenIddictServerHandlers.cs

@@ -9,6 +9,7 @@
 using System.Diagnostics;
 using System.Globalization;
 using System.Security.Claims;
+using System.Security.Cryptography;
 using System.Security.Cryptography.X509Certificates;
 using System.Text;
 using System.Text.Json;
@@ -1004,7 +1005,13 @@ public async ValueTask HandleAsync(ProcessAuthenticationContext context)
 
                     case OpenIddictServerEndpointType.Introspection when context.Options.AcceptAnonymousClients:
                     case OpenIddictServerEndpointType.Revocation    when context.Options.AcceptAnonymousClients:
-                    case OpenIddictServerEndpointType.Token         when context.Options.AcceptAnonymousClients:
+                        return;
+
+                    // Note: the authorization code and client credentials grant types never
+                    // allow anonymous clients, even if the corresponding option is enabled.
+                    case OpenIddictServerEndpointType.Token when context.Options.AcceptAnonymousClients &&
+                        !context.Request.IsAuthorizationCodeGrantType() &&
+                        !context.Request.IsClientCredentialsGrantType():
                         return;
 
                     // Note: despite being conceptually similar to the token endpoint, the pushed authorization
@@ -1240,9 +1247,9 @@ OpenIddictServerEndpointType.EndUserVerification or
     /// </summary>
     public sealed class ValidateClientCertificate : IOpenIddictServerHandler<ProcessAuthenticationContext>
     {
-        private readonly IOpenIddictApplicationManager _applicationManager;
+        private readonly IOpenIddictApplicationManager? _applicationManager;
 
-        public ValidateClientCertificate() => throw new InvalidOperationException(SR.GetResourceString(SR.ID0016));
+        public ValidateClientCertificate() { }
 
         public ValidateClientCertificate(IOpenIddictApplicationManager applicationManager)
             => _applicationManager = applicationManager ?? throw new ArgumentNullException(nameof(applicationManager));
@@ -1252,9 +1259,18 @@ public ValidateClientCertificate(IOpenIddictApplicationManager applicationManage
         /// </summary>
         public static OpenIddictServerHandlerDescriptor Descriptor { get; }
             = OpenIddictServerHandlerDescriptor.CreateBuilder<ProcessAuthenticationContext>()
-                .AddFilter<RequireClientIdParameter>()
                 .AddFilter<RequireClientCertificate>()
-                .AddFilter<RequireDegradedModeDisabled>()
+                .UseScopedHandler(static provider =>
+                {
+                    // Note: the application manager is only resolved if the degraded mode was not enabled to ensure
+                    // invalid core configuration exceptions are not thrown even if the managers were registered.
+                    var options = provider.GetRequiredService<IOptionsMonitor<OpenIddictServerOptions>>().CurrentValue;
+
+                    return options.EnableDegradedMode ?
+                        new ValidateClientCertificate() :
+                        new ValidateClientCertificate(provider.GetService<IOpenIddictApplicationManager>() ??
+                            throw new InvalidOperationException(SR.GetResourceString(SR.ID0016)));
+                })
                 .UseScopedHandler<ValidateClientCertificate>()
                 .SetOrder(ValidateClientSecret.Descriptor.Order + 1_000)
                 .SetType(OpenIddictServerHandlerType.BuiltIn)
@@ -1265,7 +1281,6 @@ public async ValueTask HandleAsync(ProcessAuthenticationContext context)
         {
             ArgumentNullException.ThrowIfNull(context);
 
-            Debug.Assert(!string.IsNullOrEmpty(context.ClientId), SR.FormatID4000(Parameters.ClientId));
             Debug.Assert(context.Transaction.RemoteCertificate is not null, SR.GetResourceString(SR.ID4020));
 
             // Don't validate the client certificate on endpoints that don't support client authentication/token binding.
@@ -1277,6 +1292,106 @@ OpenIddictServerEndpointType.EndUserVerification or
                 return;
             }
 
+            // If the client is anonymous, assume the provided certificate will exclusively be used for mTLS token
+            // binding, validate the certificate using the base chain policy specified in the options and ensure
+            // the certificate is self-signed as PKI certificates cannot be used for token binding exclusively.
+            if (string.IsNullOrEmpty(context.ClientId))
+            {
+                // Note: to avoid building and introspecting a X.509 certificate chain and reduce the cost
+                // of this check, a certificate is always assumed to be self-signed when it is self-issued.
+                //
+                // A second pass is performed once the chain is built to validate whether the certificate is self-signed or not.
+                if (context.Options.SelfSignedTlsClientAuthenticationPolicy is not X509ChainPolicy policy ||
+                    !OpenIddictHelpers.IsSelfIssuedCertificate(context.Transaction.RemoteCertificate))
+                {
+                    context.Reject(
+                        error: Errors.InvalidRequest,
+                        description: SR.GetResourceString(SR.ID2205),
+                        uri: SR.FormatID8000(SR.ID2205));
+
+                    return;
+                }
+
+                // Always clone the X.509 chain policy to ensure the original instance is never mutated.
+                policy = policy.Clone();
+
+                // Note: to allow validating certificates that are exclusively used for mTLS token binding, the chain policy
+                // is amended to consider the specified self-signed certificate as a trusted root and basically disable chain
+                // validation while still validating the other aspects of the certificate (e.g expiration date, key usage, etc).
+#if SUPPORTS_X509_CHAIN_POLICY_CUSTOM_TRUST_STORE
+                policy.CustomTrustStore.Add(context.Transaction.RemoteCertificate);
+#else
+                policy.ExtraStore.Add(context.Transaction.RemoteCertificate);
+#endif
+
+                using var chain = new X509Chain()
+                {
+                    ChainPolicy = policy
+                };
+
+                try
+                {
+                    // Ensure the specified certificate is valid based on the chain policy.
+                    if (!chain.Build(context.Transaction.RemoteCertificate))
+                    {
+                        context.Logger.LogInformation(6293, SR.GetResourceString(SR.ID6293),
+                            chain.ChainStatus.Select(static status => status.Status).ToArray());
+
+                        context.Reject(
+                            error: Errors.InvalidRequest,
+                            description: SR.GetResourceString(SR.ID2197),
+                            uri: SR.FormatID8000(SR.ID2197));
+
+                        return;
+                    }
+
+                    if (chain.ChainElements is not [X509ChainElement])
+                    {
+                        context.Reject(
+                            error: Errors.InvalidRequest,
+                            description: SR.GetResourceString(SR.ID2205),
+                            uri: SR.FormatID8000(SR.ID2205));
+
+                        return;
+                    }
+                }
+
+                catch (CryptographicException exception) when (!OpenIddictHelpers.IsFatal(exception))
+                {
+                    context.Logger.LogWarning(6288, exception, SR.GetResourceString(SR.ID6288));
+
+                    context.Reject(
+                        error: Errors.InvalidRequest,
+                        description: SR.GetResourceString(SR.ID2197),
+                        uri: SR.FormatID8000(SR.ID2197));
+
+                    return;
+                }
+
+                finally
+                {
+                    // Dispose the certificates instantiated internally while building the chain.
+                    for (var index = 0; index < chain.ChainElements.Count; index++)
+                    {
+                        chain.ChainElements[index].Certificate.Dispose();
+                    }
+                }
+
+                return;
+            }
+
+            // Note: when the degraded mode is enabled, the application is responsible for manually
+            // validating the client certificate provided by the client using a custom event handler.
+            if (context.Options.EnableDegradedMode)
+            {
+                return;
+            }
+
+            if (_applicationManager is null)
+            {
+                throw new InvalidOperationException(SR.GetResourceString(SR.ID0016));
+            }
+
             var application = await _applicationManager.FindByClientIdAsync(context.ClientId) ??
                 throw new InvalidOperationException(SR.GetResourceString(SR.ID0032));
 
@@ -1291,7 +1406,12 @@ OpenIddictServerEndpointType.EndUserVerification or
             {
                 if (context.Options.SelfSignedTlsClientAuthenticationPolicy is null)
                 {
-                    throw new InvalidOperationException(SR.GetResourceString(SR.ID0506));
+                    context.Reject(
+                        error: Errors.InvalidRequest,
+                        description: SR.GetResourceString(SR.ID2205),
+                        uri: SR.FormatID8000(SR.ID2205));
+
+                    return;
                 }
 
                 if (await _applicationManager.GetSelfSignedTlsClientAuthenticationPolicyAsync(
@@ -1346,7 +1466,12 @@ OpenIddictServerEndpointType.EndUserVerification or
             {
                 if (context.Options.PublicKeyInfrastructureTlsClientAuthenticationPolicy is null)
                 {
-                    throw new InvalidOperationException(SR.GetResourceString(SR.ID0505));
+                    context.Reject(
+                        error: Errors.InvalidRequest,
+                        description: SR.GetResourceString(SR.ID2205),
+                        uri: SR.FormatID8000(SR.ID2205));
+
+                    return;
                 }
 
                 if (await _applicationManager.GetPublicKeyInfrastructureTlsClientAuthenticationPolicyAsync(