Add a new option allowing to make PKCE mandatory

src/OpenIddict.Server/OpenIddictServerBuilder.cs

@@ -674,6 +674,16 @@ namespace Microsoft.Extensions.DependencyInjection
             return Configure(options => options.Scopes.UnionWith(scopes));
         }
 
+        /// <summary>
+        /// Configures OpenIddict to force client applications to use Proof Key for Code Exchange
+        /// (PKCE) when requesting an authorization code (e.g when using the code or hybrid flows).
+        /// When enforced, authorization requests that lack the code_challenge or
+        /// code_challenge_method parameters will be automatically rejected by OpenIddict.
+        /// </summary>
+        /// <returns>The <see cref="OpenIddictServerBuilder"/>.</returns>
+        public OpenIddictServerBuilder RequireProofKeyForCodeExchange()
+            => Configure(options => options.RequireProofKeyForCodeExchange = true);
+
         /// <summary>
         /// Sets the access token lifetime, after which client applications must retrieve
         /// a new access token by making a grant_type=refresh_token token request

src/OpenIddict.Server/OpenIddictServerOptions.cs

@@ -122,6 +122,14 @@ namespace OpenIddict.Server
             SlidingExpiration = TimeSpan.FromMinutes(30)
         };
 
+        /// <summary>
+        /// Gets or sets a boolean indicating whether PKCE must be used by client applications
+        /// when requesting an authorization code (e.g when using the code or hybrid flows).
+        /// If this property is set to <c>true</c>, authorization requests that lack the
+        /// code_challenge/code_challenge_method parameters will be automatically rejected.
+        /// </summary>
+        public bool RequireProofKeyForCodeExchange { get; set; }
+
         /// <summary>
         /// Gets the OAuth2/OpenID Connect scopes enabled for this application.
         /// </summary>

src/OpenIddict.Server/OpenIddictServerProvider.Authentication.cs

@@ -239,6 +239,21 @@ namespace OpenIddict.Server
                 return;
             }
 
+            // If OpenIddict was configured to require PKCE, reject the request if the code challenge
+            // is missing and if an authorization code was requested by the client application.
+            if (options.RequireProofKeyForCodeExchange && string.IsNullOrEmpty(context.Request.CodeChallenge) &&
+                context.Request.HasResponseType(OpenIddictConstants.ResponseTypes.Code))
+            {
+                _logger.LogError("The authorization request was rejected because the " +
+                                 "required 'code_challenge' parameter was missing.");
+
+                context.Reject(
+                    error: OpenIddictConstants.Errors.InvalidRequest,
+                    description: "The mandatory 'code_challenge' parameter is missing.");
+
+                return;
+            }
+
             // Note: the OpenID Connect server middleware always ensures a
             // code_challenge_method can't be specified without code_challenge.
             if (!string.IsNullOrEmpty(context.Request.CodeChallenge))

src/OpenIddict.Server/OpenIddictServerProvider.Exchange.cs

@@ -54,18 +54,36 @@ namespace OpenIddict.Server
                 return;
             }
 
-            // Optimization: the OpenID Connect server middleware automatically rejects grant_type=authorization_code
-            // requests missing the redirect_uri parameter if one was specified in the initial authorization request.
-            // Since OpenIddict doesn't allow redirect_uri-less authorization requests, an earlier check can be made here,
-            // which saves the OpenID Connect server middleware from having to deserialize the authorization code ticket.
-            // See http://openid.net/specs/openid-connect-core-1_0.html#TokenRequestValidation for more information.
-            if (context.Request.IsAuthorizationCodeGrantType() && string.IsNullOrEmpty(context.Request.RedirectUri))
+            if (context.Request.IsAuthorizationCodeGrantType())
             {
-                context.Reject(
-                    error: OpenIddictConstants.Errors.InvalidRequest,
-                    description: "The mandatory 'redirect_uri' parameter is missing.");
+                // Optimization: the OpenID Connect server middleware automatically rejects grant_type=authorization_code
+                // requests missing the redirect_uri parameter if one was specified in the initial authorization request.
+                // Since OpenIddict doesn't allow redirect_uri-less authorization requests, an earlier check can be made here,
+                // which saves the OpenID Connect server middleware from having to deserialize the authorization code ticket.
+                // See http://openid.net/specs/openid-connect-core-1_0.html#TokenRequestValidation for more information.
+                if (string.IsNullOrEmpty(context.Request.RedirectUri))
+                {
+                    context.Reject(
+                        error: OpenIddictConstants.Errors.InvalidRequest,
+                        description: "The mandatory 'redirect_uri' parameter is missing.");
 
-                return;
+                    return;
+                }
+
+                // Optimization: the OpenID Connect server middleware automatically rejects grant_type=authorization_code
+                // requests missing the code_verifier parameter when a challenge was specified in the authorization request.
+                // That check requires decrypting the authorization code and determining whether a code challenge was set.
+                // If OpenIddict was configured to require PKCE, this can be potentially avoided by making an early check here.
+                if (options.RequireProofKeyForCodeExchange && string.IsNullOrEmpty(context.Request.CodeVerifier))
+                {
+                    _logger.LogError("The token request was rejected because the required 'code_verifier' parameter was missing.");
+
+                    context.Reject(
+                        error: OpenIddictConstants.Errors.InvalidRequest,
+                        description: "The mandatory 'code_verifier' parameter is missing.");
+
+                    return;
+                }
             }
 
             // Note: the OpenID Connect server middleware allows returning a refresh token with grant_type=client_credentials,

test/OpenIddict.Server.Tests/OpenIddictServerBuilderTests.cs

@@ -848,6 +848,22 @@ namespace OpenIddict.Server.Tests
             Assert.Contains("custom_scope_2", options.Scopes);
         }
 
+        [Fact]
+        public void RequireProofKeyForCodeExchange_PkceIsEnforced()
+        {
+            // Arrange
+            var services = CreateServices();
+            var builder = CreateBuilder(services);
+
+            // Act
+            builder.RequireProofKeyForCodeExchange();
+
+            var options = GetOptions(services);
+
+            // Assert
+            Assert.True(options.RequireProofKeyForCodeExchange);
+        }
+
         [Fact]
         public void UseDataProtectionProvider_DefaultProviderIsReplaced()
         {

test/OpenIddict.Server.Tests/OpenIddictServerProviderTests.Authentication.cs

@@ -392,6 +392,28 @@ namespace OpenIddict.Server.Tests
             Assert.Equal("The mandatory 'redirect_uri' parameter is missing.", response.ErrorDescription);
         }
 
+        [Fact]
+        public async Task ValidateAuthorizationRequest_RequestIsRejectedWhenPkceIsRequiredAndCodeChallengeIsMissing()
+        {
+            // Arrange
+            var server = CreateAuthorizationServer(builder => builder.RequireProofKeyForCodeExchange());
+
+            var client = new OpenIdConnectClient(server.CreateClient());
+
+            // Act
+            var response = await client.PostAsync(AuthorizationEndpoint, new OpenIdConnectRequest
+            {
+                ClientId = "Fabrikam",
+                CodeChallenge = null,
+                RedirectUri = "http://www.fabrikam.com/path",
+                ResponseType = OpenIddictConstants.ResponseTypes.Code
+            });
+
+            // Assert
+            Assert.Equal(OpenIddictConstants.Errors.InvalidRequest, response.Error);
+            Assert.Equal("The mandatory 'code_challenge' parameter is missing.", response.ErrorDescription);
+        }
+
         [Fact]
         public async Task ValidateAuthorizationRequest_RequestIsRejectedWhenCodeChallengeMethodIsMissing()
         {

test/OpenIddict.Server.Tests/OpenIddictServerProviderTests.Exchange.cs

@@ -99,6 +99,29 @@ namespace OpenIddict.Server.Tests
             Assert.Equal("The mandatory 'redirect_uri' parameter is missing.", response.ErrorDescription);
         }
 
+        [Fact]
+        public async Task ValidateTokenRequest_AuthorizationCodeRequestIsRejectedWhenCodeVerifierIsMissing()
+        {
+            // Arrange
+            var server = CreateAuthorizationServer(builder => builder.RequireProofKeyForCodeExchange());
+
+            var client = new OpenIdConnectClient(server.CreateClient());
+
+            // Act
+            var response = await client.PostAsync(TokenEndpoint, new OpenIdConnectRequest
+            {
+                ClientId = "Fabrikam",
+                Code = "SplxlOBeZQQYbYS6WxSbIA",
+                CodeVerifier = null,
+                GrantType = OpenIddictConstants.GrantTypes.AuthorizationCode,
+                RedirectUri = "http://www.fabrikam.com/path"
+            });
+
+            // Assert
+            Assert.Equal(OpenIddictConstants.Errors.InvalidRequest, response.Error);
+            Assert.Equal("The mandatory 'code_verifier' parameter is missing.", response.ErrorDescription);
+        }
+
         [Fact]
         public async Task ValidateTokenRequest_RequestIsRejectedWhenUnregisteredScopeIsSpecified()
         {