diff --git a/src/OpenIddict.Core/Infrastructure/OpenIddictHelpers.cs b/src/OpenIddict.Core/Infrastructure/OpenIddictHelpers.cs
index 2bc61ca8..fa7a628f 100644
--- a/src/OpenIddict.Core/Infrastructure/OpenIddictHelpers.cs
+++ b/src/OpenIddict.Core/Infrastructure/OpenIddictHelpers.cs
@@ -12,6 +12,14 @@ using Microsoft.AspNetCore.Identity;
 
 namespace OpenIddict.Infrastructure {
     public static class OpenIddictHelpers {
+        /// <summary>
+        /// Tries to find the given claim in the user claims.
+        /// </summary>
+        /// <typeparam name="TUser">The type of the User entity.</typeparam>
+        /// <param name="manager">The user manager.</param>
+        /// <param name="user">The user.</param>
+        /// <param name="type">The claim type.</param>
+        /// <returns>The claim value, or <c>null</c> if it cannot be found.</returns>
         public static async Task<string> FindClaimAsync<TUser>(
             [NotNull] this UserManager<TUser> manager,
             [NotNull] TUser user, [NotNull] string type) where TUser : class {
@@ -33,5 +41,51 @@ namespace OpenIddict.Infrastructure {
                     where string.Equals(claim.Type, type, StringComparison.OrdinalIgnoreCase)
                     select claim.Value).FirstOrDefault();
         }
+
+        /// <summary>
+        /// Determines whether an application is a confidential client.
+        /// </summary>
+        /// <typeparam name="TApplication">The type of the Application entity.</typeparam>
+        /// <param name="manager">The application manager.</param>
+        /// <param name="application">The application.</param>
+        /// <returns><c>true</c> if the application is a confidential client, <c>false</c> otherwise.</returns>
+        public static async Task<bool> IsConfidentialAsync<TApplication>(
+            [NotNull] this OpenIddictApplicationManager<TApplication> manager,
+            [NotNull] TApplication application) where TApplication : class {
+            if (manager == null) {
+                throw new ArgumentNullException(nameof(manager));
+            }
+
+            if (application == null) {
+                throw new ArgumentNullException(nameof(application));
+            }
+
+            var type = await manager.GetClientTypeAsync(application);
+
+            return string.Equals(type, OpenIddictConstants.ClientTypes.Confidential, StringComparison.OrdinalIgnoreCase);
+        }
+
+        /// <summary>
+        /// Determines whether an application is a public client.
+        /// </summary>
+        /// <typeparam name="TApplication">The type of the Application entity.</typeparam>
+        /// <param name="manager">The application manager.</param>
+        /// <param name="application">The application.</param>
+        /// <returns><c>true</c> if the application is a public client, <c>false</c> otherwise.</returns>
+        public static async Task<bool> IsPublicAsync<TApplication>(
+            [NotNull] this OpenIddictApplicationManager<TApplication> manager,
+            [NotNull] TApplication application) where TApplication : class {
+            if (manager == null) {
+                throw new ArgumentNullException(nameof(manager));
+            }
+
+            if (application == null) {
+                throw new ArgumentNullException(nameof(application));
+            }
+
+            var type = await manager.GetClientTypeAsync(application);
+
+            return string.Equals(type, OpenIddictConstants.ClientTypes.Public, StringComparison.OrdinalIgnoreCase);
+        }
     }
 }
diff --git a/src/OpenIddict.Core/Infrastructure/OpenIddictProvider.Authentication.cs b/src/OpenIddict.Core/Infrastructure/OpenIddictProvider.Authentication.cs
index e7fd8264..324606d1 100644
--- a/src/OpenIddict.Core/Infrastructure/OpenIddictProvider.Authentication.cs
+++ b/src/OpenIddict.Core/Infrastructure/OpenIddictProvider.Authentication.cs
@@ -139,6 +139,16 @@ namespace OpenIddict.Infrastructure {
                 return;
             }
 
+            // Reject authorization requests that specify scope=offline_access if the refresh token flow is not enabled.
+            if (context.Request.HasScope(OpenIdConnectConstants.Scopes.OfflineAccess) &&
+               !services.Options.GrantTypes.Contains(OpenIdConnectConstants.GrantTypes.RefreshToken)) {
+                context.Reject(
+                    error: OpenIdConnectConstants.Errors.InvalidRequest,
+                    description: "The 'offline_access' scope is not allowed.");
+
+                return;
+            }
+
             // Note: the OpenID Connect server middleware supports the query, form_post and fragment response modes
             // and doesn't reject unknown/custom modes until the ApplyAuthorizationResponse event is invoked.
             // To ensure authorization requests are rejected early enough, an additional check is made by OpenIddict.
@@ -167,14 +177,46 @@ namespace OpenIddict.Infrastructure {
                 return;
             }
 
-            // Reject authorization requests that specify scope=offline_access if the refresh token flow is not enabled.
-            if (context.Request.HasScope(OpenIdConnectConstants.Scopes.OfflineAccess) &&
-               !services.Options.GrantTypes.Contains(OpenIdConnectConstants.GrantTypes.RefreshToken)) {
-                context.Reject(
-                    error: OpenIdConnectConstants.Errors.InvalidRequest,
-                    description: "The 'offline_access' scope is not allowed.");
+            // 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)) {
+                // Since the default challenge method (plain) is explicitly disallowed,
+                // reject the authorization request if the code_challenge_method is missing.
+                if (string.IsNullOrEmpty(context.Request.CodeChallengeMethod)) {
+                    services.Logger.LogError("The authorization request was rejected because the " +
+                                             "required 'code_challenge_method' parameter was missing.");
 
-                return;
+                    context.Reject(
+                        error: OpenIdConnectConstants.Errors.InvalidRequest,
+                        description: "The 'code_challenge_method' parameter must be specified.");
+
+                    return;
+                }
+
+                // Disallow the use of the unsecure code_challenge_method=plain method.
+                // See https://tools.ietf.org/html/rfc7636#section-7.2 for more information.
+                if (context.Request.CodeChallengeMethod == OpenIdConnectConstants.CodeChallengeMethods.Plain) {
+                    services.Logger.LogError("The authorization request was rejected because the " +
+                                             "'code_challenge_method' parameter was set to 'plain'.");
+
+                    context.Reject(
+                        error: OpenIdConnectConstants.Errors.InvalidRequest,
+                        description: "The specified response_type parameter is not allowed when using PKCE.");
+
+                    return;
+                }
+
+                // Reject authorization requests that contain response_type=token when a code_challenge is specified.
+                if (context.Request.HasResponseType(OpenIdConnectConstants.ResponseTypes.Token)) {
+                    services.Logger.LogError("The authorization request was rejected because the " +
+                                             "specified response type was not compatible with PKCE.");
+
+                    context.Reject(
+                        error: OpenIdConnectConstants.Errors.InvalidRequest,
+                        description: "The specified response_type parameter is not allowed when using PKCE.");
+
+                    return;
+                }
             }
 
             // Retrieve the application details corresponding to the requested client_id.
@@ -205,9 +247,7 @@ namespace OpenIddict.Infrastructure {
             // flow are rejected if the client identifier corresponds to a confidential application.
             // Note: when using the authorization code grant, ValidateTokenRequest is responsible of
             // rejecting the token request if the client_id corresponds to an unauthenticated confidential client.
-            var type = await services.Applications.GetClientTypeAsync(application);
-            if (!string.Equals(type, OpenIddictConstants.ClientTypes.Public, StringComparison.OrdinalIgnoreCase) &&
-                !context.Request.IsAuthorizationCodeFlow()) {
+            if (await services.Applications.IsPublicAsync(application) && !context.Request.IsAuthorizationCodeFlow()) {
                 context.Reject(
                     error: OpenIdConnectConstants.Errors.InvalidRequest,
                     description: "Confidential clients can only use response_type=code.");
diff --git a/src/OpenIddict.Core/Infrastructure/OpenIddictProvider.Discovery.cs b/src/OpenIddict.Core/Infrastructure/OpenIddictProvider.Discovery.cs
index 6b62f4f4..82b4417a 100644
--- a/src/OpenIddict.Core/Infrastructure/OpenIddictProvider.Discovery.cs
+++ b/src/OpenIddict.Core/Infrastructure/OpenIddictProvider.Discovery.cs
@@ -17,7 +17,11 @@ namespace OpenIddict.Infrastructure {
         public override Task HandleConfigurationRequest([NotNull] HandleConfigurationRequestContext context) {
             var services = context.HttpContext.RequestServices.GetRequiredService<OpenIddictServices<TUser, TApplication, TAuthorization, TScope, TToken>>();
 
-            Debug.Assert(services.Options.GrantTypes.Count != 0, "At least one flow should be enabled.");
+            // Note: though it's natively supported by the OpenID Connect server middleware,
+            // OpenIddict disallows the use of the unsecure code_challenge_method=plain method,
+            // which must be manually removed from the code_challenge_methods_supported property.
+            // See https://tools.ietf.org/html/rfc7636#section-7.2 for more information.
+            context.CodeChallengeMethods.Remove(OpenIdConnectConstants.CodeChallengeMethods.Plain);
 
             // Note: the OpenID Connect server middleware automatically populates grant_types_supported
             // by determining whether the authorization and token endpoints are enabled or not but
diff --git a/src/OpenIddict.Core/Infrastructure/OpenIddictProvider.Exchange.cs b/src/OpenIddict.Core/Infrastructure/OpenIddictProvider.Exchange.cs
index 4bc34c42..bee83461 100644
--- a/src/OpenIddict.Core/Infrastructure/OpenIddictProvider.Exchange.cs
+++ b/src/OpenIddict.Core/Infrastructure/OpenIddictProvider.Exchange.cs
@@ -161,8 +161,7 @@ namespace OpenIddict.Infrastructure {
                 return;
             }
 
-            var type = await services.Applications.GetClientTypeAsync(application);
-            if (string.Equals(type, OpenIddictConstants.ClientTypes.Public, StringComparison.OrdinalIgnoreCase)) {
+            if (await services.Applications.IsPublicAsync(application)) {
                 // Note: public applications are not allowed to use the client credentials grant.
                 if (context.Request.IsClientCredentialsGrantType()) {
                     services.Logger.LogError("The token request was rejected because the public client application '{ClientId}' " +
diff --git a/src/OpenIddict.Core/Infrastructure/OpenIddictProvider.Introspection.cs b/src/OpenIddict.Core/Infrastructure/OpenIddictProvider.Introspection.cs
index 331f1f09..17b458e6 100644
--- a/src/OpenIddict.Core/Infrastructure/OpenIddictProvider.Introspection.cs
+++ b/src/OpenIddict.Core/Infrastructure/OpenIddictProvider.Introspection.cs
@@ -55,8 +55,7 @@ namespace OpenIddict.Infrastructure {
             }
 
             // Reject non-confidential applications.
-            var type = await services.Applications.GetClientTypeAsync(application);
-            if (!string.Equals(type, OpenIddictConstants.ClientTypes.Confidential, StringComparison.OrdinalIgnoreCase)) {
+            if (!await services.Applications.IsConfidentialAsync(application)) {
                 services.Logger.LogError("The introspection request was rejected because the public application " +
                                          "'{ClientId}' was not allowed to use this endpoint.", context.ClientId);
 
diff --git a/src/OpenIddict.Core/Infrastructure/OpenIddictProvider.Revocation.cs b/src/OpenIddict.Core/Infrastructure/OpenIddictProvider.Revocation.cs
index 87d0a2ec..9c24f466 100644
--- a/src/OpenIddict.Core/Infrastructure/OpenIddictProvider.Revocation.cs
+++ b/src/OpenIddict.Core/Infrastructure/OpenIddictProvider.Revocation.cs
@@ -55,8 +55,7 @@ namespace OpenIddict.Infrastructure {
             }
 
             // Reject revocation requests containing a client_secret if the client application is not confidential.
-            var type = await services.Applications.GetClientTypeAsync(application);
-            if (string.Equals(type, OpenIddictConstants.ClientTypes.Public, StringComparison.OrdinalIgnoreCase)) {
+            if (await services.Applications.IsPublicAsync(application)) {
                 // Reject tokens requests containing a client_secret when the client is a public application.
                 if (!string.IsNullOrEmpty(context.ClientSecret)) {
                     context.Reject(