diff --git a/src/OpenIddict.Core/OpenIddictProvider.Authentication.cs b/src/OpenIddict.Core/OpenIddictProvider.Authentication.cs
index cb173f97..79336d4d 100644
--- a/src/OpenIddict.Core/OpenIddictProvider.Authentication.cs
+++ b/src/OpenIddict.Core/OpenIddictProvider.Authentication.cs
@@ -63,6 +63,34 @@ namespace OpenIddict {
                 return;
             }
 
+            // If the user is connected, ensure that a corresponding profile exists and that
+            // the appropriate set of scopes is requested to prevent personal data leakage.
+            if (context.HttpContext.User.Identities.Any(identity => identity.IsAuthenticated)) {
+                // Ensure the user profile still exists in the database.
+                var user = await manager.FindByIdAsync(context.HttpContext.User.GetUserId());
+                if (user == null) {
+                    context.Reject(
+                        error: OpenIdConnectConstants.Errors.ServerError,
+                        description: "An internal error has occurred.");
+
+                    return;
+                }
+
+                // Return an error if the username corresponds to the registered
+                // email address and if the "email" scope has not been requested.
+                if (context.Request.HasScope(OpenIdConnectConstants.Scopes.Profile) &&
+                   !context.Request.HasScope(OpenIdConnectConstants.Scopes.Email) &&
+                    string.Equals(await manager.GetUserNameAsync(user),
+                                  await manager.GetEmailAsync(user),
+                                  StringComparison.OrdinalIgnoreCase)) {
+                    context.Reject(
+                        error: OpenIdConnectConstants.Errors.InvalidRequest,
+                        description: "The 'email' scope is required.");
+
+                    return;
+                }
+            }
+
             // Run additional checks for prompt=none requests.
             if (string.Equals(context.Request.Prompt, "none", StringComparison.Ordinal)) {
                 // If the user is not authenticated, return an error to the client application.
@@ -96,30 +124,6 @@ namespace OpenIddict {
 
                     return;
                 }
-
-                // Ensure the user profile still exists in the database.
-                var user = await manager.FindByIdAsync(principal.GetUserId());
-                if (user == null) {
-                    context.Reject(
-                        error: OpenIdConnectConstants.Errors.InvalidRequest,
-                        description: "The id_token_hint parameter is invalid.");
-
-                    return;
-                }
-
-                // Return an error if the username corresponds to the registered
-                // email address and if the "email" scope has not been requested.
-                if (context.Request.HasScope(OpenIdConnectConstants.Scopes.Profile) &&
-                   !context.Request.HasScope(OpenIdConnectConstants.Scopes.Email) &&
-                    string.Equals(await manager.GetUserNameAsync(user),
-                                  await manager.GetEmailAsync(user),
-                                  StringComparison.OrdinalIgnoreCase)) {
-                    context.Reject(
-                        error: OpenIdConnectConstants.Errors.InvalidRequest,
-                        description: "The 'email' scope is required.");
-
-                    return;
-                }
             }
 
             context.Validate();
diff --git a/src/OpenIddict.Mvc/OpenIddictController.cs b/src/OpenIddict.Mvc/OpenIddictController.cs
index 21689604..c337c394 100644
--- a/src/OpenIddict.Mvc/OpenIddictController.cs
+++ b/src/OpenIddict.Mvc/OpenIddictController.cs
@@ -96,6 +96,18 @@ namespace OpenIddict.Mvc {
 
         [Authorize, HttpPost, ValidateAntiForgeryToken]
         public virtual async Task<IActionResult> Accept() {
+            // Note: when a fatal error occurs during the request processing, an OpenID Connect response
+            // is prematurely forged and added to the ASP.NET context by OpenIdConnectServerHandler.
+            // In this case, the OpenID Connect request is null and cannot be used.
+            // When the user agent can be safely redirected to the client application,
+            // OpenIdConnectServerHandler automatically handles the error and MVC is not invoked.
+            // You can safely remove this part and let AspNet.Security.OpenIdConnect.Server automatically
+            // handle the unrecoverable errors by switching ApplicationCanDisplayErrors to false.
+            var response = HttpContext.GetOpenIdConnectResponse();
+            if (response != null) {
+                return View("Error", response);
+            }
+
             // Extract the authorization request from the cache,
             // the query string or the request form.
             var request = HttpContext.GetOpenIdConnectRequest();
@@ -115,19 +127,6 @@ namespace OpenIddict.Mvc {
                 });
             }
 
-            // Return an error if the username corresponds to the registered
-            // email address and if the "email" scope has not been requested.
-            if (request.HasScope(OpenIdConnectConstants.Scopes.Profile) &&
-               !request.HasScope(OpenIdConnectConstants.Scopes.Email) &&
-                string.Equals(await Manager.GetUserNameAsync(user),
-                              await Manager.GetEmailAsync(user),
-                              StringComparison.OrdinalIgnoreCase)) {
-                return View("Error", new OpenIdConnectMessage {
-                    Error = OpenIdConnectConstants.Errors.InvalidRequest,
-                    ErrorDescription = "The 'email' scope is required."
-                });
-            }
-
             // Create a new ClaimsIdentity containing the claims that
             // will be used to create an id_token, a token or a code.
             var identity = await Manager.CreateIdentityAsync(user, request.GetScopes());
@@ -172,6 +171,18 @@ namespace OpenIddict.Mvc {
 
         [Authorize, HttpPost, ValidateAntiForgeryToken]
         public virtual IActionResult Deny() {
+            // Note: when a fatal error occurs during the request processing, an OpenID Connect response
+            // is prematurely forged and added to the ASP.NET context by OpenIdConnectServerHandler.
+            // In this case, the OpenID Connect request is null and cannot be used.
+            // When the user agent can be safely redirected to the client application,
+            // OpenIdConnectServerHandler automatically handles the error and MVC is not invoked.
+            // You can safely remove this part and let AspNet.Security.OpenIdConnect.Server automatically
+            // handle the unrecoverable errors by switching ApplicationCanDisplayErrors to false.
+            var response = HttpContext.GetOpenIdConnectResponse();
+            if (response != null) {
+                return View("Error", response);
+            }
+
             // Extract the authorization request from the cache,
             // the query string or the request form.
             var request = HttpContext.GetOpenIdConnectRequest();