diff --git a/app/models/work_package_custom_fields/scopes/on_visible_type_and_project.rb b/app/models/work_package_custom_fields/scopes/on_visible_type_and_project.rb
index 27aa76d8885..c584f539204 100644
--- a/app/models/work_package_custom_fields/scopes/on_visible_type_and_project.rb
+++ b/app/models/work_package_custom_fields/scopes/on_visible_type_and_project.rb
@@ -34,12 +34,10 @@ module WorkPackageCustomFields::Scopes
 
     class_methods do
       def on_visible_type_and_project(user = User.current)
-        with(
-          visible_projects: Project.visible(user)
-        ).where(<<~SQL.squish)
+        where(<<~SQL.squish)
           EXISTS (
             SELECT 1
-            FROM visible_projects vp
+            FROM (#{Project.visible(user).select(:id).to_sql}) vp
             JOIN projects_types pt
               ON pt.project_id = vp.id
             JOIN custom_fields_types cft
diff --git a/docs/api/apiv3/paths/custom_field_item.yml b/docs/api/apiv3/paths/custom_field_item.yml
index c1ac90dda38..5eabd3fba64 100644
--- a/docs/api/apiv3/paths/custom_field_item.yml
+++ b/docs/api/apiv3/paths/custom_field_item.yml
@@ -36,7 +36,10 @@ get:
                 errorIdentifier: urn:openproject-org:api:v3:errors:MissingPermission
                 message: You are not authorized to access this resource.
     '404':
-      description: Returned if the custom field item does not exist or the user is not logged in.
+      description: |-
+        Returned if the custom field item does not exist or the user lacks permission to see it.
+
+        The permission required to view the item depends on the custom field it belongs to.
       content:
         application/hal+json:
           schema:
diff --git a/docs/api/apiv3/paths/custom_field_item_branch.yml b/docs/api/apiv3/paths/custom_field_item_branch.yml
index 9b84412b585..3518a98be46 100644
--- a/docs/api/apiv3/paths/custom_field_item_branch.yml
+++ b/docs/api/apiv3/paths/custom_field_item_branch.yml
@@ -38,7 +38,7 @@ get:
                 errorIdentifier: urn:openproject-org:api:v3:errors:MissingPermission
                 message: You are not authorized to access this resource.
     '404':
-      description: Returned if the custom field does not exist.
+      description: Returned if the custom field does not exist or the user lacks permission to view it.
       content:
         application/hal+json:
           schema:
diff --git a/docs/api/apiv3/paths/custom_field_items.yml b/docs/api/apiv3/paths/custom_field_items.yml
index e51e9e3bffe..01fe867ef90 100644
--- a/docs/api/apiv3/paths/custom_field_items.yml
+++ b/docs/api/apiv3/paths/custom_field_items.yml
@@ -62,7 +62,7 @@ get:
                 errorIdentifier: urn:openproject-org:api:v3:errors:MissingPermission
                 message: You are not authorized to access this resource.
     '404':
-      description: Returned if the custom field does not exist.
+      description: Returned if the custom field does not exist or the user lacks the permission to view it.
       content:
         application/hal+json:
           schema:
diff --git a/lib/api/v3/custom_fields/custom_fields_api.rb b/lib/api/v3/custom_fields/custom_fields_api.rb
index 64cc6b6514b..90b7a31fdaa 100644
--- a/lib/api/v3/custom_fields/custom_fields_api.rb
+++ b/lib/api/v3/custom_fields/custom_fields_api.rb
@@ -35,7 +35,7 @@ module API
             after_validation do
               authorize_logged_in
 
-              @custom_field = CustomField.find(params[:id])
+              @custom_field = CustomField.visible.find(params[:id])
             end
 
             mount Hierarchy::ItemsAPI
diff --git a/lib/api/v3/custom_fields/hierarchy/item_api.rb b/lib/api/v3/custom_fields/hierarchy/item_api.rb
index ab2574ec16b..a4ea81c1b64 100644
--- a/lib/api/v3/custom_fields/hierarchy/item_api.rb
+++ b/lib/api/v3/custom_fields/hierarchy/item_api.rb
@@ -37,6 +37,9 @@ module API
                 authorize_logged_in
 
                 @custom_field_item = CustomField::Hierarchy::Item.find(params[:id])
+
+                # Only for checking the authorization.
+                CustomField.visible.find(@custom_field_item.root.custom_field_id)
               end
 
               get &::API::V3::Utilities::Endpoints::Show
diff --git a/spec/requests/api/v3/custom_fields/hierarchy/item_api_spec.rb b/spec/requests/api/v3/custom_fields/hierarchy/item_api_spec.rb
index 39e72aafe49..2ab1e8ff3e5 100644
--- a/spec/requests/api/v3/custom_fields/hierarchy/item_api_spec.rb
+++ b/spec/requests/api/v3/custom_fields/hierarchy/item_api_spec.rb
@@ -33,7 +33,9 @@ require "spec_helper"
 RSpec.describe "API v3 custom field items", :webmock, content_type: :json, with_ee: [:custom_field_hierarchies] do
   include API::V3::Utilities::PathHelper
 
-  let(:custom_field) { create(:custom_field, field_format: "hierarchy", hierarchy_root: nil) }
+  shared_let(:project) { create(:project) }
+
+  let(:custom_field) { create(:wp_custom_field, field_format: "hierarchy", hierarchy_root: nil) }
   let(:service) { CustomFields::Hierarchy::HierarchicalItemService.new }
   let!(:root) { service.generate_root(custom_field).value! }
   let(:contract_class) { CustomFields::Hierarchy::InsertListItemContract }
@@ -52,8 +54,14 @@ RSpec.describe "API v3 custom field items", :webmock, content_type: :json, with_
       it_behaves_like "unauthenticated access"
     end
 
-    context "if user is logged in" do
-      before { login_as create(:user) }
+    context "if user is logged in but lacks permissions" do
+      current_user { create(:user, member_with_permissions: { project => [] }) }
+
+      it_behaves_like "not found"
+    end
+
+    context "if user is logged in with the necessary permissions" do
+      current_user { create(:user, member_with_permissions: { project => [:select_custom_fields] }) }
 
       it_behaves_like "successful response"
 
@@ -78,8 +86,14 @@ RSpec.describe "API v3 custom field items", :webmock, content_type: :json, with_
       it_behaves_like "unauthenticated access"
     end
 
-    context "if user is logged in" do
-      before { login_as create(:user) }
+    context "if user is logged in but lacks permissions" do
+      current_user { create(:user, member_with_permissions: { project => [] }) }
+
+      it_behaves_like "not found"
+    end
+
+    context "if user is logged in with the necessary permissions" do
+      current_user { create(:user, member_with_permissions: { project => [:select_custom_fields] }) }
 
       it_behaves_like "API V3 collection response", 4, 4, "HierarchyItem", "Collection" do
         let(:elements) { [root, luke, r2d2, mouse] }
diff --git a/spec/requests/api/v3/custom_fields/hierarchy/items_api_spec.rb b/spec/requests/api/v3/custom_fields/hierarchy/items_api_spec.rb
index 4907ef5557b..11060112c83 100644
--- a/spec/requests/api/v3/custom_fields/hierarchy/items_api_spec.rb
+++ b/spec/requests/api/v3/custom_fields/hierarchy/items_api_spec.rb
@@ -34,7 +34,9 @@ RSpec.describe "API v3 custom field hierarchy items", :webmock, content_type: :j
   include API::V3::Utilities::PathHelper
 
   describe "GET /api/v3/custom_fields/:id/items" do
-    let(:custom_field) { create(:custom_field, field_format: "hierarchy", hierarchy_root: nil) }
+    shared_let(:project) { create(:project) }
+
+    let(:custom_field) { create(:wp_custom_field, field_format: "hierarchy", hierarchy_root: nil) }
     let!(:root) { service.generate_root(custom_field).value! }
     let(:contract_class) { CustomFields::Hierarchy::InsertListItemContract }
     let!(:luke) { service.insert_item(contract_class:, parent: root, label: "Luke", short: "LS").value! }
@@ -52,8 +54,14 @@ RSpec.describe "API v3 custom field hierarchy items", :webmock, content_type: :j
       it_behaves_like "unauthenticated access"
     end
 
-    context "if user is logged in" do
-      before { login_as create(:user) }
+    context "if the user is not allowed to view the custom field" do
+      current_user { create(:user, member_with_permissions: { project => [] }) }
+
+      it_behaves_like "not found"
+    end
+
+    context "if user is logged in with the necessary permissions" do
+      current_user { create(:user, member_with_permissions: { project => [:select_custom_fields] }) }
 
       it_behaves_like "API V3 collection response", 6, 6, "HierarchyItem", "Collection" do
         let(:elements) { [root, luke, r2d2, mouse, c3po, mara] }