Fixes #20239: Prevent shared mutable state in PluginMenuItem and PluginMenuButton

PluginMenuItem and PluginMenuButton classes used mutable class-level
defaults for `permissions` and `buttons` attributes, causing permission
leakage between instances when these attributes were modified without
explicit parameters.

Changed to initialize these attributes as fresh lists per instance in
__init__ when not explicitly provided, following standard Python pattern
for avoiding mutable default arguments.

netbox/netbox/plugins/navigation.py

@@ -37,8 +37,6 @@ class PluginMenuItem:
     Alternatively, a pre-generated url can be set on the object which will be rendered literally.
     Buttons are each specified as a list of PluginMenuButton instances.
     """
-    permissions = []
-    buttons = []
     _url = None
 
     def __init__(
@@ -54,10 +52,14 @@ class PluginMenuItem:
             if type(permissions) not in (list, tuple):
                 raise TypeError(_("Permissions must be passed as a tuple or list."))
             self.permissions = permissions
+        else:
+            self.permissions = []
         if buttons is not None:
             if type(buttons) not in (list, tuple):
                 raise TypeError(_("Buttons must be passed as a tuple or list."))
             self.buttons = buttons
+        else:
+            self.buttons = []
 
     @property
     def url(self):
@@ -74,7 +76,6 @@ class PluginMenuButton:
     ButtonColorChoices.
     """
     color = ButtonColorChoices.DEFAULT
-    permissions = []
     _url = None
 
     def __init__(self, link, title, icon_class, color=None, permissions=None):
@@ -87,6 +88,8 @@ class PluginMenuButton:
             if type(permissions) not in (list, tuple):
                 raise TypeError(_("Permissions must be passed as a tuple or list."))
             self.permissions = permissions
+        else:
+            self.permissions = []
         if color is not None:
             if color not in ButtonColorChoices.values():
                 raise ValueError(_("Button color must be a choice within ButtonColorChoices."))

netbox/netbox/tests/test_plugins.py

@@ -11,7 +11,7 @@ from netbox.tests.dummy_plugin import config as dummy_config
 from netbox.tests.dummy_plugin.data_backends import DummyBackend
 from netbox.tests.dummy_plugin.jobs import DummySystemJob
 from netbox.tests.dummy_plugin.webhook_callbacks import set_context
-from netbox.plugins.navigation import PluginMenu
+from netbox.plugins.navigation import PluginMenu, PluginMenuItem, PluginMenuButton
 from netbox.plugins.utils import get_plugin_config
 from netbox.graphql.schema import Query
 from netbox.registry import registry
@@ -227,3 +227,46 @@ class PluginTest(TestCase):
         Test the registration of webhook callbacks.
         """
         self.assertIn(set_context, registry['webhook_callbacks'])
+
+
+class PluginNavigationTest(TestCase):
+
+    def test_plugin_menu_item_independent_permissions(self):
+        item1 = PluginMenuItem(link='test1', link_text='Test 1')
+        item1.permissions.append('leaked_permission')
+
+        item2 = PluginMenuItem(link='test2', link_text='Test 2')
+
+        self.assertIsNot(item1.permissions, item2.permissions)
+        self.assertEqual(item1.permissions, ['leaked_permission'])
+        self.assertEqual(item2.permissions, [])
+
+    def test_plugin_menu_item_independent_buttons(self):
+        item1 = PluginMenuItem(link='test1', link_text='Test 1')
+        button = PluginMenuButton(link='button1', title='Button 1', icon_class='mdi-test')
+        item1.buttons.append(button)
+
+        item2 = PluginMenuItem(link='test2', link_text='Test 2')
+
+        self.assertIsNot(item1.buttons, item2.buttons)
+        self.assertEqual(len(item1.buttons), 1)
+        self.assertEqual(item1.buttons[0], button)
+        self.assertEqual(item2.buttons, [])
+
+    def test_plugin_menu_button_independent_permissions(self):
+        button1 = PluginMenuButton(link='button1', title='Button 1', icon_class='mdi-test')
+        button1.permissions.append('leaked_permission')
+
+        button2 = PluginMenuButton(link='button2', title='Button 2', icon_class='mdi-test')
+
+        self.assertIsNot(button1.permissions, button2.permissions)
+        self.assertEqual(button1.permissions, ['leaked_permission'])
+        self.assertEqual(button2.permissions, [])
+
+    def test_explicit_permissions_remain_independent(self):
+        item1 = PluginMenuItem(link='test1', link_text='Test 1', permissions=['explicit_permission'])
+        item2 = PluginMenuItem(link='test2', link_text='Test 2', permissions=['different_permission'])
+
+        self.assertIsNot(item1.permissions, item2.permissions)
+        self.assertEqual(item1.permissions, ['explicit_permission'])
+        self.assertEqual(item2.permissions, ['different_permission'])