diff --git a/app/apps/accounts/views/account_groups.py b/app/apps/accounts/views/account_groups.py
index d9b0825..75687e0 100644
--- a/app/apps/accounts/views/account_groups.py
+++ b/app/apps/accounts/views/account_groups.py
@@ -1,13 +1,19 @@
 from django.contrib import messages
 from django.contrib.auth.decorators import login_required
+from django.core.exceptions import PermissionDenied
 from django.http import HttpResponse
-from django.shortcuts import render, get_object_or_404
+from django.shortcuts import render
 from django.utils.translation import gettext_lazy as _
 from django.views.decorators.http import require_http_methods
 
 from apps.accounts.forms import AccountGroupForm
 from apps.accounts.models import AccountGroup
 from apps.common.decorators.htmx import only_htmx
+from apps.common.functions.permissions import (
+    EDIT,
+    READ,
+    get_shared_object_or_error,
+)
 from apps.common.models import SharedObject
 from apps.common.forms import SharedObjectForm
 
@@ -63,17 +69,7 @@ def account_group_add(request, **kwargs):
 @login_required
 @require_http_methods(["GET", "POST"])
 def account_group_edit(request, pk):
-    account_group = get_object_or_404(AccountGroup, id=pk)
-
-    if account_group.owner and account_group.owner != request.user:
-        messages.error(request, _("Only the owner can edit this"))
-
-        return HttpResponse(
-            status=204,
-            headers={
-                "HX-Trigger": "updated, hide_offcanvas",
-            },
-        )
+    account_group = get_shared_object_or_error(AccountGroup, request, id=pk, level=EDIT)
 
     if request.method == "POST":
         form = AccountGroupForm(request.POST, instance=account_group)
@@ -101,17 +97,18 @@ def account_group_edit(request, pk):
 @login_required
 @require_http_methods(["DELETE"])
 def account_group_delete(request, pk):
-    account_group = get_object_or_404(AccountGroup, id=pk)
+    account_group = get_shared_object_or_error(AccountGroup, request, id=pk, level=READ)
 
-    if (
-        account_group.owner != request.user
-        and request.user in account_group.shared_with.all()
-    ):
+    if account_group.is_editable_by(request.user):
+        account_group.delete()
+        messages.success(request, _("Account Group deleted successfully"))
+    elif account_group.shared_with.filter(pk=request.user.pk).exists():
+        # Someone else's object shared with us: we can drop our own access
+        # to it, but never delete it.
         account_group.shared_with.remove(request.user)
         messages.success(request, _("Item no longer shared with you"))
     else:
-        account_group.delete()
-        messages.success(request, _("Account Group deleted successfully"))
+        raise PermissionDenied
 
     return HttpResponse(
         status=204,
@@ -125,7 +122,7 @@ def account_group_delete(request, pk):
 @login_required
 @require_http_methods(["GET"])
 def account_group_take_ownership(request, pk):
-    account_group = get_object_or_404(AccountGroup, id=pk)
+    account_group = get_shared_object_or_error(AccountGroup, request, id=pk, level=EDIT)
 
     if not account_group.owner:
         account_group.owner = request.user
@@ -146,17 +143,7 @@ def account_group_take_ownership(request, pk):
 @login_required
 @require_http_methods(["GET", "POST"])
 def account_group_share(request, pk):
-    obj = get_object_or_404(AccountGroup, id=pk)
-
-    if obj.owner and obj.owner != request.user:
-        messages.error(request, _("Only the owner can edit this"))
-
-        return HttpResponse(
-            status=204,
-            headers={
-                "HX-Trigger": "updated, hide_offcanvas",
-            },
-        )
+    obj = get_shared_object_or_error(AccountGroup, request, id=pk, level=EDIT)
 
     if request.method == "POST":
         form = SharedObjectForm(request.POST, instance=obj, user=request.user)
diff --git a/app/apps/accounts/views/accounts.py b/app/apps/accounts/views/accounts.py
index 4908bb7..bfab255 100644
--- a/app/apps/accounts/views/accounts.py
+++ b/app/apps/accounts/views/accounts.py
@@ -1,13 +1,19 @@
 from django.contrib import messages
 from django.contrib.auth.decorators import login_required
+from django.core.exceptions import PermissionDenied
 from django.http import HttpResponse
-from django.shortcuts import render, get_object_or_404
+from django.shortcuts import render
 from django.utils.translation import gettext_lazy as _
 from django.views.decorators.http import require_http_methods
 
 from apps.accounts.forms import AccountForm
 from apps.accounts.models import Account
 from apps.common.decorators.htmx import only_htmx
+from apps.common.functions.permissions import (
+    EDIT,
+    READ,
+    get_shared_object_or_error,
+)
 from apps.common.models import SharedObject
 from apps.common.forms import SharedObjectForm
 
@@ -63,16 +69,7 @@ def account_add(request, **kwargs):
 @login_required
 @require_http_methods(["GET", "POST"])
 def account_edit(request, pk):
-    account = get_object_or_404(Account, id=pk)
-    if account.owner and account.owner != request.user:
-        messages.error(request, _("Only the owner can edit this"))
-
-        return HttpResponse(
-            status=204,
-            headers={
-                "HX-Trigger": "updated, hide_offcanvas",
-            },
-        )
+    account = get_shared_object_or_error(Account, request, id=pk, level=EDIT)
 
     if request.method == "POST":
         form = AccountForm(request.POST, instance=account)
@@ -100,17 +97,7 @@ def account_edit(request, pk):
 @login_required
 @require_http_methods(["GET", "POST"])
 def account_share(request, pk):
-    obj = get_object_or_404(Account, id=pk)
-
-    if obj.owner and obj.owner != request.user:
-        messages.error(request, _("Only the owner can edit this"))
-
-        return HttpResponse(
-            status=204,
-            headers={
-                "HX-Trigger": "updated, hide_offcanvas",
-            },
-        )
+    obj = get_shared_object_or_error(Account, request, id=pk, level=EDIT)
 
     if request.method == "POST":
         form = SharedObjectForm(request.POST, instance=obj, user=request.user)
@@ -138,14 +125,18 @@ def account_share(request, pk):
 @login_required
 @require_http_methods(["DELETE"])
 def account_delete(request, pk):
-    account = get_object_or_404(Account, id=pk)
+    account = get_shared_object_or_error(Account, request, id=pk, level=READ)
 
-    if account.owner != request.user and request.user in account.shared_with.all():
+    if account.is_editable_by(request.user):
+        account.delete()
+        messages.success(request, _("Account deleted successfully"))
+    elif account.shared_with.filter(pk=request.user.pk).exists():
+        # Someone else's object shared with us: we can drop our own access
+        # to it, but never delete it.
         account.shared_with.remove(request.user)
         messages.success(request, _("Item no longer shared with you"))
     else:
-        account.delete()
-        messages.success(request, _("Account deleted successfully"))
+        raise PermissionDenied
 
     return HttpResponse(
         status=204,
@@ -159,7 +150,9 @@ def account_delete(request, pk):
 @login_required
 @require_http_methods(["GET"])
 def account_toggle_untracked(request, pk):
-    account = get_object_or_404(Account, id=pk)
+    # Only flips the calling user's own row in untracked_by, so visibility --
+    # not ownership -- is the right bar here.
+    account = get_shared_object_or_error(Account, request, id=pk, level=READ)
     if account.is_untracked_by():
         account.untracked_by.remove(request.user)
         messages.success(request, _("Account is now tracked"))
@@ -179,7 +172,7 @@ def account_toggle_untracked(request, pk):
 @login_required
 @require_http_methods(["GET"])
 def account_take_ownership(request, pk):
-    account = get_object_or_404(Account, id=pk)
+    account = get_shared_object_or_error(Account, request, id=pk, level=EDIT)
 
     if not account.owner:
         account.owner = request.user
diff --git a/app/apps/api/permissions.py b/app/apps/api/permissions.py
index c8e19c5..b3d3995 100644
--- a/app/apps/api/permissions.py
+++ b/app/apps/api/permissions.py
@@ -1,4 +1,8 @@
-from rest_framework.permissions import BasePermission
+from rest_framework.permissions import (
+    SAFE_METHODS,
+    BasePermission,
+    DjangoModelPermissions,
+)
 from django.conf import settings
 
 
@@ -8,3 +12,37 @@ class NotInDemoMode(BasePermission):
             return False
         else:
             return True
+
+
+class SharedObjectPermission(BasePermission):
+    """Object-level ownership check for SharedObject-backed viewsets.
+
+    DjangoModelPermissions is model-level: a user holding ``change_account``
+    may write any object the viewset's queryset returns, and for SharedObject
+    that queryset includes other people's public and shared-with-them objects.
+    Sharing grants read access only, so writes are restricted to the owner
+    here as well.
+
+    Set ``shared_object_via`` on the viewset when the governing SharedObject is
+    reached through a relation (e.g. ``"strategy"`` for a DCA entry).
+    """
+
+    def has_object_permission(self, request, view, obj):
+        if request.method in SAFE_METHODS:
+            return True
+
+        guard = obj
+        via = getattr(view, "shared_object_via", None)
+        for attr in via.split(".") if via else []:
+            guard = getattr(guard, attr)
+
+        return guard.is_editable_by(request.user)
+
+
+#: Default permissions plus the object-level ownership check. Assigning
+#: ``permission_classes`` replaces the defaults, so they are repeated here.
+SHARED_OBJECT_PERMISSIONS = [
+    NotInDemoMode,
+    DjangoModelPermissions,
+    SharedObjectPermission,
+]
diff --git a/app/apps/api/tests/__init__.py b/app/apps/api/tests/__init__.py
index 9ca8b36..d67d54c 100644
--- a/app/apps/api/tests/__init__.py
+++ b/app/apps/api/tests/__init__.py
@@ -3,3 +3,4 @@ from .test_imports import *
 from .test_accounts import *
 from .test_data_isolation import *
 from .test_shared_access import *
+from .test_object_permissions import *
diff --git a/app/apps/api/tests/test_object_permissions.py b/app/apps/api/tests/test_object_permissions.py
new file mode 100644
index 0000000..54d4fb9
--- /dev/null
+++ b/app/apps/api/tests/test_object_permissions.py
@@ -0,0 +1,171 @@
+"""Object-level ownership on the SharedObject API viewsets.
+
+DjangoModelPermissions is model-level: a user holding ``change_account`` could
+write any object the viewset's queryset returned, and for SharedObject that
+queryset includes other people's public and shared-with-them objects. Ordinary
+users hold no model permissions, so this was not reachable for them, but the
+object-level check was missing entirely.
+
+Reads are unaffected -- they stay governed by SharedObjectManager.
+"""
+
+from datetime import date
+from decimal import Decimal
+
+from django.contrib.auth import get_user_model
+from django.contrib.auth.models import Permission
+from django.test import TestCase, override_settings
+from rest_framework import status
+from rest_framework.test import APIClient
+
+from apps.accounts.models import Account
+from apps.currencies.models import Currency
+from apps.dca.models import DCAEntry, DCAStrategy
+from apps.transactions.models import TransactionCategory
+
+
+@override_settings(
+    STORAGES={
+        "default": {"BACKEND": "django.core.files.storage.FileSystemStorage"},
+        "staticfiles": {
+            "BACKEND": "django.contrib.staticfiles.storage.StaticFilesStorage"
+        },
+    },
+    WHITENOISE_AUTOREFRESH=True,
+    DEMO=False,
+)
+class SharedObjectAPIPermissionTests(TestCase):
+    """The attacker here deliberately HOLDS the Django model permissions.
+
+    Without them DjangoModelPermissions already answers 403 and the
+    object-level check is never consulted, so the test would pass whether or
+    not it exists.
+    """
+
+    def setUp(self):
+        User = get_user_model()
+        self.owner = User.objects.create_user(
+            email="owner@test.com", password="testpass123"
+        )
+        self.attacker = User.objects.create_user(
+            email="attacker@test.com", password="testpass123"
+        )
+        self.attacker.user_permissions.set(
+            Permission.objects.filter(
+                codename__in=[
+                    "add_account",
+                    "change_account",
+                    "delete_account",
+                    "add_transactioncategory",
+                    "change_transactioncategory",
+                    "delete_transactioncategory",
+                    "add_dcaentry",
+                    "change_dcaentry",
+                    "delete_dcaentry",
+                ]
+            )
+        )
+        # Permissions are cached on the user instance.
+        self.attacker = User.objects.get(pk=self.attacker.pk)
+
+        self.currency = Currency.objects.create(
+            code="USD", name="US Dollar", decimal_places=2
+        )
+
+        self.api = APIClient()
+        self.api.force_authenticate(user=self.attacker)
+
+    def test_cannot_modify_public_account(self):
+        account = Account.all_objects.create(
+            name="Public account",
+            currency=self.currency,
+            owner=self.owner,
+            visibility="public",
+        )
+
+        response = self.api.patch(
+            f"/api/accounts/{account.id}/", {"name": "HIJACKED"}, format="json"
+        )
+
+        self.assertEqual(response.status_code, status.HTTP_403_FORBIDDEN)
+        account.refresh_from_db()
+        self.assertEqual(account.name, "Public account")
+
+    def test_cannot_delete_account_shared_with_them(self):
+        account = Account.all_objects.create(
+            name="Shared account",
+            currency=self.currency,
+            owner=self.owner,
+            visibility="private",
+        )
+        account.shared_with.add(self.attacker)
+
+        response = self.api.delete(f"/api/accounts/{account.id}/")
+
+        self.assertEqual(response.status_code, status.HTTP_403_FORBIDDEN)
+        self.assertTrue(Account.all_objects.filter(pk=account.pk).exists())
+
+    def test_cannot_delete_public_category(self):
+        category = TransactionCategory.all_objects.create(
+            name="Public category", owner=self.owner, visibility="public"
+        )
+
+        response = self.api.delete(f"/api/categories/{category.id}/")
+
+        self.assertEqual(response.status_code, status.HTTP_403_FORBIDDEN)
+        self.assertTrue(TransactionCategory.all_objects.filter(pk=category.pk).exists())
+
+    def test_cannot_delete_entry_on_public_strategy(self):
+        strategy = DCAStrategy.all_objects.create(
+            name="Public strategy",
+            owner=self.owner,
+            visibility="public",
+            target_currency=self.currency,
+            payment_currency=self.currency,
+        )
+        entry = DCAEntry.objects.create(
+            strategy=strategy,
+            date=date(2025, 1, 1),
+            amount_paid=Decimal("100"),
+            amount_received=Decimal("1"),
+        )
+
+        response = self.api.delete(f"/api/dca/entries/{entry.id}/")
+
+        self.assertEqual(response.status_code, status.HTTP_403_FORBIDDEN)
+        self.assertTrue(DCAEntry.objects.filter(pk=entry.pk).exists())
+
+    def test_reads_of_shared_objects_still_work(self):
+        account = Account.all_objects.create(
+            name="Public account",
+            currency=self.currency,
+            owner=self.owner,
+            visibility="public",
+        )
+
+        response = self.api.get(f"/api/accounts/{account.id}/")
+
+        self.assertEqual(response.status_code, status.HTTP_200_OK)
+
+    def test_owner_can_still_write_their_own_objects(self):
+        owner_client = APIClient()
+        self.owner.user_permissions.set(
+            Permission.objects.filter(codename__in=["change_account", "delete_account"])
+        )
+        owner = get_user_model().objects.get(pk=self.owner.pk)
+        owner_client.force_authenticate(user=owner)
+
+        account = Account.all_objects.create(
+            name="Own account",
+            currency=self.currency,
+            owner=owner,
+            visibility="private",
+        )
+
+        response = owner_client.patch(
+            f"/api/accounts/{account.id}/", {"name": "Renamed"}, format="json"
+        )
+
+        self.assertEqual(response.status_code, status.HTTP_200_OK)
+        account.refresh_from_db()
+        self.assertEqual(account.name, "Renamed")
diff --git a/app/apps/api/views/accounts.py b/app/apps/api/views/accounts.py
index 757001c..f7e44e1 100644
--- a/app/apps/api/views/accounts.py
+++ b/app/apps/api/views/accounts.py
@@ -6,6 +6,7 @@ from rest_framework.response import Response
 
 from apps.accounts.models import AccountGroup, Account
 from apps.accounts.services import get_account_balance
+from apps.api.permissions import SHARED_OBJECT_PERMISSIONS
 from apps.api.serializers import (
     AccountGroupSerializer,
     AccountSerializer,
@@ -16,6 +17,7 @@ from apps.api.serializers import (
 class AccountGroupViewSet(viewsets.ModelViewSet):
     """ViewSet for managing account groups."""
 
+    permission_classes = SHARED_OBJECT_PERMISSIONS
     queryset = AccountGroup.objects.all()
     serializer_class = AccountGroupSerializer
     filterset_fields = {
@@ -40,6 +42,7 @@ class AccountGroupViewSet(viewsets.ModelViewSet):
 class AccountViewSet(viewsets.ModelViewSet):
     """ViewSet for managing accounts."""
 
+    permission_classes = SHARED_OBJECT_PERMISSIONS
     queryset = Account.objects.all()
     serializer_class = AccountSerializer
     filterset_fields = {
diff --git a/app/apps/api/views/dca.py b/app/apps/api/views/dca.py
index 9360682..4fc2e8d 100644
--- a/app/apps/api/views/dca.py
+++ b/app/apps/api/views/dca.py
@@ -2,10 +2,12 @@ from rest_framework import viewsets
 from rest_framework.decorators import action
 from rest_framework.response import Response
 from apps.dca.models import DCAStrategy, DCAEntry
+from apps.api.permissions import SHARED_OBJECT_PERMISSIONS
 from apps.api.serializers import DCAStrategySerializer, DCAEntrySerializer
 
 
 class DCAStrategyViewSet(viewsets.ModelViewSet):
+    permission_classes = SHARED_OBJECT_PERMISSIONS
     queryset = DCAStrategy.objects.all()
     serializer_class = DCAStrategySerializer
     filterset_fields = {
@@ -43,6 +45,8 @@ class DCAStrategyViewSet(viewsets.ModelViewSet):
 
 
 class DCAEntryViewSet(viewsets.ModelViewSet):
+    permission_classes = SHARED_OBJECT_PERMISSIONS
+    shared_object_via = "strategy"
     queryset = DCAEntry.objects.all()
     serializer_class = DCAEntrySerializer
     filterset_fields = {
diff --git a/app/apps/api/views/transactions.py b/app/apps/api/views/transactions.py
index 068ef0d..fdc0003 100644
--- a/app/apps/api/views/transactions.py
+++ b/app/apps/api/views/transactions.py
@@ -19,6 +19,7 @@ from apps.transactions.models import (
     RecurringTransaction,
 )
 from apps.rules.signals import transaction_updated, transaction_created
+from apps.api.permissions import SHARED_OBJECT_PERMISSIONS
 
 
 class TransactionViewSet(viewsets.ModelViewSet):
@@ -68,6 +69,7 @@ class TransactionViewSet(viewsets.ModelViewSet):
 
 
 class TransactionCategoryViewSet(viewsets.ModelViewSet):
+    permission_classes = SHARED_OBJECT_PERMISSIONS
     queryset = TransactionCategory.objects.all()
     serializer_class = TransactionCategorySerializer
     filterset_fields = {
@@ -85,6 +87,7 @@ class TransactionCategoryViewSet(viewsets.ModelViewSet):
 
 
 class TransactionTagViewSet(viewsets.ModelViewSet):
+    permission_classes = SHARED_OBJECT_PERMISSIONS
     queryset = TransactionTag.objects.all()
     serializer_class = TransactionTagSerializer
     filterset_fields = {
@@ -101,6 +104,7 @@ class TransactionTagViewSet(viewsets.ModelViewSet):
 
 
 class TransactionEntityViewSet(viewsets.ModelViewSet):
+    permission_classes = SHARED_OBJECT_PERMISSIONS
     queryset = TransactionEntity.objects.all()
     serializer_class = TransactionEntitySerializer
     filterset_fields = {
diff --git a/app/apps/common/functions/permissions.py b/app/apps/common/functions/permissions.py
new file mode 100644
index 0000000..713c9f9
--- /dev/null
+++ b/app/apps/common/functions/permissions.py
@@ -0,0 +1,58 @@
+from django.core.exceptions import PermissionDenied
+from django.http import Http404
+from django.shortcuts import get_object_or_404
+
+READ = "read"
+EDIT = "edit"
+
+
+def get_shared_object_or_error(klass, request, *, level=EDIT, via=None, **kwargs):
+    """Fetch an object like ``get_object_or_404`` while enforcing access control.
+
+    ``SharedObjectManager`` scopes querysets to what a user may *see*, which is
+    not the same as what they may *change*. Views that resolve an object from a
+    URL id must state which of the two they need, otherwise a shared or public
+    object becomes writable by anyone who can see it.
+
+    ``level`` selects the check applied to the governing ``SharedObject``:
+
+    ``READ``
+        The object must be visible to the user. Denial raises :class:`Http404`
+        so the response does not confirm that the id exists.
+    ``EDIT``
+        The object must be owned by the user. Denial raises
+        :class:`~django.core.exceptions.PermissionDenied` (HTTP 403), which the
+        frontend surfaces as an "Access Denied" dialog. An object the user
+        cannot even see raises :class:`Http404` instead, so 403 never confirms
+        the existence of an object they were not allowed to know about.
+
+    Objects with no owner stay accessible to everyone, preserving the existing
+    behaviour for legacy/unowned objects.
+
+    ``via`` is a dotted path to the ``SharedObject`` that governs access, for
+    models owned through a relation, e.g. a rule action governed by its parent
+    rule::
+
+        get_shared_object_or_error(
+            TransactionRuleAction, request, id=pk, level=EDIT, via="rule"
+        )
+
+    The path is resolved with a plain ``getattr``, so a path that does not
+    resolve raises ``AttributeError`` rather than silently granting access.
+    """
+    obj = get_object_or_404(klass, **kwargs)
+
+    guard = obj
+    for attr in via.split(".") if via else []:
+        guard = getattr(guard, attr)
+
+    if level not in (READ, EDIT):
+        raise ValueError(f"Unknown access level: {level!r}")
+
+    if not guard.is_visible_to(request.user):
+        raise Http404
+
+    if level == EDIT and not guard.is_editable_by(request.user):
+        raise PermissionDenied
+
+    return obj
diff --git a/app/apps/common/models.py b/app/apps/common/models.py
index 10f79c8..f9a876f 100644
--- a/app/apps/common/models.py
+++ b/app/apps/common/models.py
@@ -58,13 +58,35 @@ class SharedObject(models.Model):
             models.Index(fields=["visibility"]),
         ]
 
-    def is_accessible_by(self, user):
-        """Check if a user can access this object"""
-        return (
-            self.visibility == "public"
-            or self.owner == user
-            or (self.visibility == "shared" and user in self.shared_with.all())
-        )
+    # NOTE: these two predicates must stay in sync with the ``Q`` objects built
+    # by ``SharedObjectManager.get_queryset`` above. The manager filters at the
+    # queryset level and these check a single instance, so they cannot share an
+    # implementation; ``SharedObjectPredicateParityTests`` asserts they agree.
+    def is_visible_to(self, user):
+        """Whether ``user`` may read this object.
+
+        Mirrors ``SharedObjectManager``: public objects, objects with no owner,
+        the owner's own objects, and objects explicitly shared with the user.
+        """
+        if self.owner is None or self.visibility == "public":
+            return True
+
+        if not user or not user.is_authenticated:
+            return False
+
+        return self.owner_id == user.pk or self.shared_with.filter(pk=user.pk).exists()
+
+    def is_editable_by(self, user):
+        """Whether ``user`` may mutate this object.
+
+        Sharing grants read access only; mutation stays with the owner. Objects
+        with no owner remain editable by everyone, preserving the behaviour of
+        legacy/unowned objects.
+        """
+        if self.owner is None:
+            return True
+
+        return bool(user and user.is_authenticated and self.owner_id == user.pk)
 
     def save(self, *args, **kwargs):
         if not self.pk and not self.owner:
diff --git a/app/apps/common/tests/test_permissions.py b/app/apps/common/tests/test_permissions.py
new file mode 100644
index 0000000..a803bed
--- /dev/null
+++ b/app/apps/common/tests/test_permissions.py
@@ -0,0 +1,269 @@
+from django.contrib.auth import get_user_model
+from django.contrib.auth.models import AnonymousUser
+from django.core.exceptions import PermissionDenied
+from django.http import Http404
+from django.test import RequestFactory, TestCase, override_settings
+
+from apps.common.functions.permissions import (
+    EDIT,
+    READ,
+    get_shared_object_or_error,
+)
+from apps.common.middleware.thread_local import delete_current_user, write_current_user
+from apps.rules.models import TransactionRule, TransactionRuleAction
+
+
+@override_settings(
+    STORAGES={
+        "default": {"BACKEND": "django.core.files.storage.FileSystemStorage"},
+        "staticfiles": {
+            "BACKEND": "django.contrib.staticfiles.storage.StaticFilesStorage"
+        },
+    },
+    WHITENOISE_AUTOREFRESH=True,
+)
+class SharedObjectPredicateTests(TestCase):
+    """Unit tests for is_visible_to / is_editable_by on SharedObject."""
+
+    def setUp(self):
+        User = get_user_model()
+        self.owner = User.objects.create_user(
+            email="owner@test.com", password="testpass123"
+        )
+        self.shared_user = User.objects.create_user(
+            email="shared@test.com", password="testpass123"
+        )
+        self.stranger = User.objects.create_user(
+            email="stranger@test.com", password="testpass123"
+        )
+
+    def _rule(self, **kwargs):
+        kwargs.setdefault("name", "Rule")
+        kwargs.setdefault("trigger", "True")
+        return TransactionRule.all_objects.create(**kwargs)
+
+    def test_owner_can_read_and_edit_own_private_rule(self):
+        rule = self._rule(owner=self.owner, visibility="private")
+
+        self.assertTrue(rule.is_visible_to(self.owner))
+        self.assertTrue(rule.is_editable_by(self.owner))
+
+    def test_private_rule_is_invisible_to_stranger(self):
+        rule = self._rule(owner=self.owner, visibility="private")
+
+        self.assertFalse(rule.is_visible_to(self.stranger))
+        self.assertFalse(rule.is_editable_by(self.stranger))
+
+    def test_shared_rule_is_readable_but_not_editable(self):
+        rule = self._rule(owner=self.owner, visibility="private")
+        rule.shared_with.add(self.shared_user)
+
+        self.assertTrue(rule.is_visible_to(self.shared_user))
+        self.assertFalse(rule.is_editable_by(self.shared_user))
+
+    def test_public_rule_is_readable_but_not_editable(self):
+        rule = self._rule(owner=self.owner, visibility="public")
+
+        self.assertTrue(rule.is_visible_to(self.stranger))
+        self.assertFalse(rule.is_editable_by(self.stranger))
+
+    def test_unowned_rule_stays_readable_and_editable_by_everyone(self):
+        rule = self._rule(owner=None, visibility="private")
+
+        self.assertTrue(rule.is_visible_to(self.stranger))
+        self.assertTrue(rule.is_editable_by(self.stranger))
+
+    def test_anonymous_user_gets_no_access_to_owned_rules(self):
+        rule = self._rule(owner=self.owner, visibility="private")
+
+        self.assertFalse(rule.is_visible_to(AnonymousUser()))
+        self.assertFalse(rule.is_editable_by(AnonymousUser()))
+
+
+@override_settings(
+    STORAGES={
+        "default": {"BACKEND": "django.core.files.storage.FileSystemStorage"},
+        "staticfiles": {
+            "BACKEND": "django.contrib.staticfiles.storage.StaticFilesStorage"
+        },
+    },
+    WHITENOISE_AUTOREFRESH=True,
+)
+class SharedObjectPredicateParityTests(TestCase):
+    """is_visible_to must agree with what SharedObjectManager returns.
+
+    The manager filters at the queryset level and the predicate checks a single
+    instance, so the two cannot share an implementation. This asserts they do
+    not drift apart.
+    """
+
+    def setUp(self):
+        User = get_user_model()
+        self.owner = User.objects.create_user(
+            email="owner@test.com", password="testpass123"
+        )
+        self.other = User.objects.create_user(
+            email="other@test.com", password="testpass123"
+        )
+        self.addCleanup(self._clear_current_user)
+
+    def _clear_current_user(self):
+        try:
+            delete_current_user()
+        except AttributeError:
+            pass
+
+    def test_manager_and_predicate_agree_over_every_combination(self):
+        combinations = []
+        for owner in (self.owner, self.other, None):
+            for visibility in ("private", "public"):
+                for shared in (True, False):
+                    rule = TransactionRule.all_objects.create(
+                        name=f"{owner}-{visibility}-{shared}",
+                        trigger="True",
+                        owner=owner,
+                        visibility=visibility,
+                    )
+                    if shared:
+                        rule.shared_with.add(self.owner)
+                    combinations.append(rule)
+
+        write_current_user(self.owner)
+        visible_ids = set(TransactionRule.objects.values_list("id", flat=True))
+
+        for rule in combinations:
+            with self.subTest(rule=rule.name):
+                self.assertEqual(
+                    rule.id in visible_ids,
+                    rule.is_visible_to(self.owner),
+                    f"manager and is_visible_to disagree for {rule.name}",
+                )
+
+
+@override_settings(
+    STORAGES={
+        "default": {"BACKEND": "django.core.files.storage.FileSystemStorage"},
+        "staticfiles": {
+            "BACKEND": "django.contrib.staticfiles.storage.StaticFilesStorage"
+        },
+    },
+    WHITENOISE_AUTOREFRESH=True,
+)
+class GetSharedObjectOrErrorTests(TestCase):
+    def setUp(self):
+        User = get_user_model()
+        self.owner = User.objects.create_user(
+            email="owner@test.com", password="testpass123"
+        )
+        self.stranger = User.objects.create_user(
+            email="stranger@test.com", password="testpass123"
+        )
+        self.factory = RequestFactory()
+
+        self.public_rule = TransactionRule.all_objects.create(
+            name="Public", trigger="True", owner=self.owner, visibility="public"
+        )
+        self.private_rule = TransactionRule.all_objects.create(
+            name="Private", trigger="True", owner=self.owner, visibility="private"
+        )
+        self.public_action = TransactionRuleAction.objects.create(
+            rule=self.public_rule, field="notes", value="x"
+        )
+        self.private_action = TransactionRuleAction.objects.create(
+            rule=self.private_rule, field="notes", value="x"
+        )
+
+        self.addCleanup(self._clear_current_user)
+
+    def _clear_current_user(self):
+        try:
+            delete_current_user()
+        except AttributeError:
+            pass
+
+    def _request(self, user):
+        request = self.factory.get("/")
+        request.user = user
+        write_current_user(user)
+        return request
+
+    def test_read_allows_visible_object(self):
+        request = self._request(self.stranger)
+
+        rule = get_shared_object_or_error(
+            TransactionRule, request, id=self.public_rule.id, level=READ
+        )
+
+        self.assertEqual(rule, self.public_rule)
+
+    def test_edit_denies_visible_but_unowned_object_with_403(self):
+        request = self._request(self.stranger)
+
+        with self.assertRaises(PermissionDenied):
+            get_shared_object_or_error(
+                TransactionRule, request, id=self.public_rule.id, level=EDIT
+            )
+
+    def test_edit_allows_owner(self):
+        request = self._request(self.owner)
+
+        rule = get_shared_object_or_error(
+            TransactionRule, request, id=self.public_rule.id, level=EDIT
+        )
+
+        self.assertEqual(rule, self.public_rule)
+
+    def test_invisible_object_raises_404_not_403(self):
+        """403 must not confirm the existence of an object the user cannot see."""
+        request = self._request(self.stranger)
+
+        with self.assertRaises(Http404):
+            get_shared_object_or_error(
+                TransactionRule, request, id=self.private_rule.id, level=EDIT
+            )
+
+    def test_via_traverses_to_the_governing_object(self):
+        request = self._request(self.stranger)
+
+        with self.assertRaises(PermissionDenied):
+            get_shared_object_or_error(
+                TransactionRuleAction,
+                request,
+                id=self.public_action.id,
+                level=EDIT,
+                via="rule",
+            )
+
+    def test_via_hides_children_of_invisible_parents_behind_404(self):
+        """TransactionRuleAction has an unscoped manager, so the id is reachable."""
+        request = self._request(self.stranger)
+
+        with self.assertRaises(Http404):
+            get_shared_object_or_error(
+                TransactionRuleAction,
+                request,
+                id=self.private_action.id,
+                level=EDIT,
+                via="rule",
+            )
+
+    def test_unresolvable_via_path_fails_closed(self):
+        """A typo'd path must raise, never silently grant access."""
+        request = self._request(self.stranger)
+
+        with self.assertRaises(AttributeError):
+            get_shared_object_or_error(
+                TransactionRuleAction,
+                request,
+                id=self.public_action.id,
+                level=EDIT,
+                via="rulee",
+            )
+
+    def test_unknown_level_is_rejected(self):
+        request = self._request(self.owner)
+
+        with self.assertRaises(ValueError):
+            get_shared_object_or_error(
+                TransactionRule, request, id=self.public_rule.id, level="write"
+            )
diff --git a/app/apps/common/tests/test_shared_object_deletion.py b/app/apps/common/tests/test_shared_object_deletion.py
new file mode 100644
index 0000000..a7bbf70
--- /dev/null
+++ b/app/apps/common/tests/test_shared_object_deletion.py
@@ -0,0 +1,161 @@
+"""Delete semantics for every SharedObject-backed model.
+
+Each of these views carried the same inverted condition::
+
+    if obj.owner != request.user and request.user in obj.shared_with.all():
+        obj.shared_with.remove(request.user)   # unshare
+    else:
+        obj.delete()                           # <- public objects landed here
+
+so an object its owner had made public could be destroyed by any authenticated
+user. The rule is: the owner deletes, a shared user revokes only their own
+access, and nobody else may do either.
+"""
+
+from django.contrib.auth import get_user_model
+from django.test import TestCase, override_settings
+from django.urls import reverse
+
+from apps.accounts.models import Account, AccountGroup
+from apps.currencies.models import Currency
+from apps.dca.models import DCAStrategy
+from apps.rules.models import TransactionRule
+from apps.transactions.models import (
+    TransactionCategory,
+    TransactionEntity,
+    TransactionTag,
+)
+
+HTMX = {"HTTP_HX_REQUEST": "true"}
+
+
+@override_settings(
+    STORAGES={
+        "default": {"BACKEND": "django.core.files.storage.FileSystemStorage"},
+        "staticfiles": {
+            "BACKEND": "django.contrib.staticfiles.storage.StaticFilesStorage"
+        },
+    },
+    WHITENOISE_AUTOREFRESH=True,
+    DEMO=False,
+)
+class SharedObjectDeletionTests(TestCase):
+    def setUp(self):
+        User = get_user_model()
+        self.owner = User.objects.create_user(
+            email="owner@test.com", password="testpass123"
+        )
+        self.shared_user = User.objects.create_user(
+            email="shared@test.com", password="testpass123"
+        )
+        self.stranger = User.objects.create_user(
+            email="stranger@test.com", password="testpass123"
+        )
+        self.currency = Currency.objects.create(
+            code="USD", name="US Dollar", decimal_places=2
+        )
+
+    def cases(self):
+        """(label, model, delete url name, url kwarg, extra create kwargs)."""
+        return [
+            (
+                "account",
+                Account,
+                "account_delete",
+                "pk",
+                {"currency": self.currency},
+            ),
+            ("account group", AccountGroup, "account_group_delete", "pk", {}),
+            (
+                "category",
+                TransactionCategory,
+                "category_delete",
+                "category_id",
+                {},
+            ),
+            ("tag", TransactionTag, "tag_delete", "tag_id", {}),
+            ("entity", TransactionEntity, "entity_delete", "entity_id", {}),
+            (
+                "rule",
+                TransactionRule,
+                "transaction_rule_delete",
+                "transaction_rule_id",
+                {"trigger": "True"},
+            ),
+            (
+                "dca strategy",
+                DCAStrategy,
+                "dca_strategy_delete",
+                "strategy_id",
+                {
+                    "target_currency": self.currency,
+                    "payment_currency": self.currency,
+                },
+            ),
+        ]
+
+    def make(self, model, visibility, extra):
+        return model.all_objects.create(
+            name="Target", owner=self.owner, visibility=visibility, **extra
+        )
+
+    def delete(self, url_name, kwarg, obj):
+        return self.client.delete(reverse(url_name, kwargs={kwarg: obj.id}), **HTMX)
+
+    def test_stranger_cannot_delete_a_public_object(self):
+        for label, model, url_name, kwarg, extra in self.cases():
+            with self.subTest(model=label):
+                obj = self.make(model, "public", extra)
+                self.client.force_login(self.stranger)
+
+                response = self.delete(url_name, kwarg, obj)
+
+                self.assertEqual(response.status_code, 403, label)
+                self.assertTrue(
+                    model.all_objects.filter(pk=obj.pk).exists(),
+                    f"{label} was deleted by a non-owner",
+                )
+
+    def test_shared_user_deleting_only_revokes_their_own_access(self):
+        for label, model, url_name, kwarg, extra in self.cases():
+            with self.subTest(model=label):
+                obj = self.make(model, "private", extra)
+                obj.shared_with.add(self.shared_user)
+                self.client.force_login(self.shared_user)
+
+                response = self.delete(url_name, kwarg, obj)
+
+                self.assertEqual(response.status_code, 204, label)
+                self.assertTrue(
+                    model.all_objects.filter(pk=obj.pk).exists(),
+                    f"{label} was deleted by a shared user",
+                )
+                self.assertNotIn(self.shared_user, obj.shared_with.all())
+
+    def test_owner_can_delete(self):
+        for label, model, url_name, kwarg, extra in self.cases():
+            with self.subTest(model=label):
+                obj = self.make(model, "private", extra)
+                self.client.force_login(self.owner)
+
+                response = self.delete(url_name, kwarg, obj)
+
+                self.assertEqual(response.status_code, 204, label)
+                self.assertFalse(
+                    model.all_objects.filter(pk=obj.pk).exists(),
+                    f"{label} was not deleted by its owner",
+                )
+
+    def test_unowned_objects_stay_deletable(self):
+        """Legacy objects with no owner are editable by everyone by design."""
+        for label, model, url_name, kwarg, extra in self.cases():
+            with self.subTest(model=label):
+                obj = model.all_objects.create(
+                    name="Legacy", owner=None, visibility="private", **extra
+                )
+                self.client.force_login(self.stranger)
+
+                response = self.delete(url_name, kwarg, obj)
+
+                self.assertEqual(response.status_code, 204, label)
+                self.assertFalse(model.all_objects.filter(pk=obj.pk).exists(), label)
diff --git a/app/apps/dca/tests.py b/app/apps/dca/tests.py
deleted file mode 100644
index 7ce503c..0000000
--- a/app/apps/dca/tests.py
+++ /dev/null
@@ -1,3 +0,0 @@
-from django.test import TestCase
-
-# Create your tests here.
diff --git a/app/apps/dca/tests/__init__.py b/app/apps/dca/tests/__init__.py
new file mode 100644
index 0000000..e69de29
diff --git a/app/apps/dca/tests/test_view_permissions.py b/app/apps/dca/tests/test_view_permissions.py
new file mode 100644
index 0000000..83f0d49
--- /dev/null
+++ b/app/apps/dca/tests/test_view_permissions.py
@@ -0,0 +1,248 @@
+"""Object-level authorization tests for the DCA views.
+
+DCAEntry has an unscoped default manager, so filtering an entry by
+``strategy__id`` alone reached entries belonging to strategies the caller could
+not see at all -- a strictly wider hole than the one reported for rules in
+GHSA-83g9-vjqf-2j5q, since it needs no public or shared strategy.
+"""
+
+from datetime import date
+from decimal import Decimal
+
+from django.contrib.auth import get_user_model
+from django.test import TestCase, override_settings
+from django.urls import reverse
+
+from apps.currencies.models import Currency
+from apps.dca.models import DCAEntry, DCAStrategy
+
+HTMX = {"HTTP_HX_REQUEST": "true"}
+
+
+@override_settings(
+    STORAGES={
+        "default": {"BACKEND": "django.core.files.storage.FileSystemStorage"},
+        "staticfiles": {
+            "BACKEND": "django.contrib.staticfiles.storage.StaticFilesStorage"
+        },
+    },
+    WHITENOISE_AUTOREFRESH=True,
+    DEMO=False,
+)
+class DCAObjectPermissionTests(TestCase):
+    def setUp(self):
+        User = get_user_model()
+        self.owner = User.objects.create_user(
+            email="owner@test.com", password="testpass123"
+        )
+        self.shared_user = User.objects.create_user(
+            email="shared@test.com", password="testpass123"
+        )
+        self.stranger = User.objects.create_user(
+            email="stranger@test.com", password="testpass123"
+        )
+        self.currency = Currency.objects.create(
+            code="USD", name="US Dollar", decimal_places=2
+        )
+
+        self.private_strategy = self._strategy("Private", visibility="private")
+        self.public_strategy = self._strategy("Public", visibility="public")
+        self.shared_strategy = self._strategy("Shared", visibility="private")
+        self.shared_strategy.shared_with.add(self.shared_user)
+
+        self.private_entry = self._entry(self.private_strategy)
+        self.public_entry = self._entry(self.public_strategy)
+        self.shared_entry = self._entry(self.shared_strategy)
+
+    def _strategy(self, name, visibility):
+        return DCAStrategy.all_objects.create(
+            name=name,
+            owner=self.owner,
+            visibility=visibility,
+            target_currency=self.currency,
+            payment_currency=self.currency,
+        )
+
+    def _entry(self, strategy):
+        return DCAEntry.objects.create(
+            strategy=strategy,
+            date=date(2025, 1, 1),
+            amount_paid=Decimal("100"),
+            amount_received=Decimal("1"),
+        )
+
+    # ------------------------------------------------------------------
+    # entries on an invisible strategy: must not even confirm they exist
+    # ------------------------------------------------------------------
+    def test_stranger_cannot_delete_entry_on_private_strategy(self):
+        self.client.force_login(self.stranger)
+
+        response = self.client.delete(
+            reverse(
+                "dca_entry_delete",
+                kwargs={
+                    "strategy_id": self.private_strategy.id,
+                    "entry_id": self.private_entry.id,
+                },
+            ),
+            **HTMX,
+        )
+
+        self.assertEqual(response.status_code, 404)
+        self.assertTrue(DCAEntry.objects.filter(pk=self.private_entry.pk).exists())
+
+    def test_stranger_cannot_open_entry_edit_form_on_private_strategy(self):
+        self.client.force_login(self.stranger)
+
+        response = self.client.get(
+            reverse(
+                "dca_entry_edit",
+                kwargs={
+                    "strategy_id": self.private_strategy.id,
+                    "entry_id": self.private_entry.id,
+                },
+            ),
+            **HTMX,
+        )
+
+        self.assertEqual(response.status_code, 404)
+
+    def test_stranger_cannot_edit_entry_on_private_strategy(self):
+        self.client.force_login(self.stranger)
+
+        response = self.client.post(
+            reverse(
+                "dca_entry_edit",
+                kwargs={
+                    "strategy_id": self.private_strategy.id,
+                    "entry_id": self.private_entry.id,
+                },
+            ),
+            data={
+                "date": "2030-01-01",
+                "amount_paid": "999",
+                "amount_received": "999",
+            },
+            **HTMX,
+        )
+
+        self.assertEqual(response.status_code, 404)
+        self.private_entry.refresh_from_db()
+        self.assertEqual(self.private_entry.amount_paid, Decimal("100"))
+
+    # ------------------------------------------------------------------
+    # entries on a visible-but-unowned strategy: 403, not 404
+    # ------------------------------------------------------------------
+    def test_stranger_cannot_delete_entry_on_public_strategy(self):
+        self.client.force_login(self.stranger)
+
+        response = self.client.delete(
+            reverse(
+                "dca_entry_delete",
+                kwargs={
+                    "strategy_id": self.public_strategy.id,
+                    "entry_id": self.public_entry.id,
+                },
+            ),
+            **HTMX,
+        )
+
+        self.assertEqual(response.status_code, 403)
+        self.assertTrue(DCAEntry.objects.filter(pk=self.public_entry.pk).exists())
+
+    def test_shared_user_cannot_add_entry_to_shared_strategy(self):
+        self.client.force_login(self.shared_user)
+
+        response = self.client.post(
+            reverse("dca_entry_add", kwargs={"strategy_id": self.shared_strategy.id}),
+            data={
+                "date": "2030-01-01",
+                "amount_paid": "5",
+                "amount_received": "5",
+            },
+            **HTMX,
+        )
+
+        self.assertEqual(response.status_code, 403)
+        self.assertEqual(self.shared_strategy.entries.count(), 1)
+
+    def test_owner_can_still_manage_own_entries(self):
+        self.client.force_login(self.owner)
+
+        response = self.client.delete(
+            reverse(
+                "dca_entry_delete",
+                kwargs={
+                    "strategy_id": self.private_strategy.id,
+                    "entry_id": self.private_entry.id,
+                },
+            ),
+            **HTMX,
+        )
+
+        self.assertEqual(response.status_code, 204)
+        self.assertFalse(DCAEntry.objects.filter(pk=self.private_entry.pk).exists())
+
+    # ------------------------------------------------------------------
+    # reads stay open to shared users
+    # ------------------------------------------------------------------
+    def test_shared_user_can_still_view_strategy_detail(self):
+        self.client.force_login(self.shared_user)
+
+        response = self.client.get(
+            reverse(
+                "dca_strategy_detail", kwargs={"strategy_id": self.shared_strategy.id}
+            ),
+            **HTMX,
+        )
+
+        self.assertEqual(response.status_code, 200)
+
+    # ------------------------------------------------------------------
+    # strategy delete: the same inversion the rules views had
+    # ------------------------------------------------------------------
+    def test_stranger_cannot_delete_public_strategy(self):
+        self.client.force_login(self.stranger)
+
+        response = self.client.delete(
+            reverse(
+                "dca_strategy_delete", kwargs={"strategy_id": self.public_strategy.id}
+            ),
+            **HTMX,
+        )
+
+        self.assertEqual(response.status_code, 403)
+        self.assertTrue(
+            DCAStrategy.all_objects.filter(pk=self.public_strategy.pk).exists()
+        )
+
+    def test_shared_user_deleting_only_revokes_their_own_access(self):
+        self.client.force_login(self.shared_user)
+
+        response = self.client.delete(
+            reverse(
+                "dca_strategy_delete", kwargs={"strategy_id": self.shared_strategy.id}
+            ),
+            **HTMX,
+        )
+
+        self.assertEqual(response.status_code, 204)
+        self.assertTrue(
+            DCAStrategy.all_objects.filter(pk=self.shared_strategy.pk).exists()
+        )
+        self.assertNotIn(self.shared_user, self.shared_strategy.shared_with.all())
+
+    def test_owner_can_delete_own_strategy(self):
+        self.client.force_login(self.owner)
+
+        response = self.client.delete(
+            reverse(
+                "dca_strategy_delete", kwargs={"strategy_id": self.public_strategy.id}
+            ),
+            **HTMX,
+        )
+
+        self.assertEqual(response.status_code, 204)
+        self.assertFalse(
+            DCAStrategy.all_objects.filter(pk=self.public_strategy.pk).exists()
+        )
diff --git a/app/apps/dca/views.py b/app/apps/dca/views.py
index 62de916..c27979f 100644
--- a/app/apps/dca/views.py
+++ b/app/apps/dca/views.py
@@ -1,13 +1,19 @@
 from django.contrib import messages
 from django.contrib.auth.decorators import login_required
+from django.core.exceptions import PermissionDenied
 from django.db.models import Sum, Avg
 from django.db.models.functions import TruncMonth
 from django.http import HttpResponse
-from django.shortcuts import render, get_object_or_404
+from django.shortcuts import render
 from django.utils.translation import gettext_lazy as _
 from django.views.decorators.http import require_http_methods
 
 from apps.common.decorators.htmx import only_htmx
+from apps.common.functions.permissions import (
+    EDIT,
+    READ,
+    get_shared_object_or_error,
+)
 from apps.dca.forms import DCAEntryForm, DCAStrategyForm
 from apps.dca.models import DCAStrategy, DCAEntry
 from apps.common.models import SharedObject
@@ -56,17 +62,9 @@ def strategy_add(request):
 @only_htmx
 @login_required
 def strategy_edit(request, strategy_id):
-    dca_strategy = get_object_or_404(DCAStrategy, id=strategy_id)
-
-    if dca_strategy.owner and dca_strategy.owner != request.user:
-        messages.error(request, _("Only the owner can edit this"))
-
-        return HttpResponse(
-            status=204,
-            headers={
-                "HX-Trigger": "updated, hide_offcanvas",
-            },
-        )
+    dca_strategy = get_shared_object_or_error(
+        DCAStrategy, request, id=strategy_id, level=EDIT
+    )
 
     if request.method == "POST":
         form = DCAStrategyForm(request.POST, instance=dca_strategy)
@@ -94,17 +92,20 @@ def strategy_edit(request, strategy_id):
 @login_required
 @require_http_methods(["DELETE"])
 def strategy_delete(request, strategy_id):
-    dca_strategy = get_object_or_404(DCAStrategy, id=strategy_id)
+    dca_strategy = get_shared_object_or_error(
+        DCAStrategy, request, id=strategy_id, level=READ
+    )
 
-    if (
-        dca_strategy.owner != request.user
-        and request.user in dca_strategy.shared_with.all()
-    ):
+    if dca_strategy.is_editable_by(request.user):
+        dca_strategy.delete()
+        messages.success(request, _("DCA strategy deleted successfully"))
+    elif dca_strategy.shared_with.filter(pk=request.user.pk).exists():
+        # Someone else's object shared with us: we can drop our own access
+        # to it, but never delete it.
         dca_strategy.shared_with.remove(request.user)
         messages.success(request, _("Item no longer shared with you"))
     else:
-        dca_strategy.delete()
-        messages.success(request, _("DCA strategy deleted successfully"))
+        raise PermissionDenied
 
     return HttpResponse(
         status=204,
@@ -118,7 +119,9 @@ def strategy_delete(request, strategy_id):
 @login_required
 @require_http_methods(["GET"])
 def strategy_take_ownership(request, strategy_id):
-    dca_strategy = get_object_or_404(DCAStrategy, id=strategy_id)
+    dca_strategy = get_shared_object_or_error(
+        DCAStrategy, request, id=strategy_id, level=EDIT
+    )
 
     if not dca_strategy.owner:
         dca_strategy.owner = request.user
@@ -139,17 +142,7 @@ def strategy_take_ownership(request, strategy_id):
 @login_required
 @require_http_methods(["GET", "POST"])
 def strategy_share(request, pk):
-    obj = get_object_or_404(DCAStrategy, id=pk)
-
-    if obj.owner and obj.owner != request.user:
-        messages.error(request, _("Only the owner can edit this"))
-
-        return HttpResponse(
-            status=204,
-            headers={
-                "HX-Trigger": "updated, hide_offcanvas",
-            },
-        )
+    obj = get_shared_object_or_error(DCAStrategy, request, id=pk, level=EDIT)
 
     if request.method == "POST":
         form = SharedObjectForm(request.POST, instance=obj, user=request.user)
@@ -175,7 +168,9 @@ def strategy_share(request, pk):
 
 @login_required
 def strategy_detail_index(request, strategy_id):
-    strategy = get_object_or_404(DCAStrategy, id=strategy_id)
+    strategy = get_shared_object_or_error(
+        DCAStrategy, request, id=strategy_id, level=READ
+    )
 
     return render(
         request,
@@ -187,7 +182,9 @@ def strategy_detail_index(request, strategy_id):
 @only_htmx
 @login_required
 def strategy_detail(request, strategy_id):
-    strategy = get_object_or_404(DCAStrategy, id=strategy_id)
+    strategy = get_shared_object_or_error(
+        DCAStrategy, request, id=strategy_id, level=READ
+    )
     entries = strategy.entries.all()
 
     # Calculate monthly aggregates
@@ -229,7 +226,9 @@ def strategy_detail(request, strategy_id):
 @only_htmx
 @login_required
 def strategy_entry_add(request, strategy_id):
-    strategy = get_object_or_404(DCAStrategy, id=strategy_id)
+    strategy = get_shared_object_or_error(
+        DCAStrategy, request, id=strategy_id, level=EDIT
+    )
     if request.method == "POST":
         form = DCAEntryForm(request.POST, strategy=strategy)
         if form.is_valid():
@@ -255,7 +254,14 @@ def strategy_entry_add(request, strategy_id):
 @only_htmx
 @login_required
 def strategy_entry_edit(request, strategy_id, entry_id):
-    dca_entry = get_object_or_404(DCAEntry, id=entry_id, strategy__id=strategy_id)
+    dca_entry = get_shared_object_or_error(
+        DCAEntry,
+        request,
+        id=entry_id,
+        strategy__id=strategy_id,
+        level=EDIT,
+        via="strategy",
+    )
 
     if request.method == "POST":
         form = DCAEntryForm(request.POST, instance=dca_entry)
@@ -283,7 +289,14 @@ def strategy_entry_edit(request, strategy_id, entry_id):
 @login_required
 @require_http_methods(["DELETE"])
 def strategy_entry_delete(request, entry_id, strategy_id):
-    dca_entry = get_object_or_404(DCAEntry, id=entry_id, strategy__id=strategy_id)
+    dca_entry = get_shared_object_or_error(
+        DCAEntry,
+        request,
+        id=entry_id,
+        strategy__id=strategy_id,
+        level=EDIT,
+        via="strategy",
+    )
 
     dca_entry.delete()
 
diff --git a/app/apps/rules/tests/test_view_permissions.py b/app/apps/rules/tests/test_view_permissions.py
new file mode 100644
index 0000000..9506dd0
--- /dev/null
+++ b/app/apps/rules/tests/test_view_permissions.py
@@ -0,0 +1,473 @@
+"""Object-level authorization tests for the transaction-rule endpoints.
+
+Regression coverage for GHSA-83g9-vjqf-2j5q: SharedObjectManager scopes rules to
+what a user may *see*, which included other people's public and shared-with-them
+rules. Several mutating endpoints treated that visibility as permission to write.
+"""
+
+from django.contrib.auth import get_user_model
+from django.test import TestCase, override_settings
+from django.urls import reverse
+
+from apps.rules.models import (
+    TransactionRule,
+    TransactionRuleAction,
+    UpdateOrCreateTransactionRuleAction,
+)
+
+HTMX = {"HTTP_HX_REQUEST": "true"}
+
+
+@override_settings(
+    STORAGES={
+        "default": {"BACKEND": "django.core.files.storage.FileSystemStorage"},
+        "staticfiles": {
+            "BACKEND": "django.contrib.staticfiles.storage.StaticFilesStorage"
+        },
+    },
+    WHITENOISE_AUTOREFRESH=True,
+    DEMO=False,
+)
+class TransactionRuleObjectPermissionTests(TestCase):
+    def setUp(self):
+        User = get_user_model()
+        self.owner = User.objects.create_user(
+            email="owner@test.com", password="testpass123"
+        )
+        self.shared_user = User.objects.create_user(
+            email="shared@test.com", password="testpass123"
+        )
+        self.stranger = User.objects.create_user(
+            email="stranger@test.com", password="testpass123"
+        )
+
+        # Public: visible to everyone through SharedObjectManager.
+        self.public_rule = TransactionRule.all_objects.create(
+            name="Public rule",
+            trigger="True",
+            owner=self.owner,
+            visibility="public",
+            active=True,
+        )
+        # Private but shared with shared_user: visible to them, not to stranger.
+        self.shared_rule = TransactionRule.all_objects.create(
+            name="Shared rule",
+            trigger="True",
+            owner=self.owner,
+            visibility="private",
+            active=True,
+        )
+        self.shared_rule.shared_with.add(self.shared_user)
+        # Private and unshared: invisible to everyone but the owner.
+        self.private_rule = TransactionRule.all_objects.create(
+            name="Private rule",
+            trigger="True",
+            owner=self.owner,
+            visibility="private",
+            active=True,
+        )
+
+        self.public_action = TransactionRuleAction.objects.create(
+            rule=self.public_rule, field="notes", value="owned by owner"
+        )
+        self.private_action = TransactionRuleAction.objects.create(
+            rule=self.private_rule, field="notes", value="owned by owner"
+        )
+        self.public_linked_action = UpdateOrCreateTransactionRuleAction.objects.create(
+            rule=self.public_rule
+        )
+        self.private_linked_action = UpdateOrCreateTransactionRuleAction.objects.create(
+            rule=self.private_rule
+        )
+
+    def login(self, user):
+        self.client.force_login(user)
+
+    # ------------------------------------------------------------------
+    # toggle-active
+    # ------------------------------------------------------------------
+    def test_stranger_cannot_toggle_public_rule(self):
+        self.login(self.stranger)
+
+        response = self.client.get(
+            reverse(
+                "transaction_rule_toggle_activity",
+                kwargs={"transaction_rule_id": self.public_rule.id},
+            ),
+            **HTMX,
+        )
+
+        self.assertEqual(response.status_code, 403)
+        self.public_rule.refresh_from_db()
+        self.assertTrue(self.public_rule.active)
+
+    def test_shared_user_cannot_toggle_shared_rule(self):
+        self.login(self.shared_user)
+
+        response = self.client.get(
+            reverse(
+                "transaction_rule_toggle_activity",
+                kwargs={"transaction_rule_id": self.shared_rule.id},
+            ),
+            **HTMX,
+        )
+
+        self.assertEqual(response.status_code, 403)
+        self.shared_rule.refresh_from_db()
+        self.assertTrue(self.shared_rule.active)
+
+    def test_owner_can_toggle_own_rule(self):
+        self.login(self.owner)
+
+        response = self.client.get(
+            reverse(
+                "transaction_rule_toggle_activity",
+                kwargs={"transaction_rule_id": self.public_rule.id},
+            ),
+            **HTMX,
+        )
+
+        self.assertEqual(response.status_code, 204)
+        self.public_rule.refresh_from_db()
+        self.assertFalse(self.public_rule.active)
+
+    # ------------------------------------------------------------------
+    # rule actions
+    # ------------------------------------------------------------------
+    def test_stranger_cannot_add_action_to_public_rule(self):
+        self.login(self.stranger)
+
+        response = self.client.post(
+            reverse(
+                "transaction_rule_action_add",
+                kwargs={"transaction_rule_id": self.public_rule.id},
+            ),
+            data={"field": "notes", "value": "injected", "order": 0},
+            **HTMX,
+        )
+
+        self.assertEqual(response.status_code, 403)
+        self.assertFalse(
+            TransactionRuleAction.objects.filter(value="injected").exists()
+        )
+
+    def test_stranger_cannot_edit_action_on_public_rule(self):
+        self.login(self.stranger)
+
+        response = self.client.post(
+            reverse(
+                "transaction_rule_action_edit",
+                kwargs={"transaction_rule_action_id": self.public_action.id},
+            ),
+            data={"field": "notes", "value": "rewritten", "order": 0},
+            **HTMX,
+        )
+
+        self.assertEqual(response.status_code, 403)
+        self.public_action.refresh_from_db()
+        self.assertEqual(self.public_action.value, "owned by owner")
+
+    def test_stranger_cannot_delete_action_on_public_rule(self):
+        self.login(self.stranger)
+
+        response = self.client.delete(
+            reverse(
+                "transaction_rule_action_delete",
+                kwargs={"transaction_rule_action_id": self.public_action.id},
+            ),
+            **HTMX,
+        )
+
+        self.assertEqual(response.status_code, 403)
+        self.assertTrue(
+            TransactionRuleAction.objects.filter(pk=self.public_action.pk).exists()
+        )
+
+    def test_stranger_cannot_delete_action_on_invisible_rule(self):
+        """The child manager is unscoped, so the id is reachable by guessing.
+
+        The parent rule is invisible to the stranger, so the response must be a
+        404 rather than a 403 that confirms the action exists.
+        """
+        self.login(self.stranger)
+
+        response = self.client.delete(
+            reverse(
+                "transaction_rule_action_delete",
+                kwargs={"transaction_rule_action_id": self.private_action.id},
+            ),
+            **HTMX,
+        )
+
+        self.assertEqual(response.status_code, 404)
+        self.assertTrue(
+            TransactionRuleAction.objects.filter(pk=self.private_action.pk).exists()
+        )
+
+    def test_owner_can_delete_own_action(self):
+        self.login(self.owner)
+
+        response = self.client.delete(
+            reverse(
+                "transaction_rule_action_delete",
+                kwargs={"transaction_rule_action_id": self.public_action.id},
+            ),
+            **HTMX,
+        )
+
+        self.assertEqual(response.status_code, 204)
+        self.assertFalse(
+            TransactionRuleAction.objects.filter(pk=self.public_action.pk).exists()
+        )
+
+    # ------------------------------------------------------------------
+    # update-or-create rule actions
+    # ------------------------------------------------------------------
+    def test_stranger_cannot_add_linked_action_to_public_rule(self):
+        self.login(self.stranger)
+
+        response = self.client.post(
+            reverse(
+                "update_or_create_transaction_rule_action_add",
+                kwargs={"transaction_rule_id": self.public_rule.id},
+            ),
+            data={},
+            **HTMX,
+        )
+
+        self.assertEqual(response.status_code, 403)
+        self.assertEqual(
+            UpdateOrCreateTransactionRuleAction.objects.filter(
+                rule=self.public_rule
+            ).count(),
+            1,
+        )
+
+    def test_stranger_cannot_edit_linked_action_on_public_rule(self):
+        self.login(self.stranger)
+
+        response = self.client.post(
+            reverse(
+                "update_or_create_transaction_rule_action_edit",
+                kwargs={"pk": self.public_linked_action.id},
+            ),
+            data={"filter": "injected"},
+            **HTMX,
+        )
+
+        self.assertEqual(response.status_code, 403)
+        self.public_linked_action.refresh_from_db()
+        self.assertEqual(self.public_linked_action.filter, "")
+
+    def test_stranger_cannot_delete_linked_action_on_public_rule(self):
+        self.login(self.stranger)
+
+        response = self.client.delete(
+            reverse(
+                "update_or_create_transaction_rule_action_delete",
+                kwargs={"pk": self.public_linked_action.id},
+            ),
+            **HTMX,
+        )
+
+        self.assertEqual(response.status_code, 403)
+        self.assertTrue(
+            UpdateOrCreateTransactionRuleAction.objects.filter(
+                pk=self.public_linked_action.pk
+            ).exists()
+        )
+
+    def test_stranger_cannot_delete_linked_action_on_invisible_rule(self):
+        self.login(self.stranger)
+
+        response = self.client.delete(
+            reverse(
+                "update_or_create_transaction_rule_action_delete",
+                kwargs={"pk": self.private_linked_action.id},
+            ),
+            **HTMX,
+        )
+
+        self.assertEqual(response.status_code, 404)
+        self.assertTrue(
+            UpdateOrCreateTransactionRuleAction.objects.filter(
+                pk=self.private_linked_action.pk
+            ).exists()
+        )
+
+    # ------------------------------------------------------------------
+    # edit / share / dry-run
+    # ------------------------------------------------------------------
+    def test_stranger_cannot_edit_public_rule(self):
+        self.login(self.stranger)
+
+        response = self.client.post(
+            reverse(
+                "transaction_rule_edit",
+                kwargs={"transaction_rule_id": self.public_rule.id},
+            ),
+            data={"name": "hijacked", "trigger": "True", "order": 0},
+            **HTMX,
+        )
+
+        self.assertEqual(response.status_code, 403)
+        self.public_rule.refresh_from_db()
+        self.assertEqual(self.public_rule.name, "Public rule")
+
+    def test_stranger_cannot_change_sharing_of_public_rule(self):
+        self.login(self.stranger)
+
+        response = self.client.post(
+            reverse(
+                "transaction_rule_share_settings", kwargs={"pk": self.public_rule.id}
+            ),
+            data={"visibility": "private"},
+            **HTMX,
+        )
+
+        self.assertEqual(response.status_code, 403)
+        self.public_rule.refresh_from_db()
+        self.assertEqual(self.public_rule.visibility, "public")
+
+    def test_stranger_cannot_dry_run_public_rule(self):
+        self.login(self.stranger)
+
+        response = self.client.get(
+            reverse(
+                "transaction_rule_dry_run_created", kwargs={"pk": self.public_rule.id}
+            ),
+            **HTMX,
+        )
+
+        self.assertEqual(response.status_code, 403)
+
+    # ------------------------------------------------------------------
+    # delete: sharing may be revoked, ownership may not be overridden
+    # ------------------------------------------------------------------
+    def test_stranger_cannot_delete_public_rule(self):
+        """The original condition fell through to delete() for public rules."""
+        self.login(self.stranger)
+
+        response = self.client.delete(
+            reverse(
+                "transaction_rule_delete",
+                kwargs={"transaction_rule_id": self.public_rule.id},
+            ),
+            **HTMX,
+        )
+
+        self.assertEqual(response.status_code, 403)
+        self.assertTrue(
+            TransactionRule.all_objects.filter(pk=self.public_rule.pk).exists()
+        )
+
+    def test_shared_user_deleting_only_revokes_their_own_access(self):
+        self.login(self.shared_user)
+
+        response = self.client.delete(
+            reverse(
+                "transaction_rule_delete",
+                kwargs={"transaction_rule_id": self.shared_rule.id},
+            ),
+            **HTMX,
+        )
+
+        self.assertEqual(response.status_code, 204)
+        self.assertTrue(
+            TransactionRule.all_objects.filter(pk=self.shared_rule.pk).exists()
+        )
+        self.assertNotIn(self.shared_user, self.shared_rule.shared_with.all())
+
+    def test_owner_can_delete_own_rule(self):
+        self.login(self.owner)
+
+        response = self.client.delete(
+            reverse(
+                "transaction_rule_delete",
+                kwargs={"transaction_rule_id": self.public_rule.id},
+            ),
+            **HTMX,
+        )
+
+        self.assertEqual(response.status_code, 204)
+        self.assertFalse(
+            TransactionRule.all_objects.filter(pk=self.public_rule.pk).exists()
+        )
+
+    # ------------------------------------------------------------------
+    # reads must keep working for shared users
+    # ------------------------------------------------------------------
+    def test_shared_user_can_still_view_shared_rule(self):
+        self.login(self.shared_user)
+
+        response = self.client.get(
+            reverse(
+                "transaction_rule_view",
+                kwargs={"transaction_rule_id": self.shared_rule.id},
+            ),
+            **HTMX,
+        )
+
+        self.assertEqual(response.status_code, 200)
+
+    def test_stranger_can_view_public_rule(self):
+        self.login(self.stranger)
+
+        response = self.client.get(
+            reverse(
+                "transaction_rule_view",
+                kwargs={"transaction_rule_id": self.public_rule.id},
+            ),
+            **HTMX,
+        )
+
+        self.assertEqual(response.status_code, 200)
+
+    def test_stranger_cannot_view_private_rule(self):
+        self.login(self.stranger)
+
+        response = self.client.get(
+            reverse(
+                "transaction_rule_view",
+                kwargs={"transaction_rule_id": self.private_rule.id},
+            ),
+            **HTMX,
+        )
+
+        self.assertEqual(response.status_code, 404)
+
+    # ------------------------------------------------------------------
+    # take ownership stays available for unowned rules only
+    # ------------------------------------------------------------------
+    def test_take_ownership_of_unowned_rule_still_works(self):
+        unowned = TransactionRule.all_objects.create(
+            name="Legacy rule", trigger="True", owner=None, visibility="private"
+        )
+        self.login(self.stranger)
+
+        response = self.client.get(
+            reverse(
+                "transaction_rule_take_ownership",
+                kwargs={"transaction_rule_id": unowned.id},
+            ),
+            **HTMX,
+        )
+
+        self.assertEqual(response.status_code, 204)
+        unowned.refresh_from_db()
+        self.assertEqual(unowned.owner, self.stranger)
+
+    def test_cannot_take_ownership_of_someone_elses_public_rule(self):
+        self.login(self.stranger)
+
+        response = self.client.get(
+            reverse(
+                "transaction_rule_take_ownership",
+                kwargs={"transaction_rule_id": self.public_rule.id},
+            ),
+            **HTMX,
+        )
+
+        self.assertEqual(response.status_code, 403)
+        self.public_rule.refresh_from_db()
+        self.assertEqual(self.public_rule.owner, self.owner)
diff --git a/app/apps/rules/views.py b/app/apps/rules/views.py
index 1fb44a6..5054c02 100644
--- a/app/apps/rules/views.py
+++ b/app/apps/rules/views.py
@@ -4,13 +4,19 @@ from copy import deepcopy
 
 from django.contrib import messages
 from django.contrib.auth.decorators import login_required
+from django.core.exceptions import PermissionDenied
 from django.db import transaction
 from django.http import HttpResponse
-from django.shortcuts import render, get_object_or_404, redirect
+from django.shortcuts import render, redirect
 from django.utils.translation import gettext_lazy as _
 from django.views.decorators.http import require_http_methods
 
 from apps.common.decorators.htmx import only_htmx
+from apps.common.functions.permissions import (
+    EDIT,
+    READ,
+    get_shared_object_or_error,
+)
 from apps.rules.forms import (
     TransactionRuleForm,
     TransactionRuleActionForm,
@@ -62,7 +68,9 @@ def rules_list(request):
 @disabled_on_demo
 @require_http_methods(["GET", "POST"])
 def transaction_rule_toggle_activity(request, transaction_rule_id, **kwargs):
-    transaction_rule = get_object_or_404(TransactionRule, id=transaction_rule_id)
+    transaction_rule = get_shared_object_or_error(
+        TransactionRule, request, id=transaction_rule_id, level=EDIT
+    )
     current_active = transaction_rule.active
     transaction_rule.active = not current_active
     transaction_rule.save(update_fields=["active"])
@@ -112,17 +120,9 @@ def transaction_rule_add(request, **kwargs):
 @disabled_on_demo
 @require_http_methods(["GET", "POST"])
 def transaction_rule_edit(request, transaction_rule_id):
-    transaction_rule = get_object_or_404(TransactionRule, id=transaction_rule_id)
-
-    if transaction_rule.owner and transaction_rule.owner != request.user:
-        messages.error(request, _("Only the owner can edit this"))
-
-        return HttpResponse(
-            status=204,
-            headers={
-                "HX-Trigger": "updated, hide_offcanvas",
-            },
-        )
+    transaction_rule = get_shared_object_or_error(
+        TransactionRule, request, id=transaction_rule_id, level=EDIT
+    )
 
     if request.method == "POST":
         form = TransactionRuleForm(request.POST, instance=transaction_rule)
@@ -151,7 +151,9 @@ def transaction_rule_edit(request, transaction_rule_id):
 @disabled_on_demo
 @require_http_methods(["GET", "POST"])
 def transaction_rule_view(request, transaction_rule_id):
-    transaction_rule = get_object_or_404(TransactionRule, id=transaction_rule_id)
+    transaction_rule = get_shared_object_or_error(
+        TransactionRule, request, id=transaction_rule_id, level=READ
+    )
 
     edit_actions = transaction_rule.transaction_actions.all()
     update_or_create_actions = (
@@ -175,17 +177,20 @@ def transaction_rule_view(request, transaction_rule_id):
 @disabled_on_demo
 @require_http_methods(["DELETE"])
 def transaction_rule_delete(request, transaction_rule_id):
-    transaction_rule = get_object_or_404(TransactionRule, id=transaction_rule_id)
+    transaction_rule = get_shared_object_or_error(
+        TransactionRule, request, id=transaction_rule_id, level=READ
+    )
 
-    if (
-        transaction_rule.owner != request.user
-        and request.user in transaction_rule.shared_with.all()
-    ):
+    if transaction_rule.is_editable_by(request.user):
+        transaction_rule.delete()
+        messages.success(request, _("Rule deleted successfully"))
+    elif transaction_rule.shared_with.filter(pk=request.user.pk).exists():
+        # Someone else's rule shared with us: we can drop our own access to it,
+        # but never delete it.
         transaction_rule.shared_with.remove(request.user)
         messages.success(request, _("Item no longer shared with you"))
     else:
-        transaction_rule.delete()
-        messages.success(request, _("Rule deleted successfully"))
+        raise PermissionDenied
 
     return HttpResponse(
         status=204,
@@ -200,7 +205,9 @@ def transaction_rule_delete(request, transaction_rule_id):
 @disabled_on_demo
 @require_http_methods(["GET"])
 def transaction_rule_take_ownership(request, transaction_rule_id):
-    transaction_rule = get_object_or_404(TransactionRule, id=transaction_rule_id)
+    transaction_rule = get_shared_object_or_error(
+        TransactionRule, request, id=transaction_rule_id, level=EDIT
+    )
 
     if not transaction_rule.owner:
         transaction_rule.owner = request.user
@@ -222,17 +229,7 @@ def transaction_rule_take_ownership(request, transaction_rule_id):
 @disabled_on_demo
 @require_http_methods(["GET", "POST"])
 def transaction_rule_share(request, pk):
-    obj = get_object_or_404(TransactionRule, id=pk)
-
-    if obj.owner and obj.owner != request.user:
-        messages.error(request, _("Only the owner can edit this"))
-
-        return HttpResponse(
-            status=204,
-            headers={
-                "HX-Trigger": "updated, hide_offcanvas",
-            },
-        )
+    obj = get_shared_object_or_error(TransactionRule, request, id=pk, level=EDIT)
 
     if request.method == "POST":
         form = SharedObjectForm(request.POST, instance=obj, user=request.user)
@@ -261,7 +258,9 @@ def transaction_rule_share(request, pk):
 @disabled_on_demo
 @require_http_methods(["GET", "POST"])
 def transaction_rule_action_add(request, transaction_rule_id):
-    transaction_rule = get_object_or_404(TransactionRule, id=transaction_rule_id)
+    transaction_rule = get_shared_object_or_error(
+        TransactionRule, request, id=transaction_rule_id, level=EDIT
+    )
 
     if request.method == "POST":
         form = TransactionRuleActionForm(request.POST, rule=transaction_rule)
@@ -289,12 +288,14 @@ def transaction_rule_action_add(request, transaction_rule_id):
 @disabled_on_demo
 @require_http_methods(["GET", "POST"])
 def transaction_rule_action_edit(request, transaction_rule_action_id):
-    transaction_rule_action = get_object_or_404(
-        TransactionRuleAction, id=transaction_rule_action_id
-    )
-    transaction_rule = get_object_or_404(
-        TransactionRule, id=transaction_rule_action.rule.id
+    transaction_rule_action = get_shared_object_or_error(
+        TransactionRuleAction,
+        request,
+        id=transaction_rule_action_id,
+        level=EDIT,
+        via="rule",
     )
+    transaction_rule = transaction_rule_action.rule
 
     if request.method == "POST":
         form = TransactionRuleActionForm(
@@ -327,8 +328,12 @@ def transaction_rule_action_edit(request, transaction_rule_action_id):
 @disabled_on_demo
 @require_http_methods(["DELETE"])
 def transaction_rule_action_delete(request, transaction_rule_action_id):
-    transaction_rule_action = get_object_or_404(
-        TransactionRuleAction, id=transaction_rule_action_id
+    transaction_rule_action = get_shared_object_or_error(
+        TransactionRuleAction,
+        request,
+        id=transaction_rule_action_id,
+        level=EDIT,
+        via="rule",
     )
 
     transaction_rule_action.delete()
@@ -348,7 +353,9 @@ def transaction_rule_action_delete(request, transaction_rule_action_id):
 @disabled_on_demo
 @require_http_methods(["GET", "POST"])
 def update_or_create_transaction_rule_action_add(request, transaction_rule_id):
-    transaction_rule = get_object_or_404(TransactionRule, id=transaction_rule_id)
+    transaction_rule = get_shared_object_or_error(
+        TransactionRule, request, id=transaction_rule_id, level=EDIT
+    )
 
     if request.method == "POST":
         form = UpdateOrCreateTransactionRuleActionForm(
@@ -380,7 +387,9 @@ def update_or_create_transaction_rule_action_add(request, transaction_rule_id):
 @disabled_on_demo
 @require_http_methods(["GET", "POST"])
 def update_or_create_transaction_rule_action_edit(request, pk):
-    linked_action = get_object_or_404(UpdateOrCreateTransactionRuleAction, id=pk)
+    linked_action = get_shared_object_or_error(
+        UpdateOrCreateTransactionRuleAction, request, id=pk, level=EDIT, via="rule"
+    )
     transaction_rule = linked_action.rule
 
     if request.method == "POST":
@@ -415,7 +424,9 @@ def update_or_create_transaction_rule_action_edit(request, pk):
 @disabled_on_demo
 @require_http_methods(["DELETE"])
 def update_or_create_transaction_rule_action_delete(request, pk):
-    linked_action = get_object_or_404(UpdateOrCreateTransactionRuleAction, id=pk)
+    linked_action = get_shared_object_or_error(
+        UpdateOrCreateTransactionRuleAction, request, id=pk, level=EDIT, via="rule"
+    )
 
     linked_action.delete()
 
@@ -436,7 +447,7 @@ def update_or_create_transaction_rule_action_delete(request, pk):
 @disabled_on_demo
 @require_http_methods(["GET", "POST"])
 def dry_run_rule_created(request, pk):
-    rule = get_object_or_404(TransactionRule, id=pk)
+    rule = get_shared_object_or_error(TransactionRule, request, id=pk, level=EDIT)
     logs = None
     results = None
 
@@ -481,7 +492,7 @@ def dry_run_rule_created(request, pk):
 @disabled_on_demo
 @require_http_methods(["GET", "POST"])
 def dry_run_rule_deleted(request, pk):
-    rule = get_object_or_404(TransactionRule, id=pk)
+    rule = get_shared_object_or_error(TransactionRule, request, id=pk, level=EDIT)
     logs = None
     results = None
 
@@ -526,7 +537,7 @@ def dry_run_rule_deleted(request, pk):
 @disabled_on_demo
 @require_http_methods(["GET", "POST"])
 def dry_run_rule_updated(request, pk):
-    rule = get_object_or_404(TransactionRule, id=pk)
+    rule = get_shared_object_or_error(TransactionRule, request, id=pk, level=EDIT)
     logs = None
     results = None
 
diff --git a/app/apps/transactions/views/categories.py b/app/apps/transactions/views/categories.py
index 43bd5ff..c539696 100644
--- a/app/apps/transactions/views/categories.py
+++ b/app/apps/transactions/views/categories.py
@@ -1,11 +1,17 @@
 from django.contrib import messages
 from django.contrib.auth.decorators import login_required
+from django.core.exceptions import PermissionDenied
 from django.http import HttpResponse
-from django.shortcuts import render, get_object_or_404
+from django.shortcuts import render
 from django.utils.translation import gettext_lazy as _
 from django.views.decorators.http import require_http_methods
 
 from apps.common.decorators.htmx import only_htmx
+from apps.common.functions.permissions import (
+    EDIT,
+    READ,
+    get_shared_object_or_error,
+)
 from apps.transactions.forms import TransactionCategoryForm
 from apps.transactions.models import TransactionCategory
 from apps.common.models import SharedObject
@@ -85,17 +91,9 @@ def category_add(request, **kwargs):
 @login_required
 @require_http_methods(["GET", "POST"])
 def category_edit(request, category_id):
-    category = get_object_or_404(TransactionCategory, id=category_id)
-
-    if category.owner and category.owner != request.user:
-        messages.error(request, _("Only the owner can edit this"))
-
-        return HttpResponse(
-            status=204,
-            headers={
-                "HX-Trigger": "updated, hide_offcanvas",
-            },
-        )
+    category = get_shared_object_or_error(
+        TransactionCategory, request, id=category_id, level=EDIT
+    )
 
     if request.method == "POST":
         form = TransactionCategoryForm(request.POST, instance=category)
@@ -123,17 +121,7 @@ def category_edit(request, category_id):
 @login_required
 @require_http_methods(["GET", "POST"])
 def category_share(request, pk):
-    obj = get_object_or_404(TransactionCategory, id=pk)
-
-    if obj.owner and obj.owner != request.user:
-        messages.error(request, _("Only the owner can edit this"))
-
-        return HttpResponse(
-            status=204,
-            headers={
-                "HX-Trigger": "updated, hide_offcanvas",
-            },
-        )
+    obj = get_shared_object_or_error(TransactionCategory, request, id=pk, level=EDIT)
 
     if request.method == "POST":
         form = SharedObjectForm(request.POST, instance=obj, user=request.user)
@@ -161,14 +149,20 @@ def category_share(request, pk):
 @login_required
 @require_http_methods(["DELETE"])
 def category_delete(request, category_id):
-    category = get_object_or_404(TransactionCategory, id=category_id)
+    category = get_shared_object_or_error(
+        TransactionCategory, request, id=category_id, level=READ
+    )
 
-    if category.owner != request.user and request.user in category.shared_with.all():
+    if category.is_editable_by(request.user):
+        category.delete()
+        messages.success(request, _("Category deleted successfully"))
+    elif category.shared_with.filter(pk=request.user.pk).exists():
+        # Someone else's object shared with us: we can drop our own access
+        # to it, but never delete it.
         category.shared_with.remove(request.user)
         messages.success(request, _("Item no longer shared with you"))
     else:
-        category.delete()
-        messages.success(request, _("Category deleted successfully"))
+        raise PermissionDenied
 
     return HttpResponse(
         status=204,
@@ -182,7 +176,9 @@ def category_delete(request, category_id):
 @login_required
 @require_http_methods(["GET"])
 def category_take_ownership(request, category_id):
-    category = get_object_or_404(TransactionCategory, id=category_id)
+    category = get_shared_object_or_error(
+        TransactionCategory, request, id=category_id, level=EDIT
+    )
 
     if not category.owner:
         category.owner = request.user
diff --git a/app/apps/transactions/views/entities.py b/app/apps/transactions/views/entities.py
index 300d3e2..8500e61 100644
--- a/app/apps/transactions/views/entities.py
+++ b/app/apps/transactions/views/entities.py
@@ -1,11 +1,17 @@
 from django.contrib import messages
 from django.contrib.auth.decorators import login_required
+from django.core.exceptions import PermissionDenied
 from django.http import HttpResponse
-from django.shortcuts import render, get_object_or_404
+from django.shortcuts import render
 from django.utils.translation import gettext_lazy as _
 from django.views.decorators.http import require_http_methods
 
 from apps.common.decorators.htmx import only_htmx
+from apps.common.functions.permissions import (
+    EDIT,
+    READ,
+    get_shared_object_or_error,
+)
 from apps.transactions.forms import TransactionEntityForm
 from apps.transactions.models import TransactionEntity
 from apps.common.models import SharedObject
@@ -85,17 +91,9 @@ def entity_add(request, **kwargs):
 @login_required
 @require_http_methods(["GET", "POST"])
 def entity_edit(request, entity_id):
-    entity = get_object_or_404(TransactionEntity, id=entity_id)
-
-    if entity.owner and entity.owner != request.user:
-        messages.error(request, _("Only the owner can edit this"))
-
-        return HttpResponse(
-            status=204,
-            headers={
-                "HX-Trigger": "updated, hide_offcanvas",
-            },
-        )
+    entity = get_shared_object_or_error(
+        TransactionEntity, request, id=entity_id, level=EDIT
+    )
 
     if request.method == "POST":
         form = TransactionEntityForm(request.POST, instance=entity)
@@ -123,14 +121,20 @@ def entity_edit(request, entity_id):
 @login_required
 @require_http_methods(["DELETE"])
 def entity_delete(request, entity_id):
-    entity = get_object_or_404(TransactionEntity, id=entity_id)
+    entity = get_shared_object_or_error(
+        TransactionEntity, request, id=entity_id, level=READ
+    )
 
-    if entity.owner != request.user and request.user in entity.shared_with.all():
+    if entity.is_editable_by(request.user):
+        entity.delete()
+        messages.success(request, _("Entity deleted successfully"))
+    elif entity.shared_with.filter(pk=request.user.pk).exists():
+        # Someone else's object shared with us: we can drop our own access
+        # to it, but never delete it.
         entity.shared_with.remove(request.user)
         messages.success(request, _("Item no longer shared with you"))
     else:
-        entity.delete()
-        messages.success(request, _("Entity deleted successfully"))
+        raise PermissionDenied
 
     return HttpResponse(
         status=204,
@@ -144,7 +148,9 @@ def entity_delete(request, entity_id):
 @login_required
 @require_http_methods(["GET"])
 def entity_take_ownership(request, entity_id):
-    entity = get_object_or_404(TransactionEntity, id=entity_id)
+    entity = get_shared_object_or_error(
+        TransactionEntity, request, id=entity_id, level=EDIT
+    )
 
     if not entity.owner:
         entity.owner = request.user
@@ -165,17 +171,7 @@ def entity_take_ownership(request, entity_id):
 @login_required
 @require_http_methods(["GET", "POST"])
 def entity_share(request, pk):
-    obj = get_object_or_404(TransactionEntity, id=pk)
-
-    if obj.owner and obj.owner != request.user:
-        messages.error(request, _("Only the owner can edit this"))
-
-        return HttpResponse(
-            status=204,
-            headers={
-                "HX-Trigger": "updated, hide_offcanvas",
-            },
-        )
+    obj = get_shared_object_or_error(TransactionEntity, request, id=pk, level=EDIT)
 
     if request.method == "POST":
         form = SharedObjectForm(request.POST, instance=obj, user=request.user)
diff --git a/app/apps/transactions/views/tags.py b/app/apps/transactions/views/tags.py
index f7709c9..cc0e487 100644
--- a/app/apps/transactions/views/tags.py
+++ b/app/apps/transactions/views/tags.py
@@ -1,11 +1,17 @@
 from django.contrib import messages
 from django.contrib.auth.decorators import login_required
+from django.core.exceptions import PermissionDenied
 from django.http import HttpResponse
-from django.shortcuts import render, get_object_or_404
+from django.shortcuts import render
 from django.utils.translation import gettext_lazy as _
 from django.views.decorators.http import require_http_methods
 
 from apps.common.decorators.htmx import only_htmx
+from apps.common.functions.permissions import (
+    EDIT,
+    READ,
+    get_shared_object_or_error,
+)
 from apps.transactions.forms import TransactionTagForm
 from apps.transactions.models import TransactionTag
 from apps.common.models import SharedObject
@@ -85,17 +91,7 @@ def tag_add(request, **kwargs):
 @login_required
 @require_http_methods(["GET", "POST"])
 def tag_edit(request, tag_id):
-    tag = get_object_or_404(TransactionTag, id=tag_id)
-
-    if tag.owner and tag.owner != request.user:
-        messages.error(request, _("Only the owner can edit this"))
-
-        return HttpResponse(
-            status=204,
-            headers={
-                "HX-Trigger": "updated, hide_offcanvas",
-            },
-        )
+    tag = get_shared_object_or_error(TransactionTag, request, id=tag_id, level=EDIT)
 
     if request.method == "POST":
         form = TransactionTagForm(request.POST, instance=tag)
@@ -123,14 +119,18 @@ def tag_edit(request, tag_id):
 @login_required
 @require_http_methods(["DELETE"])
 def tag_delete(request, tag_id):
-    tag = get_object_or_404(TransactionTag, id=tag_id)
+    tag = get_shared_object_or_error(TransactionTag, request, id=tag_id, level=READ)
 
-    if tag.owner != request.user and request.user in tag.shared_with.all():
+    if tag.is_editable_by(request.user):
+        tag.delete()
+        messages.success(request, _("Tag deleted successfully"))
+    elif tag.shared_with.filter(pk=request.user.pk).exists():
+        # Someone else's object shared with us: we can drop our own access
+        # to it, but never delete it.
         tag.shared_with.remove(request.user)
         messages.success(request, _("Item no longer shared with you"))
     else:
-        tag.delete()
-        messages.success(request, _("Tag deleted successfully"))
+        raise PermissionDenied
 
     return HttpResponse(
         status=204,
@@ -144,7 +144,7 @@ def tag_delete(request, tag_id):
 @login_required
 @require_http_methods(["GET"])
 def tag_take_ownership(request, tag_id):
-    tag = get_object_or_404(TransactionTag, id=tag_id)
+    tag = get_shared_object_or_error(TransactionTag, request, id=tag_id, level=EDIT)
 
     if not tag.owner:
         tag.owner = request.user
@@ -165,17 +165,7 @@ def tag_take_ownership(request, tag_id):
 @login_required
 @require_http_methods(["GET", "POST"])
 def tag_share(request, pk):
-    obj = get_object_or_404(TransactionTag, id=pk)
-
-    if obj.owner and obj.owner != request.user:
-        messages.error(request, _("Only the owner can edit this"))
-
-        return HttpResponse(
-            status=204,
-            headers={
-                "HX-Trigger": "updated, hide_offcanvas",
-            },
-        )
+    obj = get_shared_object_or_error(TransactionTag, request, id=pk, level=EDIT)
 
     if request.method == "POST":
         form = SharedObjectForm(request.POST, instance=obj, user=request.user)
diff --git a/app/templates/rules/fragments/list.html b/app/templates/rules/fragments/list.html
index 07cb0f7..998708e 100644
--- a/app/templates/rules/fragments/list.html
+++ b/app/templates/rules/fragments/list.html
@@ -64,14 +64,21 @@
                   </div>
                 </td>
                 <td class="table-col-auto">
-                  <a class="no-underline cursor-pointer"
-                     role="button"
-                     data-tippy-content="
-                       {% if rule.active %}{% translate "Deactivate" %}{% else %}{% translate "Activate" %}{% endif %}"
-                     hx-get="{% url 'transaction_rule_toggle_activity' transaction_rule_id=rule.id %}">
-                    {% if rule.active %}<i class="fa-solid fa-toggle-on text-success"></i>{% else %}
-                      <i class="fa-solid fa-toggle-off text-error"></i>{% endif %}
-                  </a>
+                  {% if not rule.owner or user == rule.owner %}
+                    <a class="no-underline cursor-pointer"
+                       role="button"
+                       data-tippy-content="
+                         {% if rule.active %}{% translate "Deactivate" %}{% else %}{% translate "Activate" %}{% endif %}"
+                       hx-get="{% url 'transaction_rule_toggle_activity' transaction_rule_id=rule.id %}">
+                      {% if rule.active %}<i class="fa-solid fa-toggle-on text-success"></i>{% else %}
+                        <i class="fa-solid fa-toggle-off text-error"></i>{% endif %}
+                    </a>
+                  {% else %}
+                    <span data-tippy-content="{% translate "Only the owner can change this" %}">
+                      {% if rule.active %}<i class="fa-solid fa-toggle-on text-success opacity-50"></i>{% else %}
+                        <i class="fa-solid fa-toggle-off text-error opacity-50"></i>{% endif %}
+                    </span>
+                  {% endif %}
                 </td>
                 <td class="table-col-auto text-center">
                   <div>{{ rule.order }}</div>