diff --git a/src/accounting/utils/permission.py b/src/accounting/utils/permission.py
index 11a4c71..071319a 100644
--- a/src/accounting/utils/permission.py
+++ b/src/accounting/utils/permission.py
@@ -21,7 +21,7 @@ This module should not import any other module from the application.
 """
 import typing as t
 
-from flask import abort, Blueprint
+from flask import abort, Blueprint, Response
 
 from accounting.utils.user import get_current_user, UserUtilityInterface
 
@@ -49,6 +49,10 @@ def has_permission(rule: t.Callable[[], bool]) -> t.Callable:
             :raise Forbidden: When the user is denied.
             """
             if not rule():
+                if get_current_user() is None:
+                    response: Response | None = _unauthorized_func()
+                    if response is not None:
+                        return response
                 abort(403)
             return view(*args, **kwargs)
 
@@ -66,6 +70,9 @@ data."""
 __can_admin_func: t.Callable[[], bool] = lambda: True
 """The callback that returns whether the current user can administrate the
 accounting settings."""
+_unauthorized_func: t.Callable[[], Response | None] \
+    = lambda: Response(status=403)
+"""The callback that returns the response to require the user to log in."""
 
 
 def can_view() -> bool:
@@ -111,10 +118,12 @@ def init_app(bp: Blueprint, user_utils: UserUtilityInterface) -> None:
     :param user_utils: The user utilities.
     :return: None.
     """
-    global __can_view_func, __can_edit_func, __can_admin_func
+    global __can_view_func, __can_edit_func, __can_admin_func, \
+        _unauthorized_func
     __can_view_func = user_utils.can_view
     __can_edit_func = user_utils.can_edit
     __can_admin_func = user_utils.can_admin
+    _unauthorized_func = user_utils.unauthorized
     bp.add_app_template_global(user_utils.can_view, "accounting_can_view")
     bp.add_app_template_global(user_utils.can_edit, "accounting_can_edit")
     bp.add_app_template_global(user_utils.can_admin, "accounting_can_admin")
diff --git a/src/accounting/utils/user.py b/src/accounting/utils/user.py
index 63b0f3a..d1d07fa 100644
--- a/src/accounting/utils/user.py
+++ b/src/accounting/utils/user.py
@@ -23,7 +23,7 @@ import typing as t
 from abc import ABC, abstractmethod
 
 import sqlalchemy as sa
-from flask import g
+from flask import g, Response
 from flask_sqlalchemy.model import Model
 
 T = t.TypeVar("T", bound=Model)
@@ -59,6 +59,17 @@ class UserUtilityInterface(t.Generic[T], ABC):
             accounting settings, or False otherwise.
         """
 
+    @abstractmethod
+    def unauthorized(self) -> Response | None:
+        """Returns the response to require the user to log in.
+
+        This may be a redirection to the login page, or an HTTP 401
+        Unauthorized response for HTTP Authentication.  If this returns None,
+        an HTTP 403 Forbidden response is return to the user.
+
+        :return: The response to require the user to log in.
+        """
+
     @property
     @abstractmethod
     def cls(self) -> t.Type[T]:
diff --git a/tests/test_site/__init__.py b/tests/test_site/__init__.py
index e1550dc..95e610a 100644
--- a/tests/test_site/__init__.py
+++ b/tests/test_site/__init__.py
@@ -22,7 +22,7 @@ import typing as t
 from secrets import token_urlsafe
 
 import click
-from flask import Flask, Blueprint, render_template
+from flask import Flask, Blueprint, render_template, redirect, Response
 from flask.cli import with_appcontext
 from flask_babel_js import BabelJS
 from flask_sqlalchemy import SQLAlchemy
@@ -82,6 +82,9 @@ def create_app(is_testing: bool = False) -> Flask:
             return auth.current_user() is not None \
                 and auth.current_user().username == "admin"
 
+        def unauthorized(self) -> Response:
+            return redirect("/login")
+
         @property
         def cls(self) -> t.Type[auth.User]:
             return auth.User