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Caveats

There are a few things to be aware of when using django-guardian to manage object permissions. If ignored, they can lead to unexpected behavior and potential security issues.

Orphaned object permissions

Note the following does not apply if using direct foreign keys, as documented in performance-direct-fk.

Permissions, in particular per object permissions, can be tricky to manage. For example, how can we manage permissions that are no longer used? In some setups, it is possible to reuse database primary keys which have been removed from the database. When permissions are at the model level only, this is not a problem; but can lead to tremendous security issues when using object permissions. How can we deal with this?

Let's imagine our table has primary key to the filesystem path. We have a record with pk equal to /home/www/joe.config. User jane has read access to joe's configuration and we store that information in database by creating guardian's object permissions. Now, joe user removes account from our site and another user creates account with joe as username. The problem is that if we haven't removed object permissions explicitly in the process of first joe account removal, jane still has read permissions for joe's configuration file - but this is another user.

There is no easy way to deal with orphaned permissions as they are not foreign keyed with objects directly. Even if they would, there are some database engines—or ON DELETE rules—which restricts removal of related objects.

Important

It is extremely important to removeUserObjectPermission and GroupObjectPermission as we delete objects for which permissions are defined.

Guardian comes with utility function which tries to help to remove orphaned object permissions. Remember - those are only helpers. Applications should remove those object permissions explicitly by itself.

From our previous example, our application should remove a user object (e.g., joe). However, permissions for the joe user assigned to jane wouldNOT be removed. In this case, it would be easy to remove user/group object permissions if we connect proper action with proper signal. This could be achieved by following snippet:

    from django.contrib.contenttypes.models import ContentType
    from django.db.models import Q
    from django.db.models.signals import pre_delete
    from guardian.models import User
    from guardian.models import UserObjectPermission
    from guardian.models import GroupObjectPermission


    def remove_obj_perms_connected_with_user(sender, instance, **kwargs):
        filters = Q(content_type=ContentType.objects.get_for_model(instance),
            object_pk=instance.pk)
        UserObjectPermission.objects.filter(filters).delete()
        GroupObjectPermission.objects.filter(filters).delete()

    pre_delete.connect(remove_obj_perms_connected_with_user, sender=User)

This signal handler would remove all object permissions connected with user just before user is actually removed.

If we forgot to add such handlers, we may still remove orphaned object permissions by using clean_orphan_obj_perms If our application uses celery, it is also straightforward to remove orphaned permissions periodically with guardian.utils.clean_orphan_obj_perms function. We would still strongly advise to remove orphaned object permissions explicitly (i.e., at view that confirms object removal or using signals as described above).

Info

  • guardian.utils.clean_orphan_obj_perms
  • clean_orphan_obj_perms

Using multiple databases

This is not supported at present time due to a Django bug. See 288 and 16281.