Directus allows updates to non-allowed fields due to overlapping policies

Summary

If there are two overlapping policies for the update action that allow access to different fields, instead of correctly checking access permissions against the item they apply for the user is allowed to update the superset of fields allowed by any of the policies.

E.g. have one policy allowing update access to field_a if the id == 1 and one policy allowing update access to field_b if the id == 2. The user with both these policies is allowed to update both field_a and field_b for the items with ids 1 and 2.

Details

Before v11, if a user was allowed to update an item they were allowed to update the fields that the single permission, that applied to that item, listed. With overlapping permissions this isn't as clear cut anymore and the union of fields might not be the fields the user is allowed to update for that specific item.

The solution that this PR introduces is to evaluate the permissions for each field that the user tries to update in the validateItemAccess DB query, instead of only verifying access to the item as a whole. This is done by, instead of returning the actual field value, returning a flag that indicates if the user has access to that field. This uses the same case/when mechanism that is used for stripping out non permitted field that is at the core of the permissions engine.

As a result, for every item that the access is validated for, the expected result is an item that has either 1 or null for all the "requested" fields instead of any of the actual field values. These results are not useful for anything other than verifying the field level access permissions.

The final check in validateItemAccess can either fail if the number of items does not match the number of items the access is checked for (ie. the user does not have access to the item at all) or if not all of the passed in fields have access permissions for any of the returned items.

Impact

This is a vulnerability that allows update access to unintended fields, potentially impacting the password field for user accounts.