The Field Permissions module enables you to set field-level permissions for viewing and editing fields, including a "Private" setting.
The module does not sufficiently restrict access to "Private" fields on entity types that have no field for an author (user account or owner). As a result, a field configured as "Private" on one of these entities can be viewed. Where the entity is editable, the entity can also be edited by anyone who can view it, including anonymous users.
This vulnerability is mitigated by the fact that an administrator must have configured a field as "Private" on an entity type that has no author, and the attacker must already be able to view that entity. Fields set to "Private" on nodes, users, media, comments, and other entity types with an author are not affected. Fields using Custom permissions are not affected.