* Drupal 7.x-1.x before 7.x-1.1-rc1
* Drupal 7.x-1.1-rc1 and later
By default, with an autoselect or a select widget, a user cannot autocomplete an entity title, nor can they select an entity that they have no access to. This will correctly throw a 'invalid id' error and does not show the title of the entity.
However, if a user (A) that has access to the referenced entity (Node 1), makes that reference on a node (Node 2), and gives edit access to another user (B), user B will be able to see the node title for the referenced node (Node 2).
This vulnerability is mitigated by the fact that an attacker must get a user with access to a private node to reference it via another node that attacker has edit access to. No other node information is leaked other than the title.