When a regular user adds a reference between objects or attributes, MISP checks whether the user can access the overall event, but it does not always check whether the individual pieces of data are also allowed for that user.
Because of this, someone who can view an event could potentially access attributes or objects inside that event that were meant to be restricted to a specific sharing group or distribution level.
The vulnerability affects authenticated users who are not site administrators and who already have access to an event containing more restricted data.
The main impact is that users may be able to view sensitive attribute values, object details, or related object data that they should not normally be allowed to see.
The fix introduces granular authorization checks in the ObjectReferencesController add() method. For non-site-admin users, the query conditions for attributes, objects, and object attributes are now augmented with OR clauses that restrict results to: (1) entities belonging to the user's own event, (2) entities with unrestricted distribution levels (1, 2, 3, 5), or (3) entities with distribution level 4 whose sharing_group_id is in the user's authorized sharing group list. This ensures that only data the user is explicitly authorized to see under MISP's distribution and sharing-group model is included in the object reference operation.