Parse Server before 8.6.105 and 9.0.0 before 9.10.5-alpha.6 does not validate the owning object reference in $relatedTo relation queries, so supplying a query operator instead of an object ID bypasses the owning-object ACL check. A client with read access to at least one object of the owning class can disclose which readable objects are linked to owning objects it cannot read.