The product does not sufficiently encapsulate critical data or functionality.
Encapsulation is about drawing strong boundaries. In a web browser that might mean ensuring that your mobile code cannot be abused by other mobile code. On the server it might mean differentiation between validated data and unvalidated data, between one user's data and another's, or between data users are allowed to see and data that they are not.
The "encapsulation" term is used in multiple ways. Within some security
sources, the term is used to describe the establishment of boundaries
between different control spheres. Within general computing circles, it is
more about hiding implementation details and maintainability than security.
Even within the security usage, there is also a question of whether
"encapsulation" encompasses the entire range of security problems.