Which type of attack is characterized by exploitation of well-known weaknesses, use of ports that are allowed through a firewall, and can never be completely eliminated?

A.
Network reconnaissance

B.
Application layer

C.
Man-in-the-middle

D.
Trust exploitation

Explanation:
The primary problem with application layer attacks is that they often use ports that are allowed through a firewall.
Ref: Safe White papers 68
SAFE: Extending the Security Blueprint to Small, Midsize, and Remote-User Networks