Final answer:
The vulnerability created by not configuring a new firewall is due to a default password, which is a well-known security risk that attackers can exploit to gain unauthorized access to the network.
Step-by-step explanation:
When an administrator plugs a new firewall into the network without making any changes, the vulnerability created is due to a default password. Manufacturers often set a standard default password for initial access that is widely known and should be changed before deployment to prevent unauthorized access. Failure to change this password can lead to potential breaches as attackers can easily find default passwords online for various devices.