Limitations of firewall
Firewalls cannot protect a network if security rules are not followed properly by an organization or if the rules are not defined properly. Firewalls are also useless against attacks coming from inside the network. An inside attack can be from any legitimate user or from a virus that has arrived though an email or by some other mean on a local machine. Lastly a firewall also cannot restrict people from disclosing their login passwords into wrong hands. If such happens an intruder can access local network as a legitimate user and can easily exploit it.