To understand stateful filters, let’s reexamine the access control list in Table 8.6.
Although rather restrictive, the access control list in Table 8.6 nevertheless allows
any packet arriving from the outside with ACK = 1 and source port 80 to get through
the filter. Such packets could be used by attackers in attempts to crash internal systems
with malformed packets, carry out denial-of-service attacks, or map the internal
network. The naive solution is to block TCP ACK packets as well, but such an
approach would prevent the organization’s internal users from surfing the Web.