FreeBSD -- Resource exhaustion due to sessions stuck in LAST_ACK state

Problem Description:
TCP connections transitioning to the LAST_ACK state can
become permanently stuck due to mishandling of protocol
state in certain situations, which in turn can lead to
accumulated consumption and eventual exhaustion of system
resources, such as mbufs and sockets.
Impact:
An attacker who can repeatedly establish TCP connections
to a victim system (for instance, a Web server) could create
many TCP connections that are stuck in LAST_ACK state and
cause resource exhaustion, resulting in a denial of service
condition. This may also happen in normal operation where
no intentional attack is conducted, but an attacker who can
send specifically crafted packets can trigger this more
reliably.