Another FreeBSD Security Advisory (FreeBSD-SA-04:04.tcp)

I. _ Background

The Transmission Control Protocol (TCP) of the TCP/IP protocol suite
provides a connection-oriented, reliable, sequence-preserving data
stream service. _When network packets making up a TCP stream (``TCP
segments'') are received out-of-sequence, they are maintained in a
reassembly queue by the destination system until they can be re-ordered
and re-assembled.

II. _Problem Description

FreeBSD does not limit the number of TCP segments that may be held in a
reassembly queue.

III. Impact

A remote attacker may conduct a low-bandwidth denial-of-service attack
against a machine providing services based on TCP (there are many such
services, including HTTP, SMTP, and FTP). _By sending many
out-of-sequence TCP segments, the attacker can cause the target machine
to consume all available memory buffers (``mbufs''), likely leading to
a system crash.