CHAPTER 20 / INTRUDERS
They agreed that Graham should set the test for Charles Mabledene. It was nei-
ther more nor less than that Dragon should get Stern’s code. If he had the ‘in’ at
Utting which he claimed to have this should be possible, only loyalty to Moscow
Centre would prevent it. If he got the key to the code he would prove his loyalty
to London Central beyond a doubt.
Talking to Strange Men
, Ruth Rendell
Unauthorized intrusion into a computer system or network is one of the
most serious threats to computer security.
Intrusion detection systems have been developed to provide early warning
of an intrusion so that defensive action can be taken to prevent or mini-
Intrusion detection involves detecting unusual patterns of activity or
patterns of activity that are known to correlate with intrusions.
One important element of intrusion prevention is password management,
with the goal of preventing unauthorized users from having access to the
passwords of others.
A significant security problem for networked systems is hostile, or at least
unwanted, trespass by users or software. User trespass can take the form of unau-
thorized logon to a machine or, in the case of an authorized user, acquisition of priv-
ileges or performance of actions beyond those that have been authorized. Software
trespass can take the form of a virus, worm, or Trojan horse.
All these attacks relate to network security because system entry can be
achieved by means of a network. However, these attacks are not confined to net-
work-based attacks. A user with access to a local terminal may attempt trespass
without using an intermediate network. A virus or Trojan horse may be introduced
into a system by means of an optical disc. Only the worm is a uniquely network phe-
nomenon.Thus, system trespass is an area in which the concerns of network security
and computer security overlap.
Because the focus of this book is network security, we do not attempt a com-
prehensive analysis of either the attacks or the security countermeasures related
to system trespass. Instead, in this Part we present a broad overview of these
This chapter covers the subject of intruders. First, we examine the nature of
the attack and then look at strategies intended for prevention and, failing that,
detection. Next we examine the related topic of password management.