Computer Systems Technology
===============================================================================
NIST Special Publication 500-166
Computer Viruses and Related Threats:
A Management Guide
John P. Wack
Lisa J. Carnahan
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COMPUTER VIRUSES AND RELATED THREATS
Table of Contents
Executive Summary . . . . . . . . . . . . . . . . . . . . . . v
1. Introduction . . . . . . . . . . . . . . . . . . . . . . 1-1
1.1 Audience and Scope . . . . . . . . . . . . . . . 1-1
1.2 How to Use This Guide . . . . . . . . . . . . . . 1-2
2. A Brief Overview on Viruses and Related Threats . . . . 2-1
2.1 Trojan Horses . . . . . . . . . . . . . . . . . . 2-1
2.2 Computer Viruses . . . . . . . . . . . . . . . . 2-3
2.3 Network Worms . . . . . . . . . . . . . . . . . . 2-5
2.4 Other Related Software Threats . . . . . . . . . 2-8
2.5 The Threat of Unauthorized Use . . . . . . . . . 2-8
3. Virus Prevention in General . . . . . . . . . . . . . . 3-1
3.1 User Education . . . . . . . . . . . . . . . . . 3-3
3.2 Software Management . . . . . . . . . . . . . . . 3-4
3.3 Technical Controls . . . . . . . . . . . . . . . 3-6
3.4 General Monitoring . . . . . . . . . . . . . . . 3-8
3.5 Contingency Planning . . . . . . . . . . . . . . 3-9
4. Virus Prevention for Multi-User Computers and Associated
Networks . . . . . . . . . . . . . . . . . . . . . . . 4-1
4.1 General Policies . . . . . . . . . . . . . . . . 4-1
4.2 Software Management . . . . . . . . . . . . . . . 4-2
4.3 Technical Controls . . . . . . . . . . . . . . . 4-4
4.4 Monitoring . . . . . . . . . . . . . . . . . . . 4-7
4.5 Contingency Planning . . . . . . . . . . . . . . 4-9
4.6 Associated Network Concerns . . . . . . . . . . . 4-10
5. Virus Prevention for Personal Computers and Associated
Networks . . . . . . . . . . . . . . . . . . . . . . . 5-1
5.1 General Policies . . . . . . . . . . . . . . . . 5-2
5.2 Software Management . . . . . . . . . . . . . . . 5-3
5.3 Technical Controls . . . . . . . . . . . . . . . 5-4
5.4 Monitoring . . . . . . . . . . . . . . . . . . . 5-6
5.5 Contingency Planning . . . . . . . . . . . . . . 5-7
5.6 Associated Network Concerns . . . . . . . . . . . 5-9
References . . . . . . . . . . . . . . . . . . . . . . . . A-1
Suggested Reading . . . . . . . . . . . . . . . . . . . . . B-1
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EXECUTIVE SUMMARY
Executive Summary
Computer viruses and related threats represent an increasingly
serious security problem in computing systems and networks. This
document presents guidelines for preventing, deterring,
containing, and recovering from attacks of viruses and related
threats. This section acquaints senior management with the
nature of the problem and outlines some of the steps that can be
taken to reduce an organization's vulnerability.
What Are Computer Viruses and Related Threats?
Computer viruses are the most widely recognized example of a
class of programs written to cause some form of intentional
damage to computer systems or networks. A computer virus
performs two basic functions: it copies itself to other programs,
thereby infecting them, and it executes the instructions the
author has included in it. Depending on the author's motives, a
program infected with a virus may cause damage immediately upon
its execution, or it may wait until a certain event has occurred,
such as a particular date and time. The damage can vary widely,
and can be so extensive as to require the complete rebuilding of
all system software and data. Because viruses can spread rapidly
to other programs and systems, the damage can multiply
geometrically.
Related threats include other forms of destructive programs such
as Trojan horses and network worms. Collectively, they are
sometimes referred to as malicious software. These programs are
often written to masquerade as useful programs, so that users are
induced into copying them and sharing them with friends and work
colleagues. The malicious software phenomena is fundamentally a
people problem, as it is authored and initially spread by
individuals who use systems in an unauthorized manner. Thus, the
threat of unauthorized use, by unauthorized and authorized users,
must be addressed as a part of virus prevention.
What Are the Vulnerabilities They Exploit?
Unauthorized users and malicious software may gain access to
systems through inadequate system security mechanisms, through
security holes in applications or systems, and through weaknesses
in computer management, such as the failure to properly use
existing security mechanisms. Malicious software can be copied
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EXECUTIVE SUMMARY
intentionally onto systems, or be spread when users unwittingly
copy and share infected software obtained from public software
repositories, such as software bulletin boards and shareware.
Because malicious software often hides its destructive nature by
performing or claiming to perform some useful function, users
generally don't suspect that they are copying and spreading the
problem.
Why Are Incidents of Viruses and Related Threats On the Rise?
Viruses and related threats, while not a recent phenomena, have
had relatively little attention focused on them in the past.
They occurred less frequently and caused relatively little
damage. For these reasons, they were frequently treated lightly
in computer design and by management, even though their potential
for harm was known to be great.
Computer users have become increasingly proficient and
sophisticated. Software applications are increasingly complex,
making their bugs and security loopholes more difficult to
initially detect and correct by the manufacturer. In conjunction
with these two factors, some brands of software are now widely
used, thus their bugs and security loopholes are often known to
users. With the widespread use of personal computers that lack
effective security mechanisms, it is relatively easy for
knowledgeable users to author malicious software and then dupe
unsuspecting users into copying it.
Steps Toward Reducing Risk
Organizations can take steps to reduce their risk to viruses and
related threats. Some of the more important steps are outlined
below.
o Include the damage potential of viruses, unauthorized
use, and related threats in risk analysis and contingency
planning. Develop a plan to deal with potential
incidents.
o Make computer security education a prerequisite to any
computer use. Teach users how to protect their systems
and detect evidence of tampering or unusual activity.
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EXECUTIVE SUMMARY
o Ensure that technically oriented security and management
staff are in place to deal with security incidents.
o Use the security mechanisms that exist in your current
software. Ensure that they are used correctly. Add to
them as necessary.
o Purchase and use software tools to aid in auditing
computing activity and detecting the presence of
tampering and damage.
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EXECUTIVE SUMMARY
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CHAPTER 1
1. Introduction
This document provides guidance for technical managers for the
reduction of risk to their computer systems and networks from
attack by computer viruses, unauthorized users, and related
threats. The guidance discusses the combined use of policies,
procedures, and controls to address security vulnerabilities that
can leave systems open to attack. The aim of this document is
not to provide solutions to the wide range of specific problems
or vulnerabilities, rather it is to help technical managers
administer their systems and networks such that manifestations of
viruses and related threats can be initially prevented, detected,
and contained.
1.1 Audience and Scope
This document is intended primarily for the managers of multi-
user systems, personal computers, and associated networks, and
managers of end-user groups. Additionally, the document is
useful for the users of such systems. The document presents an
overview of computer viruses and related threats, how they
typically work, the methods by which they can attack, and the
harm they can potentially cause. It then presents guidance in
the following areas:
o Multi-User Systems and Associated Networks - with
guidance directed at managers of medium to small
systems (as opposed to mainframes that already provide
generally effective security controls or are by their
nature more secure) and associated wide area and large
local area networks, as well as managers of end-users
of such systems
o Personal Computer Systems and Networks - guidance is
directed at those responsible for the management of
personal computers and personal computer networks, as
well as the managers of personal computer end-users
Within these general categories, individual computing
environments will vary widely, from size of computer to user
population to type of software and computing requirements. To
accommodate these differences, the guidance presented here is
general in nature. It attempts to address computer security
problems and vulnerabilities that are likely to be found in most
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computing environments. This document does not address problems
directly related to specific brands of software or hardware. A
reading list at the end of the document contains references and
pointers to other literature that address specific systems and
software.
Recommended control measures are grouped according to categories
that include general policies and procedures, education, software
management, technical controls, monitoring, and contingency
planning. The guidance emphasizes the need for a strong security
program as a means for protection from manifestations of viruses
and related threats, and as a means for providing detection,
containment, and recovery. Such a security program requires
personal involvement on the part of management to ensure that the
proper policies, procedures, and technical controls exist, and
that users are educated so that they can follow safe computing
practices and understand the proper actions to take if they
detect the presence of viruses or related threats. The
guidelines recommend that network managers, multi-user system
managers, end-users, and end-user managers work with each other
and approach virus protection from an organizationally consistent
basis.
1.2 How to Use This Guide
This document is divided into five chapters and two appendices.
Chapter 2 describes in general how viruses and related software
operate, the vulnerabilities they exploit, and how they can be
introduced into systems and networks. Chapter 3 discusses
general protection strategies and control measures that apply to
technical and end-user management in general; this is done so
that the same guidance need not be repeated for each of the
succeeding chapters that deal with specific environments.
Chapters 4 and 5 present guidance specific to multi-user and
personal computer environments, respectively. The guidance in
these chapters is directed at the respective technical managers
and managers of associated networks, as well as the managers of
end-user groups that use such systems and networks. It is
recommended that all readers, regardless of their management
perspective, examine Chapters 3, 4, and 5 to gain a fuller
appreciation of the whole environment with regard to threats,
vulnerabilities, and controls.
Appendix A contains document references, while Appendix B
contains a reading list with references to general and specific
information on various types of viruses, systems, and protective
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measures. Readers can use these documents to obtain information
specific to their individual systems and software.
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2. A Brief Overview on Viruses and Related Threats
The term computer virus is often used in a general sense to
indicate any software that can cause harm to systems or networks.
However, computer viruses are just one example of many different
but related forms of software that can act with great speed and
power to cause extensive damage -other important examples are
Trojan horses and network worms. In this document, the term
malicious software refers to such software.
2.1 Trojan Horses
A Trojan horse1 program is a useful or apparently useful program
or command procedure containing hidden code that, when invoked,
performs some unwanted function. An author of a Trojan horse
program might first create or gain access to the source code of a
useful program that is attractive to other users, and then add
code so that the program performs some harmful function in
addition to its useful function. A simple example of a Trojan
horse program might be a calculator program that performs
functions similar to that of a pocket calculator. When a user
invokes the program, it appears to be performing calculations and
nothing more, however it may also be quietly deleting the user's
files, or performing any number of harmful actions. An example
of an even simpler Trojan horse program is one that performs only
a harmful function, such as a program that does nothing but
delete files. However, it may appear to be a useful program by
having a name such as CALCULATOR or something similar to promote
acceptability.
Trojan horse programs can be used to accomplish functions
indirectly that an unauthorized user could not accomplish
directly. For example, a user of a multi-user system who wishes
to gain access to other users' files could create a Trojan horse
program to circumvent the users' file security mechanisms. The
Trojan horse program, when run, changes the invoking user's file
permissions so that the files are readable by any user. The
author could then induce users to run this program by placing it
in a common directory and naming it such that users will think
1 named after the use of a hollow wooden horse filled with
enemy soldiers used to gain entry into the city of Troy in
ancient Greece.
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the program is a useful utility. After a user runs the program,
the author can then access the information in the user's files,
which in this example could be important work or personal
information. Affected users may not notice the changes for long
periods of time unless they are very observant.
An example of a Trojan horse program that would be very difficult
to detect would be a compiler on a multi-user system that has
been modified to insert additional code into certain programs as
they are compiled, such as a login program. The code creates a
trap door in the login program which permits the Trojan horse's
author to log onto the system using a special password. Whenever
the login program is recompiled, the compiler will always insert
the trap door code into the program, thus the Trojan horse code
can never be discovered by reading the login program's source
code. For more information on this example, see [THOMPSON84].
Trojan horse programs are introduced into systems in two ways:
they are initially planted, and unsuspecting users copy and run
them. They are planted in software repositories that many people
can access, such as on personal computer network servers,
publicly-accessible directories in a multi-user environment, and
software bulletin boards. Users are then essentially duped into
copying Trojan horse programs to their own systems or
directories. If a Trojan horse program performs a useful
function and causes no immediate or obvious damage, a user may
continue to spread it by sharing the program with other friends
and co-workers. The compiler that copies hidden code to a login
program might be an example of a deliberately planted Trojan
horse that could be planted by an authorized user of a system,
such as a user assigned to maintain compilers and software tools.
2.2 Computer Viruses
Computer viruses, like Trojan horses, are programs that contain
hidden code which performs some usually unwanted function.
Whereas the hidden code in a Trojan horse program has been
deliberately placed by the program's author, the hidden code in a
computer virus program has been added by another program, that
program itself being a computer virus or Trojan horse. Thus,
computer viruses are programs that copy their hidden code to
other programs, thereby infecting them. Once infected, a program
may continue to infect even more programs. In due time, a
computer could be completely overrun as the viruses spread in a
geometric manner.
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An example illustrating how a computer virus works might be an
operating system program for a personal computer, in which an
infected version of the operating system exists on a diskette
that contains an attractive game. For the game to operate, the
diskette must be used to boot the computer, regardless of whether
the computer contains a hard disk with its own copy of the
(uninfected) operating system program. When the computer is
booted using the diskette, the infected program is loaded into
memory and begins to run. It immediately searches for other
copies of the operating system program, and finds one on the hard
disk. It then copies its hidden code to the program on the hard
disk. This happens so quickly that the user may not notice the
slight delay before his game is run. Later, when the computer is
booted using the hard disk, the newly infected version of the
operating system will be loaded into memory. It will in turn
look for copies to infect. However, it may also perform any
number of very destructive actions, such as deleting or
scrambling all the files on the disk.
A computer virus exhibits three characteristics: a replication
mechanism, an activation mechanism, and an objective. The
replication mechanism performs the following functions:
o searches for other programs to infect
o when it finds a program, possibly determines whether
the program has been previously infected by checking a
flag
o inserts the hidden instructions somewhere in the
program
o modifies the execution sequence of the program's
instructions such that the hidden code will be
executed whenever the program is invoked
o possibly creates a flag to indicate that the program
has been infected
The flag may be necessary because without it, programs could be
repeatedly infected and grow noticeably large. The replication
mechanism could also perform other functions to help disguise
that the file has been infected, such as resetting the program
file's modification date to its previous value, and storing the
hidden code within the program so that the program's size remains
the same.
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The activation mechanism checks for the occurrence of some event.
When the event occurs, the computer virus executes its objective,
which is generally some unwanted, harmful action. If the
activation mechanism checks for a specific date or time before
executing its objective, it is said to contain a time bomb. If
it checks for a certain action, such as if an infected program
has been executed a preset number of times, it is said to contain
a logic bomb. There may be any number of variations, or there
may be no activation mechanism other than the initial execution
of the infected program.
As mentioned, the objective is usually some unwanted, possibly
destructive event. Previous examples of computer viruses have
varied widely in their objectives, with some causing irritating
but harmless displays to appear, whereas others have erased or
modified files or caused system hardware to behave differently.
Generally, the objective consists of whatever actions the author
has designed into the virus.
As with Trojan horse programs, computer viruses can be introduced
into systems deliberately and by unsuspecting users. For
example, a Trojan horse program whose purpose is to infect other
programs could be planted on a software bulletin board that
permits users to upload and download programs. When a user
downloads the program and then executes it, the program proceeds
to infect other programs in the user's system. If the computer
virus hides itself well, the user may continue to spread it by
copying the infected program to other disks, by backing it up,
and by sharing it with other users. Other examples of how
computer viruses are introduced include situations where
authorized users of systems deliberately plant viruses, often
with a time bomb mechanism. The virus may then activate itself
at some later point in time, perhaps when the user is not logged
onto the system or perhaps after the user has left the
organization. For more information on computer viruses, see
[DENNING88]
2.3 Network Worms
Network worm programs use network connections to spread from
system to system, thus network worms attack systems that are
linked via communications lines. Once active within a system, a
network worm can behave as a computer virus, or it could implant
Trojan horse programs or perform any number of disruptive or
destructive actions. In a sense, network worms are like computer
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viruses with the ability to infect other systems as well as other
programs. Some people use the term virus to include both cases.
To replicate themselves, network worms use some sort of network
vehicle, depending on the type of network and systems. Examples
of network vehicles include (a) a network mail facility, in which
a worm can mail a copy of itself to other systems, or (b), a
remote execution capability, in which a worm can execute a copy
of itself on another system, or (c) a remote login capability,
whereby a worm can log into a remote system as a user and then
use commands to copy itself from one system to the other. The
new copy of the network worm is then run on the remote system,
where it may continue to spread to more systems in a like manner.
Depending on the size of a network, a network worm can spread to
many systems in a relatively short amount of time, thus the
damage it can cause to one system is multiplied by the number of
systems to which it can spread.
A network worm exhibits the same characteristics as a computer
virus: a replication mechanism, possibly an activation mechanism,
and an objective. The replication mechanism generally performs
the following functions:
o searches for other systems to infect by examining host
tables or similar repositories of remote system
addresses
o establishes a connection with a remote system,
possibly by logging in as a user or using a mail
facility or remote execution capability
o copies itself to the remote system and causes the copy
to be run
The network worm may also attempt to determine whether a system
has previously been infected before copying itself to the system.
In a multi-tasking computer, it may also disguise its presence by
naming itself as a system process or using some other name that
may not be noticed by a system operator.
The activation mechanism might use a time bomb or logic bomb or
any number of variations to activate itself. Its objective, like
all malicious software, is whatever the author has designed into
it. Some network worms have been designed for a useful purpose,
such as to perform general "house-cleaning" on networked systems,
or to use extra machine cycles on each networked system to
perform large amounts of computations not practical on one
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system. A network worm with a harmful objective could perform a
wide range of destructive functions, such as deleting files on
each affected computer, or by implanting Trojan horse programs or
computer viruses.
Two examples of actual network worms are presented here. The
first involved a Trojan horse program that displayed a Christmas
tree and a message of good cheer (this happened during the
Christmas season). When a user executed this program, it
examined network information files which listed the other
personal computers that could receive mail from this user. The
program then mailed itself to those systems. Users who received
this message were invited to run the Christmas tree program
themselves, which they did. The network worm thus continued to
spread to other systems until the network was nearly saturated
with traffic. The network worm did not cause any destructive
action other than disrupting communications and causing a loss in
productivity [BUNZEL88].
The second example concerns the incident whereby a network worm
used the collection of networks known as the Internet to spread
itself to several thousands of computers located throughout the
United States. This worm spread itself automatically, employing
somewhat sophisticated techniques for bypassing the systems'
security mechanisms. The worm's replication mechanism accessed
the systems by using one of three methods:
o it employed password cracking, in which it attempted
to log into systems using usernames for passwords, as
well as using words from an on-line dictionary
o it exploited a trap door mechanism in mail programs
which permitted it to send commands to a remote
system's command interpreter
o it exploited a bug in a network information program
which permitted it to access a remote system's command
interpreter
By using a combination of these methods, the network worm was
able to copy itself to different brands of computers which used
similar versions of a widely-used operating system. Many system
managers were unable to detect its presence in their systems,
thus it spread very quickly, affecting several thousands of
computers within two days. Recovery efforts were hampered
because many sites disconnected from the network to prevent
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further infections, thus preventing those sites from receiving
network mail that explained how to correct the problems.
It was unclear what the network worm's objective was, as it did
not destroy information, steal passwords, or plant viruses or
Trojan horses. The potential for destruction was very high, as
the worm could have contained code to effect many forms of
damage, such as to destroy all files on each system. For more
information, see [DENNING89] and [SPAFFORD88].
2.4 Other Related Software Threats
The number of variations of Trojan horses, computer viruses, and
network worms is apparently endless. Some have names, such as a
rabbit, whose objective is to spread wildly within or among other
systems and disrupt network traffic, or a bacterium, whose
objective is to replicate within a system and eat up processor
time until computer throughput is halted [DENNING88]. It is
likely that many new forms will be created, employing more
sophisticated techniques for spreading and causing damage.
2.5 The Threat of Unauthorized Use
In that computer viruses and related forms of malicious software
are intriguing issues in themselves, it is important not to
overlook that they are created by people, and are fundamentally a
people problem. In essence, examples of malicious software are
tools that people use to extend and enhance their ability to
create mischief and various other forms of damage. Such software
can do things that the interactive user often cannot directly
effect, such as working with great speed, or maintaining
anonymity, or doing things that require programmatic system
calls. But in general, malicious software exploits the same
vulnerabilities as can knowledgeable users. Thus, any steps
taken to reduce the likelihood of attack by malicious software
should address the likelihood of unauthorized use by computer
users.
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3. Virus Prevention in General
To provide general protection from attacks by computer viruses,
unauthorized users, and related threats, users and managers need
to eliminate or reduce vulnerabilities. A general summary of the
vulnerabilities that computer viruses and related threats are
most likely to exploit is as follows:
o lack of user awareness - users copy and share infected
software, fail to detect signs of virus activity, do not
understand proper security techniques
o absence of or inadequate security controls - personal
computers generally lack software and hardware security
mechanisms that help to prevent and detect unauthorized
use, existing controls on multi-user systems can
sometimes be surmounted by knowledgeable users
o ineffective use of existing security controls - using
easily guessed passwords, failing to use access controls,
granting users more access to resources than necessary
o bugs and loopholes in system software - enabling
knowledgeable users to break into systems or exceed their
authorized privileges
o unauthorized use - unauthorized users can break in to
systems, authorized users can exceed levels of privilege
and misuse systems
o susceptibility of networks to misuse - networks can
provide anonymous access to systems, many are in general
only as secure as the systems which use them
As can be seen from this summary, virus prevention requires that
many diverse vulnerabilities be addressed. Some of the
vulnerabilities can be improved upon significantly, such as
security controls that can be added or improved, while others are
somewhat inherent in computing, such as the risk that users will
not use security controls or follow policies, or the risk of
unauthorized use of computers and networks. Thus, it may not be
possible to completely protect systems from all virus-like
attacks. However, to attain a realistic degree of protection,
all areas of vulnerability must be addressed; improving upon some
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areas at the expense of others will still leave significant holes
in security.
To adequately address all areas of vulnerability, the active
involvement of individual users, the management structure, and
the organization in a virus prevention program is essential.
Such a program, whether formal or informal, depends on the mutual
cooperation of the three groups to identify vulnerabilities, to
take steps to correct them, and to monitor the results.
A virus prevention program must be initially based upon effective
system computer administration that restricts access to
authorized users, ensures that hardware and software are
regularly monitored and maintained, makes backups regularly, and
maintains contingency procedures for potential problems. Sites
that do not maintain a basic computer administration program need
to put one into place, regardless of their size or the types of
computers used. Many system vendors supply system administration
manuals that describe the aspects of a basic program, and one can
consult documents such as [FIPS73], or [NBS120].
Once a basic administration program is in place, management and
users need to incorporate virus prevention measures that will
help to deter attacks by viruses and related threats, detect when
they occur, contain the attacks to limit damage, and recover in a
reasonable amount of time without loss of data. To accomplish
these aims, attention needs to be focused on the following areas:
o educating users about malicious software in general, the
risks that it poses, how to use control measures,
policies, and procedures to protect themselves and the
organization
o software management policies and procedures that address
public-domain software, and the use and maintenance of
software in general
o use of technical controls that help to prevent and deter
attacks by malicious software and unauthorized users
o monitoring of user and software activity to detect signs
of attacks, to detect policy violations, and to monitor
the overall effectiveness of policies, procedures, and
controls
o contingency policies and procedures for containing and
recovering from attacks
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General guidance in each of these areas is explained in the
following sections.
3.1 User Education
Education is one of the primary methods by which systems and
organizations can achieve greater protection from incidents of
malicious software and unauthorized use. In situations where
technical controls do not provide complete protection (i.e., most
computers), it is ultimately people and their willingness to
adhere to security policies that will determine whether systems
and organizations are protected. By educating users about the
general nature of computer viruses and related threats, an
organization can improve its ability to deter, detect, contain
and recover from potential incidents.
Users should be educated about the following:
o how malicious software operates, methods by which it is
planted and spread, the vulnerabilities exploited by
malicious software and unauthorized users
o general security policies and procedures and how to use
them
o the policies to follow regarding the backup, storage, and
use of software, especially public-domain software and
shareware
o how to use the technical controls they have at their
disposal to protect themselves
o how to monitor their systems and software to detect signs
of abnormal activity, what to do or whom to contact for
more information
o contingency procedures for containing and recovering from
potential incidents
User education, while perhaps expensive in terms of time and
resources required, is ultimately a cost-effective measure for
protecting against incidents of malicious software and
unauthorized use. Users who are better acquainted with the
destructive potential of malicious software and the methods by
which it can attack systems may in turn be prompted to take
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measures to protect themselves. The purpose of security policies
and procedures will be more clear, thus users may be more willing
to actively use them. By educating users how to detect abnormal
system activity and the resultant steps to follow for containing
and recovering from potential incidents, organizations will save
money and time if and when actual incidents occur.
3.2 Software Management
As shown by examples in Chapter 2, one of the prime methods by
which malicious software is initially copied onto systems is by
unsuspecting users. When users download programs from sources
such as software bulletin boards, or public directories on
systems or network servers, or in general use and share software
that has not been obtained from a reputable source, users are in
danger of spreading malicious software. To prevent users from
potentially spreading malicious software, managers need to
o ensure that users understand the nature of malicious
software, how it is generally spread, and the technical
controls to use to protect themselves
o develop policies for the downloading and use of public-
domain and shareware software
o create some mechanism for validating such software prior
to allowing users to copy and use it
o minimize the exchange of executable software within an
organization as much as possible
o do not create software repositories on LAN servers or in
multi-user system directories unless technical controls
exist to prevent users from freely uploading or
downloading the software
The role of education is important, as users who do not
understand the risks yet who are asked to follow necessarily
restrictive policies may share and copy software anyway. Where
technical controls cannot prevent placing new software onto a
system, users are then primarily responsible for the success or
failure of whatever policies are developed.
A policy that prohibits any copying or use of public-domain
software may be overly restrictive, as some public domain
programs have proved to be useful. A less restrictive policy
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would allow some copying, however a user might first require
permission from the appropriate manager. A special system should
be used from which to perform the copy and then to test the
software. This type of system, called an isolated system, should
be configured so that there is no risk of spreading a potentially
malicious program to other areas of an organization. The system
should not be used by other users, should not connect to
networks, and should not contain any valuable data. An isolated
system should also be used to test internally developed software
and updates to vendor software.
Other policies for managing vendor software should be developed.
These policies should control how and where software is
purchased, and should govern where the software is installed and
how it is to be used. The following policies and procedures are
suggested:
o purchase vendor software only from reputable sources
o maintain the software properly and update it as necessary
o don't use pirated software, as it may have been modified
o keep records of where software is installed readily
available for contingency purposes
o ensure that vendors can be contacted quickly if problems
occur
o store the original disks or tapes from the vendor in a
secure location
3.3 Technical Controls
Technical controls are the mechanisms used to protect the
security and integrity of systems and associated data. The use
of technical controls can help to prevent occurrences of viruses
and related threats by deterring them or making it more difficult
for them to gain access to systems and data. Examples of
technical controls include user authentication mechanisms such as
passwords, mechanisms which provide selective levels of access to
files and directories (read-only, no access, access to certain
users, etc.), and write-protection mechanisms on tapes and
diskettes.
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The different types of technical controls and the degree to which
they can provide protection and deterrence varies from system to
system, thus the use of specific types of controls is discussed
in Chapters 4 and 5. However, the following general points are
important to note:
o technical controls should be used as available to
restrict system access to authorized users only
o in the multi-user environment, technical controls should
be used to limit users' privileges to the minimum
practical level; they should work automatically and need
not be initiated by users
o users and system managers must be educated as to how and
when to use technical controls
o where technical controls are weak or non-existent (i.e.,
personal computers), they should be supplemented with
alternative physical controls or add-on control
mechanisms
Managers need to determine which technical controls are available
on their systems, and then the degree to which they should be
used and whether additional add-on controls are necessary. One
way to answer these questions is to first categorize the
different classes of data being processed by a system or systems,
and then to rank the categories according to criteria such as
sensitivity to the organization and vulnerability of the system
to attack. The rankings should then help determine the degree to
which the controls should be applied and whether additional
controls are necessary. Ideally, those systems with the most
effective controls should be used to process the most sensitive
data, and vice-versa. As an example, a personal computer which
processes sensitive employee information should require add-on
user authentication mechanisms, whereas a personal computer used
for general word processing may not need additional controls.
It is important to note that technical controls do not generally
provide complete protection against viruses and related threats.
They may be cracked by determined users who are knowledgeable of
hidden bugs and weaknesses, and they may be surmounted through
the use of Trojan horse programs, as shown by examples in Chapter
2. An inherent weakness in technical controls is that, while
deterring users and software from objects to which they do not
have access, they may be totally ineffective against attacks
which target objects that are accessible. For example, technical
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controls may not prevent an authorized user from destroying files
to which the user has authorized access. Most importantly, when
technical controls are not used properly, they may increase a
system's degree of vulnerability. It is generally agreed that
fully effective technical controls will not be widely available
for some time. Because of the immediate nature of the computer
virus threat, technical controls must be supplemented by less
technically-oriented control measures such as described in this
chapter.
3.4 General Monitoring
An important aspect of computer viruses and related threats is
that they potentially can cause extensive damage within a very
small amount of time, such as minutes or seconds. Through proper
monitoring of software, system activity, and in some cases user
activity, managers can increase their chances that they will
detect early signs of malicious software and unauthorized
activity. Once the presence is noted or suspected, managers can
then use contingency procedures to contain the activity and
recover from whatever damage has been caused. An additional
benefit of general monitoring is that over time, it can aid in
determining the necessary level or degree of security by
indicating whether security policies, procedures, and controls
are working as planned.
Monitoring is a combination of continual system and system
management activity. Its effectiveness depends on cooperation
between management and users. The following items are necessary
for effective monitoring:
o user education - users must know, specific to their
computing environment, what constitutes normal and
abnormal system activity and whom to contact for further
information - this is especially important for users of
personal computers, which generally lack automated
methods for monitoring
o automated system monitoring tools - generally on multi-
user systems, to automate logging or accounting of user
and software accesses to accounts, files, and other
system objects - can sometimes be tuned to record only
certain types of accesses such as "illegal" accesses
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o anti-viral software - generally on personal computers,
these tools alert users of certain types of system access
that are indicative of "typical" malicious software
o system-sweep programs - programs to automatically check
files for changes in size, date, or content
o network monitoring tools - as with system monitoring
tools, to record network accesses or attempts to access
The statistics gained from monitoring activities should be used
as input for periodic reviews of security programs. The reviews
should evaluate the effectiveness of general system management,
and associated security policies, procedures, and controls. The
statistics will indicate the need for changes and will help to
fine tune the program so that security is distributed to where it
is most necessary. The reviews should also incorporate users'
suggestions, and to ensure that the program is not overly
restrictive, their criticisms.
3.5 Contingency Planning
The purpose of contingency planning with regard to computer
viruses and related threats is to be able to contain and recover
completely from actual attacks. In many ways, effective system
management that includes user education, use of technical
controls, software management, and monitoring activities, is a
form of contingency planning, generally because a well-run,
organized system or facility is better able to withstand the
disruption that could result from a computer virus attack. In
addition to effective system management activities, managers need
to consider other contingency procedures that specifically take
into account the nature of computer viruses and related threats.
Possibly the most important contingency planning activity
involves the use of backups. The ability to recover from a virus
attack depends upon maintaining regular, frequent backups of all
system data. Each backup should be checked to ensure that the
backup media has not been corrupted. Backup media could easily
be corrupted because of defects, because the backup procedure was
incorrect, or perhaps because the backup software itself has been
attacked and modified to corrupt backups as they are made.
Contingency procedures for restoring from backups after a virus
attack are equally important. Backups may contain copies of
malicious software that have been hiding in the system.
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Restoring the malicious software to a system that has been
attacked could cause a recurrence of the problem. To avoid this
possibility, software should be restored only from its original
media: the tapes or diskettes from the vendor. In some cases,
this may involve reconfiguring the software, therefore managers
must maintain copies of configuration information for system and
application software. Because data is not directly executable,
it can be restored from routine backups. However, data that has
been damaged may need to be restored manually or from older
backups. Command files such as batch procedures and files
executed when systems boot or when user log on should be
inspected to ensure that they have not been damaged or modified.
Thus, managers will need to retain successive versions of
backups, and search through them when restoring damaged data and
command files.
Other contingency procedures for containing virus attacks need to
be developed. The following are suggested; they are discussed in
more detail in Chapters 4 and 5:
o ensure that accurate records are kept of each system's
configuration, including the system's location, the
software it runs, the system's network and modem
connections, and the name of the system's manager or
responsible individual
o create a group of skilled users to deal with virus
incidents and ensure that users can quickly contact this
group if they suspect signs of viral activity
o maintain a security distribution list at each site with
appropriate telephone numbers of managers to contact when
problems occur
o isolate critical systems from networks and other sources
of infection
o place outside network connections on systems with the
best protections, use central gateways to facilitate
rapid disconnects
4. Virus Prevention for Multi-User Computers
and Associated Networks
Virus prevention in the multi-user computer environment is aided
by the centralized system and user management, and the relative
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richness of technical controls. Unlike personal computers, many
multi-user systems possess basic controls for user
authentication, for levels of access to files and directories,
and for protected regions of memory. By themselves, these
controls are not adequate, but combined with other policies and
procedures that specifically target viruses and related threats,
multi-user systems can greatly reduce their vulnerabilities to
exploitation and attack.
However, some relatively powerful multi-user machines are now so
compact as to be able to be located in an office or on a desk-
top. These machines are still fully able to support a small user
population, to connect to major networks, and to perform complex
real-time operations. But due to their size and increased ease
of operation, they are more vulnerable to unauthorized access.
Also, multi-user machines are sometimes managed by untrained
personnel who do not have adequate time to devote to proper
system management and who may not possess a technical background
or understanding of the system's operation. Thus, it is
especially important for organizations who use or are considering
machines of this nature to pay particular attention to the risks
of attack by unauthorized users, viruses, and related software.
The following sections offer guidance and recommendations for
improving the management and reducing the risk of attack for
multi-user computers and associated networks.
4.1 General Policies
Two general policies are suggested here. They are intended for
uniform adoption throughout an organization, i.e., they will not
be entirely effective if they are not uniformly followed. These
policies are as follows:
o An organization must assign a dedicated system manager to
operate each multi-user computer. The manager should be
trained, if necessary, to operate the system in a
practical and secure manner. This individual should be
assigned the management duties as part of his job
description; the management duties should not be assigned
"on top" of the individual's other duties, but rather
adequate time should be taken from other duties. System
management is a demanding and time-consuming operation
that can unexpectedly require complete dedication. As
systems are increasingly inter-connected via networks, a
poorly managed system that can be used as a pathway for
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unauthorized access to other systems will present a
significant vulnerability to an organization. Thus, the
job of system manager should be assigned carefully, and
adequate time be given so that the job can be performed
completely.
o Management needs to impress upon users the need for their
involvement and cooperation in computer security. A
method for doing this is to create an organizational
security policy. This policy should be a superset of all
other computer-related policy, and should serve to
clearly define what is expected of the user. It should
detail how systems are to be used and what sorts of
computing are permitted and not permitted. Users should
read this policy and agree to it as a prerequisite to
computer use. It would also be helpful to use this
policy to create other policies specific to each multi-
user system.
4.2 Software Management
Effective software management can help to make a system less
vulnerable to attack and can make containment and recovery more
successful. Carefully controlled access to software will prevent
or discourage unauthorized access. If accurate records and
backups are maintained, software restoral can be accomplished
with a minimum of lost time and data. A policy of testing all
new software, especially public-domain software, will help
prevent accidental infection of a system by viruses and related
software. Thus, the following policies and procedures are
recommended:
o Use only licensed copies of vendor software, or software
that can be verified to be free of harmful code or other
destructive aspects. Maintain complete information about
the software, such as the vendor address and telephone
number, the license number and version, and update
information. Store the software in a secure, tamper-
proof location.
o Maintain configuration reports of all installed software,
including the operating system. This information will be
necessary if the software must be re-installed later.
o Prevent user access to system software and data. Ensure
that such software is fully protected, and that
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appropriate monitoring is done to detect attempts at
unauthorized access.
o Prohibit users from installing software. Users should
first contact the system manager regarding new software.
The software should then be tested on an isolated system
to determine whether the software may contain destructive
elements. The isolated system should be set up so that,
to a practical degree, it replicates the target system,
but does not connect to networks or process sensitive
data. A highly-skilled user knowledgeable about viruses
and related threats should perform the testing and ensure
that the software does not change or delete other
software or data. Do not allow users to directly add any
software to the system, whether from public software
repositories, or other systems, or their home systems.
o Teach users to protect their data from unauthorized
access. Ensure that they know how to use access controls
or file protection mechanisms to prevent others from
reading or modifying their files. As possible, set
default file protections such that when a user creates a
file, the file can be accessed only by that user, and no
others. Each user should not permit others to use his or
her account.
o Do not set-up directories to serve as software
repositories unless technical controls are used to
prevent users from writing to the directory. Make sure
that users contact the system manager regarding software
they wish to place in a software repository. It would be
helpful to track where the software is installed by
setting up a process whereby users must first register
their names before they can copy software from the
directory.
o If developing software, control the update process so
that the software is not modified without authorization.
Use a software management and control application to
control access to the software and to automate the
logging of modifications.
o Accept system and application bug fixes or patches only
from highly reliable sources, such as the software
vendor. Do not accept patches from anonymous sources,
such as received via a network. Test the new software on
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an isolated system to ensure that the software does not
make an existing problem worse.
4.3 Technical Controls
Many multi-user computers contain basic built-in technical
controls. These include user authentication via passwords,
levels of user privilege, and file access controls. By using
these basic controls effectively, managers can significantly
reduce the risk of attack by preventing or deterring viruses and
related threats from accessing a system.
Perhaps the most important technical control is user
authentication, with the most widely form of user authentication
being a username associated with a password. Every user account
should use a password that is deliberately chosen so that simple
attempts at password cracking cannot occur. An effective
password should not consist of a person's name or a recognizable
word, but rather should consist of alphanumeric characters and/or
strings of words that cannot easily be guessed. The passwords
should be changed at regular intervals, such as every three to
six months. Some systems include or can be modified to include a
password history, to prevent users from reusing old passwords.
For more information on effective password practices, see
[FIPS73].
The username/password mechanism can sometimes be modified to
reduce opportunities for password cracking. One method is to
increase the running time of the password encryption to several
seconds. Another method is to cause the user login program to
accept from three to five incorrect password attempts in a row
before disabling the user account for several minutes. Both
methods significantly increase the amount of time a password
cracker would spend when making repeated attempts at guessing a
password. A method for ensuring that passwords are difficult to
crack involves the use of a program that could systematically
guess passwords, and then send warning messages to the system
manager and corresponding users if successful. The program could
attempt passwords that are permutations of each user's name, as
well as using words from an on-line dictionary.
Besides user authentication, access control mechanisms are
perhaps the next most important technical control. Access
control mechanisms permit a system manager to selectively permit
or bar user access to system resources regardless of the user's
level of privilege. For example, a user at a low-level of system
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privilege can be granted access to a resource at a higher level
of privilege without raising the user's privilege through the use
of an access control that specifically grants that user access.
Usually, the access control can determine the type of access,
e.g., read or write. Some access controls can send alarm
messages to audit logs or the system manager when unsuccessful
attempts are made to access resources protected by an access
control.
Systems which do not use access controls usually contain another
more basic form that grants access based on user categories.
Usually, there are four: owner, where only the user who "owns" or
creates the resource can access it; group, where anyone in the
same group as the owner can access the resource; world, where all
users can access the resource, and system, which supersedes all
other user privileges. Usually, a file or directory can be set
up to allow any combination of the four. Unlike access controls,
this scheme doesn't permit access to resources on a specific user
basis, thus if a user at a low level of privilege requires access
to a system level resource, the user must be granted system
privilege. However, if used carefully, this scheme can
adequately protect users' files from being accessed without
authorization. The most effective mode is to create a unique
group for each user. Some systems may permit a default file
permission mask to be set so that every file created would be
accessible only by the file's owner.
Other technical control guidelines are as follows:
o Do not use the same password on several systems.
Additionally, sets of computers that are mutually
trusting in the sense that login to one constitutes login
to all should be carefully controlled.
o Disable or remove old or unnecessary user accounts.
Whenever users leave an organization or no longer use a
system, change all passwords that the users had knowledge
of.
o Practice a "least privilege" policy, whereby users are
restricted to accessing resources on a need-to-know basis
only. User privileges should be as restricting as
possible without adversely affecting the performance of
their work. To determine what level of access is
required, err first by setting privileges to their most
restrictive, and upgrade them as necessary. If the
system uses access controls, attempt to maintain a user's
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system privileges at a low level while using the access
controls to specifically grant access to the required
resources.
o Users are generally able to determine other users' access
to their files and directories, thus instruct users to
carefully maintain their files and directories such that
they are not accessible, or at a minimum, not writable,
by other users. As possible, set default file
protections such that files and directories created by
each user are accessible by only that user.
o When using modems, do not provide more access to the
system than is necessary. For example, if only dial-out
service is required, set up the modem or telephone line
so that dial-in service is not possible. If dial-in
service is necessary, use modems that require an
additional passwords or modems that use a call-back
mechanism. These modems may work such that a caller must
first identify himself to the system. If the
identification has been pre-recorded with the system and
therefore valid, the system then calls back at a pre-
recorded telephone number.
o If file encryption mechanisms are available, make them
accessible to users. Users may wish to use encryption as
a further means of protecting the confidentiality of
their files, especially if the system is accessible via
networks or modems.
o Include software so that users can temporarily "lock"
their terminals from accepting keystrokes while they are
away. Use software that automatically disables a user's
account if no activity occurs after a certain interval,
such as 10 - 15 minutes.
4.4 Monitoring
Many multi-user systems provide a mechanism for automatically
recording some aspects of user and system activity. This
monitoring mechanism, if used regularly, can help to detect
evidence of viruses and related threats. Early detection is of
great value, because malicious software potentially can cause
significant damage within a matter of minutes. Once evidence of
an attack has been verified, managers can use contingency
procedures to contain and recover from any resultant damage.
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Effective monitoring also requires user involvement, and
therefore, user education. Users must have some guidelines for
what constitutes normal and abnormal system activity. They need
to be aware of such items as whether files have been changed in
content, date, or by access permissions, whether disk space has
become suddenly full, and whether abnormal error messages occur.
They need to know whom to contact to report signs of trouble and
then the steps to take to contain any damage.
The following policies and procedures for effective monitoring
are recommended:
o Use the system monitoring/auditing tools that are
available. Follow the procedures recommended by the
system vendor, or start out by enabling the full level or
most detailed level of monitoring. Use tools as
available to help read the logs, and determine what level
of monitoring is adequate, and cut back on the level of
detail as necessary. Be on the guard for excessive
attempts to access accounts or other resources that are
protected. Examine the log regularly, at least weekly if
not more often.
o As a further aid to monitoring, use alarm mechanisms
found in some access controls. These mechanisms send a
message to the audit log whenever an attempt is made to
access a resource protected by an access control.
o If no system monitoring is available, or if the present
mechanism is unwieldy or not sufficient, investigate and
purchase other monitoring tools as available. Some
third-party software companies sell monitoring tools for
major operating systems with capabilities that supersede
those of the vendor's.
o Educate users so that they understand the normal
operating aspects of the system. Ensure that they have
quick access to an individual or group who can answer
their questions and investigate potential virus
incidents.
o Purchase or build system sweep programs to checksum files
at night, and report differences from previous runs. Use
a password checker to monitor whether passwords are being
used effectively.
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o Always report, log, and investigate security problems,
even when the problems appear insignificant. Use the log
as input into regular security reviews. Use the reviews
as a means for evaluating the effectiveness of security
policies and procedures.
o Enforce some form of sanctions against users who
consistently violate or attempt to violate security
policies and procedures. Use the audit logs as evidence,
and bar the users from system use.
4.5 Contingency Planning
As stressed in Chapter 3, backups are the most important
contingency planning activity. A system manager must plan for
the eventuality of having to restore all software and data from
backup tapes for any number of reasons, such as disk drive
failure or upgrades. It has been shown that viruses and related
threats could potentially and unexpectedly destroy all system
information or render it useless, thus managers should pay
particular attention to the effectiveness of their backup
policies. Backup policies will vary from system to system,
however they should be performed daily, with a minimum of several
months backup history. Backup tapes should be verified to be
accurate, and should be stored off-site in a secured location.
Viruses and related software threats could go undetected in a
system for months to years, and thus could be backed up along
with normal system data. If such a program would suddenly
trigger and cause damage, it may require much searching through
old backups to determine when the program first appeared or was
infected. Therefore the safest policy is to restore programs,
i.e., executable and command files, from their original vendor
media only. Only system data that is non-executable should be
restored from regular backups. Of course, in the case of command
files or batch procedures that are developed or modified in the
course of daily system activity, these may need to be inspected
manually to ensure that they have not been modified or damaged.
Other recommended contingency planning activities are as follows:
o Create a security distribution list for hand-out to each
user. The list should include the system manager's name
and number, and other similar information for individuals
who can answer users' questions about suspicious or
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unusual system activity. The list should indicate when
to contact these individuals, and where to reach them in
emergencies.
o Coordinate with other system managers, especially if
their computers are connected to the same network.
Ensure that all can be contacted quickly in the event of
a network emergency by using some mechanism other than
the network.
o Besides observing physical security for the system as
well as its software and backup media, locate terminals
in offices that can be locked or in other secure areas.
o If users are accessing the system via personal computers
and terminal emulation software, keep a record of where
the personal computers are located and their network or
port address for monitoring purposes. Control carefully
whether such users are uploading software to the system.
o Exercise caution when accepting system patches. Do not
accept patches that arrive over a network unless there is
a high degree of certainty as to their validity. It is
best to accept patches only from the appropriate software
vendor.
4.6 Associated Network Concerns
Multi-user computers are more often associated with relatively
large networks than very localized local area networks or
personal computer networks that may use dedicated network
servers. The viewpoint taken here is that wide area network and
large local area network security is essentially a collective
function of the systems connected to the network, i.e., it is not
practical for a controlling system to monitor all network traffic
and differentiate between authorized and unauthorized use. A
system manager should generally assume that network connections
pose inherent risks of unauthorized access to the system in the
forms of unauthorized users and malicious software. Thus, a
system manager needs to protect the system from network-borne
threats and likewise exercise responsibility by ensuring that his
system is not a source of such threats, while at the same time
making network connections available to users as necessary. The
accomplishment of these aims will require the use of technical
controls to restrict certain types of access, monitoring to
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detect violations, and a certain amount of trust that users will
use the controls and follow the policies.
Some guidelines for using networks in a more secure manner are as
follows:
o Assume that network connections elevate the risk of
unauthorized access. Place network connections on system
which provide adequate controls, such as strong user
authentication and access control mechanisms. Avoid
placing network connections on system which process
sensitive data.
o If the system permits, require an additional password or
form of authentication for accounts accessed from network
ports. If possible, do not permit access to system
manager accounts from network ports.
o If anonymous or guest accounts are used, place
restrictions on the types of commands that can be
executed from the account. Don't permit access to
software tools, commands that can increase privileges,
and so forth.
o As possible, monitor usage of the network. Check if
network connections are made at odd hours, such as during
the night, or if repeated attempts are made to log in to
the system from a network port.
o When more than one computer is connected to the same
network, arrange the connections so that one machine
serves as a central gateway for the other machines. This
will allow a rapid disconnect from the network in case of
an attack.
o Ensure that users are fully educated in network usage.
Make them aware of the additional risks involved in
network access. Instruct them to be on the alert for any
signs of tampering, and to contact an appropriate person
if they detect any suspicious activity. Create a policy
for responsible network usage that details what sort of
computing activity will and will not be tolerated. Have
users read the policy as a prerequisite to network use.
o Warn users to be suspicious of any messages that are
received from unidentified or unknown sources.
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o Don't advertise a system to network users by printing
more information than necessary on a welcome banner. For
example, don't include messages such as "Welcome to the
Payroll Accounting System" that may cause the system to
be more attractive to unauthorized users.
o Don't network to outside organizations without a mutual
review of security practices
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5. Virus Prevention for Personal Computers
and Associated Networks
Virus prevention in the personal computer environment differs
from that of the multi-user computer environment mainly in the
following two respects: the relative lack of technical controls,
and the resultant emphasis this places on less-technically
oriented means of protection which necessitates more reliance on
user involvement. Personal computers typically do not provide
technical controls for such things as user authorization, access
controls, or memory protection that differentiates between system
memory and memory used by user applications. Because of the lack
of controls and the resultant freedom with which users can share
and modify software, personal computers are more prone to attack
by viruses, unauthorized users, and related threats.
Virus prevention in the personal computer environment must rely
on continual user awareness to adequately detect potential
threats and then to contain and recover from the damage.
Personal computer users are in essence personal computer
managers, and must practice their management as a part of their
general computing. Personal computers generally do not contain
auditing features, thus a user needs to be aware at all times of
the computer's performance, i.e., what it is doing, or what is
normal or abnormal activity. Ultimately, personal computer users
need to understand some of the technical aspects of their
computers in order to protect, deter, contain, and recover. Not
all personal computer users are technically oriented, thus this
poses some problems and places even more emphasis on user
education and involvement in virus prevention.
Because of the dependance on user involvement, policies for the
personal computer environment are more difficult to implement
than in the multi-user computer environment. However,
emphasizing these policies as part of a user education program
will help to ingrain them in users' behavior. Users should be
shown via examples what can happen if they don't follow the
policies. An example where users share infected software and
then spread the software throughout an organization would serve
to effectively illustrate the point, thus making the purpose of
the policy more clear and more likely to be followed. Another
effective method for increasing user cooperation is to create a
list of effective personal computer management practices specific
to each personal computing environment. Creating such a list
would save users the problem of determining how best to enact the
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policies, and would serve as a convenient checklist that users
could reference as necessary.
It will likely be years before personal computers incorporate
strong technical controls in their architectures. In the
meantime, managers and users must be actively involved in
protecting their computers from viruses and related threats. The
following sections provide guidance to help achieve that aim.
5.1 General Policies
Two general policies are suggested here. The first requires that
management make firm, unambiguous decisions as to how users
should operate personal computers, and state that policy in
writing. This policy will be a general re-statement of all other
policies affecting personal computer use. It is important that
users read this policy and agree to its conditions as a
prerequisite to personal computer use. The purposes of the
policy are to (1) ensure that users are aware of all policies,
and (2) impress upon users the need for their active involvement
in computer security.
The second policy is that every personal computer should have an
"owner" or "system manager" who is responsible for the
maintenance and security of the computer, and for following all
policies and procedures associated with the use of the computer.
It would be preferable that the primary user of the computer fill
this role. It would not be too extreme to make this
responsibility a part of the user's job description. This policy
will require that resources be spent on educating users so that
they can adequately follow all policies and procedures.
5.2 Software Management
Due to the wide variety of software available for many types of
personal computers, it is especially important that software be
carefully controlled. The following policies are suggested:
o Use only licensed copies of vendor software for personal
computers. Ensure that the license numbers are logged,
that warranty information is completed, and that updates
or update notices will be mailed to the appropriate
users. Ensure that software versions are uniform on all
personal computers. Purchase software from known,
reputable sources - do not purchase software that is
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CHAPTER 5
priced suspiciously low and do not use pirated software,
even on a trial basis. As possible, buy software with
built-in security features.
o Do not install software that is not clearly needed. For
example, software tools such as compilers or debuggers
should not be installed on machines where they are not
needed.
o Store the original copies of vendor software in a secure
location for use when restoring the software.
o Develop a clear policy for use of public-domain software
and shareware. It is recommended that the policy
prohibit indiscriminate downloading from software
bulletin boards. A special isolated system should be
configured to perform the downloading, as well as for
testing downloaded and other software or shareware. The
operation of the system should be managed by a
technically skilled user who can use anti-virus software
and other techniques to test new software before it is
released for use by other users.
o Maintain an easily-updated database of installed
software. For each type of software, the database should
list the computers where the software is installed, the
license numbers, software version number, the vendor
contact information, and the responsible person for each
computer listed. This database should be used to quickly
identify users, machines, and software when problems or
emergencies arise, such as when a particular type of
software is discovered to contain a virus or other
harmful aspects.
o Minimize software sharing within the organization. Do
not permit software to be placed on computers unless the
proper manager is notified and the software database is
updated. If computer networks permit software to be
mailed or otherwise transferred among machines, prohibit
this as a policy. Instruct users not to run software
that has been mailed to them.
o If using software repositories on LAN servers, set up the
server directory such that users can copy from the
directory, but not add software to the directory. Assign
a user to manage the repository; all updates to the
repository should be cleared through this individual.
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The software should be tested on an isolated system as
described earlier.
o If developing software, consider the use of software
management and control programs that automate record
keeping for software updates, and that provide a degree
of protection against unauthorized modifications to the
software under development.
o Prohibit users from using software or disks from their
home systems. A home system that is used to access
software bulletin boards or that uses shared copies of
software could be infected with viruses or other
malicious software.
5.3 Technical Controls
As stated earlier, personal computers suffer from a relative lack
of technical controls. There are usually no mechanisms for user
authentication and for preventing users or software from
modifying system and application software. Generally, all
software and hardware is accessible by the personal computer
user, thus the potential for misuse is substantially greater than
in the multi-user computer environment.
However, some technical controls can be added to personal
computers, e.g., user authentication devices. The technical
controls that do not exist can be simulated by other controls,
such as a lock on an office door to substitute for a user
authentication device, or anti-virus software to take the place
of system auditing software. Lastly, some of the personal
computer's accessibility can be reduced, such as by the removal
of floppy diskette drives or by the use of diskless computers
that must download their software from a LAN server. The
following items are suggested:
o Where technical controls exist, use them. If basic file
access controls are available to make files read-only,
make sure that operating system files and other
executable files are marked as read-only. Use write-
protect tabs on floppy diskettes and tapes. If LAN
access requires a password, ensure that passwords are
used carefully - follow the guidelines for password
usage presented in Chapter 4 or see [FIPS73].
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o Use new cost-effective forms of user identification such
as magnetic access cards. Or, setup other software such
as password mechanism that at a minimum deters
unauthorized users.
o If using a LAN, consider downloading the personal
computer's operating system and other applications from a
read-only directory on the LAN server (instead of the
personal computer's hard disk). If the LAN server is
well protected, this arrangement would significantly
reduce chances of the software becoming infected, and
would simplify software management.
o Consider booting personal computers from write-protected
floppy diskettes (instead of the computer's hard disk).
Use a unique diskette per computer, and keep the diskette
secured when not in use.
o Do not leave a personal computer running but unattended.
Lock the computer with a hardware lock (if possible), or
purchase vendor add-on software to "lock" the keyboard
using a password mechanism. Alternatively, turn off the
computer and lock the office door. Shut down and lock
the computer at the end of the day.
o When using modems connected to personal computers, do not
provide more access to the computer than necessary. If
only dial-out service is required, configure the modem so
that it won't answer calls. If dial-in service is
necessary, consider purchasing modems that require a
password or that use a call-back mechanism to force a
caller to call from a telephone number that is known to
the modem.
o Consider using "limited-use" systems, whereby the
capabilities of a system are restricted to only what is
absolutely required. For example, users who run only a
certain application (such as word-processor) may not
require the flexibility of a personal computer. At the
minimum, do not install applications or network
connections where they are not needed.
5.4 Monitoring
Personal computer operating systems typically do not provide any
software or user monitoring/auditing features. Monitoring, then,
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CHAPTER 5
is largely a user function whereby the user must be aware of what
the computer is doing, such as when the computer is accessing the
disk or the general speed of its response to commands, and then
must decide whether the activity is normal or abnormal. Anti-
viral software can be added to the operating system and run in
such a way that the software flags or in some way alerts a user
when suspicious activity occurs, such as when critical files or
memory regions are written.
Effective monitoring depends on user education. Users must know
what constitutes normal and abnormal activity on their personal
computers. They need to have a reporting structure available so
that they can alert an informed individual to determine whether
there is indeed a problem. They need to know the steps to take
to contain the damage, and how to recover. Thus, the following
policies and procedures are recommended:
o Form a team of skilled technical people to investigate
problems reported by users. This same group could be
responsible for other aspects of virus prevention, such
as testing new software and handling the containment and
recovery from virus-related incidents. Ensure that users
have quick access to this group, e.g., via a telephone
number.
o Educate users so that they are familiar with how their
computers function. Show them how to use such items as
anti-viral software. Acquaint them with how their
computers boot, what files are loaded, whether start-up
batch files are executed, and so forth.
o Users need to watch for changes in patterns of system
activity. They need to watch for program loads that
suddenly take longer, whether disk accesses seem
excessive for simple tasks, do unusual error messages
occur, do access lights for disks turn on when no disk
activity should occur, is less memory available than
usual, do files disappear mysteriously, is there less
disk space than normal?
o Users also need to examine whether important files have
changed in size, date, or content. Such files would
include the operating system, regularly-run applications,
and other batch files. System sweep programs may be
purchased or built to perform checksums on selected
files, and then to report whether changes have occurred
since the last time the program was run.
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CHAPTER 5
o Purchase virus prevention software as applicable. At a
minimum, use anti-viral software to test new software
before releasing it to other users. However, do not
download or use pirated copies of anti-viral software.
o Always report, log, and investigate security problems,
even when the problems appear insignificant. Then use
the log as input into regular security reviews. Use the
reviews as a means for evaluating the effectiveness of
security policies and procedures.
5.5 Contingency Planning
As described in Chapter 3, backups are the single most important
contingency procedure. It is especially important to emphasize
regular backups for personal computers, due to their greater
susceptibility to misuse and due to the usual requirement of
direct user involvement in the backup procedure, unlike that of
multi-user computers. Because of the second factor, where users
must directly copy files to one or more floppy diskettes,
personal computer backups are sometimes ignored or not done
completely. To help ensure that backups are done regularly,
external backup mechanisms that use a high-density tape cartridge
can be purchased and a user assigned to run the backup procedure
on a regular basis. Additionally, some personal computer
networks contain a personal computer backup feature, where a
computer can directly access a network server's backup mechanism,
sometimes in an off-line mode at a selected time. If neither of
these mechanisms are available, then users must be supplied with
an adequate number of diskettes to make complete backups and to
maintain a reasonable amount of backup history, with a minimum of
several weeks.
Users should maintain the original installation media for
software applications and store it in a secure area, such as a
locked cabinet, container, or desk. If a user needs to restore
software, the user should use only the original media; the user
should not use any other type of backup or a copy belonging to
another user, as they could be infected or damaged by some form
of malicious software.
The effectiveness of a backup policy can be judged by whether a
user is able to recover with a minimum loss of data from a
situation whereby the user would have to format the computer's
disk and reload all software. Several incidents of malicious
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CHAPTER 5
software have required that users go to this length to recover -
see [MACAFEE89].
Other important contingency procedures are described below:
o Maintain a database of personal computer information.
Each record should include items such as the computer's
configuration, i.e., network connections, disks, modems,
etc., the computer's location, how it is used, the
software it runs, and the name of the computer's primary
user/manager. Maintain this database to facilitate rapid
communication and identification when security problems
arise.
o Create a security distribution list for each user. The
list should include names of people to contact who can
help identify the cause of unusual computer activity, and
other appropriate security personnel to contact when
actual problems arise.
o Create a group of skilled users who can respond to users'
inquiries regarding virus detection. This group should
be able to determine when a computer has been attacked,
and how best to contain and recover from the problem.
o Set up some means of distributing information rapidly to
all affected users in the event of an emergency. This
should not rely upon a computer network, as the network
could actually be attacked, but could use other means
such as telephone mail or a general announcement
mechanism.
o Observe physical security for personal computers. Locate
them in offices that can be locked. Do not store
software and backups in unsecured cabinets.
5.6 Associated Network Concerns
Personal computer networks offer many advantages to users,
however they must be managed carefully so that they do not
increase vulnerability to viruses and related threats. Used
incorrectly, they can become an additional pathway to
unauthorized access to systems, and can be used to plant
malicious software such as network worms. This section does not
provide specific management guidance, as there are many different
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COMPUTER VIRUSES AND RELATED THREATS
CHAPTER 5
types of personal computer networks with widely varying degrees
of similarity. However, some general suggestions for improving
basic management are listed below:
o Assign a network administrator, and make the required
duties part of the administrator's job description.
Personal computer networks are becoming increasingly
complex to administer, thus the administration should not
be left to an individual who cannot dedicate time as
necessary.
o Protect the network server(s) by locating them in secure
areas. Make sure that physical access is restricted
during off-hours. If possible, lock or remove a server's
keyboard to prevent tampering.
o Do not provide for more than one administrator account,
i.e., do not give other users administrator privileges.
Similar to the problem of multiple system manager
accounts on multi-user systems, this situation makes it
more likely that a password will become known, and makes
overall management more difficult to control. Users
should coordinate their requests through a single network
administrator.
o Do not permit users to connect personal computers to the
network cable without permission. The administrator
should keep an updated diagram of the network's topology,
complete with corresponding network addresses and users.
o Use the network monitoring tools that are available.
Track network usage and access to resources, and pinpoint
unauthorized access attempts. Take appropriate action
when violations consistently occur, such as requiring the
user in question to attend a network user class or
disabling the user's network account.
o Ensure that users know how to properly use the network.
Show them how to use all security features. Ensure that
users know how to use passwords and access controls
effectively - see [FIPS73] for information on password
usage. Show them the difference between normal and
abnormal network activity or response. Encourage users
to contact the administrator if they detect unusual
activity. Log and investigate all problems.
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o Do not give users more access to network resources than
they require. If using shared directories, make them
read-only if write permission is not required, or use a
password. Encourage users to do the same with their
shared directories.
o Do not set up directories for software repository unless
(1) someone can first verify whether the software is not
infected, and (2) users are not permitted to write to the
directory without prior approval.
o Backup the network server(s) regularly. If possible or
practical, backup personal computers using the network
server backup mechanism.
o Disable the network mail facility from transferring
executable files, if possible. This will prevent
software from being indiscriminately shared, and may
prevent network worm programs from accessing personal
computers.
o For network guest or anonymous accounts, limit the types
of commands that can be executed.
o Warn network users to be suspicious of any messages or
programs that are received from unidentified sources -
network users should have a critical and suspicious
attitude towards anything received from an unknown
source.
o Always remove old accounts or change passwords. Change
important passwords immediately when users leave the
organization or no longer require access to the network.
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COMPUTER VIRUSES AND RELATED THREATS
APPENDIX A
References
BUNZEL88 Bunzel, Rick; Flu Season; Connect, Summer 1988.
DENNING88 Denning, Peter J.; Computer Viruses; American
Scientist, Vol 76, May-June, 1988.
DENNING89 Denning, Peter J.; The Internet Worm; American
Scientist, Vol 77, March-April, 1989.
FIPS73 Federal Information Processing Standards
Publication 73, Guidelines for Security of
Computer Applications; National Bureau of
Standards, June, 1980.
FIPS112 Federal Information Processing Standards
Publication 112, Password Usage; National Bureau
of Standards, May, 1985.
MACAFEE89 McAfee, John; The Virus Cure; Datamation, Feb 15,
1989.
NBS120 NBS Special Publication 500-120; Security of
Personal Computer Systems: A Management Guide;
National Bureau of Standards, Jan 1985.
SPAFFORD88 Spafford, Eugene H.; The Internet Worm Program: An
Analysis; Purdue Technical Report CSD-TR-823, Nov
28, 1988.
THOMPSON84 Thompson, Ken; Reflections on Trusting Trust
(Deliberate Software Bugs); Communications of the
ACM, Vol 27, Aug 1984.
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COMPUTER VIRUSES AND RELATED THREATS
APPENDIX A
A-2
COMPUTER VIRUSES AND RELATED THREATS
APPENDIX B
Suggested Reading
In addition to the references listed in Appendix A, the following
documents are suggested reading for specific and general
information on computer viruses and related forms, and other
related security information.
Brenner, Aaron; LAN Security; LAN Magazine, Aug 1989.
Cohen, Fred; Computer Viruses, Theory and Experiments; 7th
Security Conference, DOD/NBS Sept 1984.
Computer Viruses - Proceedings of an Invitational Symposium, Oct
10/11, 1988;
Deloitte, Haskins, and Sells; 1989
Dvorak, John; Virus Wars: A Serious Warning; PC Magazine; Feb 29,
1988.
Federal Information Processing Standards Publication 83,
Guideline on User Authentication Techniques for Computer Network
Access Control; National Bureau of Standards, Sept, 1980.
Federal Information Processing Standards Publication 87,
Guidelines for ADP Contingency Planning; National Bureau of
Standards, March, 1981.
Fiedler, David and Hunter, Bruce M.; Unix System Administration;
Hayden Books, 1987
Fitzgerald, Jerry; Business Data Communications: Basic Concepts,
Security, and Design; John Wiley and Sons, Inc., 1984
Gasser, Morrie; Building a Secure Computer System; Van Nostrand
Reinhold, New York, 1988.
Grampp, F. T. and Morris, R. H.; UNIX Operating System Security;
AT&T Bell Laboratories Technical Journal, Oct 1984.
Highland, Harold J.; From the Editor -- Computer Viruses;
Computers & Security; Aug 1987.
Longley, Dennis and Shain, Michael; Data and Computer Security
B-1
COMPUTER VIRUSES AND RELATED THREATS
APPENDIX B
NBS Special Publication 500-120; Security of Personal Computer
Systems: A Management Guide; National Bureau of Standards, Jan
1985.
Parker, T.; Public domain software review: Trojans revisited,
CROBOTS, and ATC; Computer Language; April 1987.
Schnaidt, Patricia; Fasten Your Safety Belt; LAN Magazine, Oct
1987.
Shoch, J. F. and Hupp, J. A.; The Worm Programs: Early Experience
with a Distributed Computation; Comm of ACM, Mar 1982.
White, Stephen and Chess, David; Coping with Computer Viruses and
Related Problems; IBM Research Report RC 14405 (#64367), Jan
1989.
Witten, I. H.; Computer (In)security: infiltrating open systems;
Abacus (USA) Summer 1987.