Network Working Group E. Guttman
Request for Comments: 2504 Sun Microsystems
FYI: 34 L. Leong
Category: Informational COLT Internet
Users' Security Handbook
Status of this Memo
This memo provides information for the Internet community. It does
not specify an Internet standard of any kind. Distribution of this
memo is unlimited.
Copyright (C) The Internet Society (1999). All Rights Reserved.
The Users' Security Handbook is the companion to the Site Security
Handbook (SSH). It is intended to provide users with the information
they need to help keep their networks and systems secure.
Table of Contents
Part One: Introduction . . . . . . . . . . . . . . . . . . . . 2
1. READ.ME . . . . . . . . . . . . . . . . . . . . . . . . . 2
2. The Wires have Ears . . . . . . . . . . . . . . . . . . . 3
Part Two: End-users in a centrally-administered network . . . 4
3. Watch Out! . . . . . . . . . . . . . . . . . . . . . . . 4
3.1. The Dangers of Downloading . . . . . . . . . . . . . . 4
3.2. Don't Get Caught in the Web . . . . . . . . . . . . . . 5
3.3. Email Pitfalls . . . . . . . . . . . . . . . . . . . . 6
3.4. Passwords . . . . . . . . . . . . . . . . . . . . . . . 7
3.5. Viruses and Other Illnesses . . . . . . . . . . . . . . 7
3.6. Modems . . . . . . . . . . . . . . . . . . . . . . . . 8
3.7. Don't Leave Me... . . . . . . . . . . . . . . . . . . . 9
3.8. File Protections . . . . . . . . . . . . . . . . . . . 9
3.9. Encrypt Everything . . . . . . . . . . . . . . . . . . 10
3.10. Shred Everything Else . . . . . . . . . . . . . . . . . 10
3.11. What Program is This, Anyway? . . . . . . . . . . . . . 11
4. Paranoia is Good . . . . . . . . . . . . . . . . . . . . 11
Part Three: End-users self administering a networked computer 14
5. Make Your Own Security Policy . . . . . . . . . . . . . . 14
6. Bad Things Happen . . . . . . . . . . . . . . . . . . . . 15
6.1. How to Prepare for the Worst in Advance . . . . . . . . 15
6.2. What To Do if You Suspect Trouble . . . . . . . . . . . 16
6.3. Email . . . . . . . . . . . . . . . . . . . . . . . . . 17
7. Home Alone . . . . . . . . . . . . . . . . . . . . . . . 17
7.1. Beware of Daemons . . . . . . . . . . . . . . . . . . . 17
7.2. Going Places . . . . . . . . . . . . . . . . . . . . . 19
7.3. Secure It! . . . . . . . . . . . . . . . . . . . . . . 20
8. A Final Note . . . . . . . . . . . . . . . . . . . . . . 20
Appendix: Glossary of Security Terms . . . . . . . . . . . . . 21
Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 31
References . . . . . . . . . . . . . . . . . . . . . . . . . . 31
Security Considerations . . . . . . . . . . . . . . . . . . . 32
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 32
Full Copyright Statement . . . . . . . . . . . . . . . . . . . 33
Part One: Introduction
This document provides guidance to the end-users of computer systems
and networks about what they can do to keep their data and
communication private, and their systems and networks secure. Part
Two of this document concerns "corporate users" in small, medium and
large corporate and campus sites. Part Three of the document
addresses users who administer their own computers, such as home
System and network administrators may wish to use this document as
the foundation of a site-specific users' security guide; however,
they should consult the Site Security Handbook first [RFC2196].
A glossary of terms is included in an appendix at the end of this
document, introducing computer network security notions to those not
familiar with them.
Before getting connected to the Internet or any other public network,
you should obtain the security policy of the site that you intend to
use as your access provider, and read it. A security policy is a
formal statement of the rules by which users who are given access to
a site's technology and information assets must abide. As a user,
you are obliged to follow the policy created by the decision makers
and administrators at your site.
A security policy exists to protect a site's hardware, software and
data. It explains what the security goals of the site are, what
users can and cannot do, what to do and who to contact when problems
arise, and generally informs users what the "rules of the game" are.
2. The Wires have Ears
It is a lot easier to eavesdrop on communications over data networks
than to tap a telephone conversation. Any link between computers may
potentially be insecure, as can any of the computers through which
data flows. All information passing over networks may be
eavesdropped on, even if you think "No one will care about this..."
Information passing over a network may be read not only by the
intended audience but can be read by others as well. This can happen
to personal Email and sensitive information that is accessed via file
transfer or the Web. Please refer to the "Don't Get Caught in the
Web" and "Email Pitfalls" sections for specific information on
protecting your privacy.
As a user, your utmost concerns should, firstly, be to protect
yourself against misuse of your computer account(s) and secondly, to
protect your privacy.
Unless precautions are taken, every time you log in over a network,
to any network service, your password or confidential information may
be stolen. It may then be used to gain illicit access to systems you
have access to. In some cases, the consequences are obvious: If
someone gains access to your bank account, you might find yourself
losing some cash, quickly. What is not so obvious is that services
which are not financial in nature may also be abused in rather costly
ways. You may be held responsible if your account is misused by
Many network services involve remote log in. A user is prompted for
his or her account ID (ie. user name) and password. If this
information is sent through the network without encryption, the
message can be intercepted and read by others. This is not really an
issue when you are logging in to a "dial-in" service where you make a
connection via telephone and log in, say to an online service
provider, as telephone lines are more difficult to eavesdrop on than
The risk is there when you are using programs to log in over a
network. Many popular programs used to log in to services or to
transfer files (such as telnet and ftp, respectively) send your user
name and password and then your data over the network without
The precaution commonly taken against password eavesdropping by
larger institutions, such as corporations, is to use one-time
Until recently, it has been far too complicated and expensive for
home systems and small businesses to employ secure log in systems.
However, an increasing number of products enable this to be done
without fancy hardware, using cryptographic techniques. An example
of such a technique is Secure Shell [SSH], which is both freely and
commercially available for a variety of platforms. Many products
(including SSH-based ones) also allow data to be encrypted before it
is passed over the network.
Part Two: End-users in a centrally-administered network
The following rules of thumb provide a summary of the most important
pieces of advice discussed in Part Two of this document:
- Know who your security point-of-contact is.
- Keep passwords secret at all times.
- Use a password-locked screensaver or log out when you leave your
- Don't let simply anyone have physical access to your computer or
- Be aware what software you run and very wary of software of
unknown origin. Think hard before you execute downloaded
- Do not panic. Consult your security point-of-contact, if
possible, before spreading alarm.
- Report security problems as soon as possible to your security
3. Watch Out!
3.1. The Dangers of Downloading
An ever expanding wealth of free software has become available on the
Internet. While this exciting development is one of the most
attractive aspects of using public networks, you should also exercise
caution. Some files may be dangerous. Downloading poses the single
Be careful to store all downloaded files so that you will remember
their (possibly dubious) origin. Do not, for example, mistake a
downloaded program for another program just because they have the
same name. This is a common tactic to fool users into activating
programs they believe to be familiar but could, in fact, be
Programs can use the network without making you aware of it. One
thing to keep in mind is that if a computer is connected, any program
has the capability of using the network, with or without informing
you. Say, for example:
You download a game program from an anonymous FTP server. This
appears to be a shoot-em-up game, but unbeknownst to you, it
transfers all your files, one by one, over the Internet to a
Many corporate environments explicitly prohibit the downloading and
running of software from the Internet.
3.2. Don't Get Caught in the Web
The greatest risk when web browsing is downloading files. Web
browsers allow any file to be retrieved from the Internet. See "The
Dangers of Downloading".
Web browsers are downloading files even when it is not entirely
obvious. Thus, the risk posed by downloading files may be present
even if you do not actively go out and retrieve files overtly. Any
file which you have loaded over the network should be considered
possibly dangerous (even files in the web browser's cache). Do not
execute them by accident, as they may be malicious programs.
(Remember, programs are files, too. You may believe you have
downloaded a text file, when in fact it is a Trojan Horse program,
Web browsers may download and execute programs on your behalf, either
automatically or after manual intervention. You may disable these
features. If you leave them enabled, be sure that you understand the
consequences. You should read the security guide which accompanies
your web browser as well as the security policy of your company. You
should be aware that downloaded programs may be risky to execute on
your machine. See "What program is this, anyway?".
Web pages often include forms. Be aware that, as with Email, data
sent from a web browser to a web server is not secure. Several
mechanisms have been created to prevent this, most notably Secure
Sockets Layer [SSL]. This facility has been built into many web
browsers. It encrypts data sent between the user's web browser and
the web server so no one along the way can read it.
It is possible that a web page will appear to be genuine, but is, in
fact, a forgery. It is easy to copy the appearance of a genuine web
page and possible to subvert the network protocols which contact the
desired web server, to misdirect a web browser to an imposter.
That threat may be guarded against using SSL to verify if a web page
is genuine. When a 'secure' page has been downloaded, the web
browser's 'lock' or 'key' will indicate so. It is good to
double-check this: View the 'certificate' associated with the web
page you have accessed. Each web browser has a different way to do
this. The certificate will list the certificate's owner and who
issued it. If these look trustworthy, you are probably OK.
3.3 Email Pitfalls
All the normal concerns apply to messages received via Email that you
could receive any other way. For example, the sender may not be who
he or she claims to be. If Email security software is not used, it
is very difficult to determine for sure who sent a message. This
means that Email itself is a not a suitable way to conduct many types
of business. It is very easy to forge an Email message to make it
appear to have come from anyone.
Another security issue you should consider when using Email is
privacy. Email passes through the Internet from computer to
computer. As the message moves between computers, and indeed as it
sits in a user's mailbox waiting to be read, it is potentially
visible to others. For this reason, it is wise to think twice before
sending confidential or extremely personal information via Email.
You should never send credit card numbers and other sensitive data
via unprotected Email. Please refer to "The Wires Have Ears".
To cope with this problem, there are privacy programs available, some
of which are integrated into Email packages.
One service many Email users like to use is Email forwarding. This
should be used very cautiously. Imagine the following scenario:
A user has an account with a private Internet Service Provider and
wishes to receive all her Email there. She sets it up so that her
Email at work is forwarded to her private address. All the Email
she would receive at work then moves across the Internet until it
reaches her private account. All along the way, the Email is
vulnerable to being read. A sensitive Email message sent to her
at work could be read by a network snoop at any of the many stops
along the way the Email takes.
Note that Email sent or received at work may not be private. Check
with your employer, as employers may (in some instances) legally both
read your Email and make use of it. The legal status of Email
depends on the privacy of information laws in force in each country.
Many mail programs allow files to be included in Email messages. The
files which come by Email are files like any other. Any way in which
a file can find its way onto a computer is possibly dangerous. If
the attached file is merely a text message, fine. But it may be more
than a text message. If the attached file is itself a program or an
executable script, extreme caution should be applied before running
it. See the section entitled "The Dangers of Downloading".
Passwords may be easily guessed by an intruder unless precautions are
taken. Your password should contain a mixture of numbers, upper and
lower case letters, and punctuation. Avoid all real words in any
language, or combinations of words, license plate numbers, names and
so on. The best password is a made-up sequence (e.g., an acronym
from a phrase you won't forget), such as "2B*Rnot2B" (but don't use
Resist the temptation to write your password down. If you do, keep
it with you until you remember it, then shred it! NEVER leave a
password taped onto a terminal or written on a whiteboard. You
wouldn't write your PIN code on your automated teller machine (ATM)
card, would you? You should have different passwords for different
accounts, but not so many passwords that you can't remember them.
You should change your passwords periodically.
You should also NEVER save passwords in scripts or login procedures
as these could be used by anyone who has access to your machine.
Be certain that you are really logging into your system. Just
because a login prompt appears and asks you for your password does
not mean you should enter it. Avoid unusual login prompts and
immediately report them to your security point-of-contact. If you
notice anything strange upon logging in, change your password.
Unless precautions have been taken to encrypt your password when it
is sent over the network, you should, if possible, use "one-time
passwords" whenever you log in to a system over a network. (Some
applications take care of that for you.) See "The Wires Have Ears"
for more information on the risks associated with logging in over a
3.5 Viruses and Other Illnesses
Viruses are essentially unwanted pieces of software that find their
way onto a computer. What the virus may do once it has entered its
host, depends on several factors: What has the virus been programmed
to do? What part of the computer system has the virus attacked?
Some viruses are 'time bombs' which activate only when given a
particular condition, such as reaching a certain date. Others remain
latent in the system until a particular afflicted program is
activated. There are still others which are continually active,
exploiting every opportunity to do mischief. A subtle virus may
simply modify a system's configuration, then hide.
Be cautious about what software you install on your system. Use
software from "trusted sources", if possible. Check your site policy
before installing any software: Some sites only allow administrators
to install software to avoid security and system maintenance
Centrally-administered sites have their own policy and tools for
dealing with the threat of viruses. Consult your site policy or find
out from your systems administrator what the correct procedures are
to stay virus free.
You should report it if a virus detection tool indicates that your
system has a problem. You should notify your site's systems
administrators as well as the person you believe passed the virus to
you. It is important to remain calm. Virus scares may cause more
delay and confusion than an actual virus outbreak. Before announcing
the virus widely, make sure you verify its presence using a virus
detection tool, if possible, with the assistance of
Trojan Horse programs and worms are often categorized with viruses.
Trojan Horse programs are dealt with in the "What Program is This,
Anyway?" section. For the purposes of this section, worms should be
considered a type of virus.
You should be careful when attaching anything to your computer, and
especially any equipment which allows data to flow. You should get
permission before you connect anything to your computer in a
centrally-administered computing environment.
Modems present a special security risk. Many networks are protected
by a set of precautions designed to prevent a frontal assault from
public networks. If your computer is attached to such a network, you
must exercise care when also using a modem. It is quite possible to
use the modem to connect to a remote network while *still* being
connected to the 'secure' net. Your computer can now act as a hole
in your network's defenses. Unauthorized users may be able to get
onto your organization's network through your computer!
Be sure you know what you are doing if you leave a modem on and set
up your computer to allow remote computers to dial in. Be sure you
use all available security features correctly. Many modems answer
calls by default. You should turn auto-answer off unless you are
prepared to have your computer respond to callers. Some 'remote
access' software requires this. Be sure to turn on all the security
features of your 'remote access' software before allowing your
computer to be accessed by phone.
Note that having an unlisted number will not protect you from someone
breaking into your computer via a phone line. It is very easy to
probe many phone lines to detect modems and then launch attacks.
3.7 Don't Leave Me...
Do not leave a terminal or computer logged in and walk away. Use
password-locked screensavers whenever possible. These can be set up
so that they activate after the computer has been idle for a while.
Sinister as it may seem, someone coming around to erase your work is
not uncommon. If you remained logged in, anyone can come by and
perform mischief for which you may be held accountable. For example,
imagine the trouble you could be in for if nasty Email were sent to
the president of your company in your name, or your account were used
to transfer illegal pornography.
Anyone who can gain physical access to your computer can almost
certainly break into it. Therefore, be cautious regarding who you
allow access to your machine. If physically securing your machine is
not possible, it is wise to encrypt your data files kept on your
local hard disk. If possible, it is also wise to lock the door to
one's office where the computer is stored.
3.8 File Protections
Data files and directories on shared systems or networked file
systems require care and maintenance. There are two categories of
- Files to share
Shared files may be visible to everyone or to a restricted group
of other users. Each system has a different way of specifying
this. Learn how to control sharing permissions of files and
implement such control without fail.
- Protected files
These include files that only you should have access to, but
which are also available to anyone with system administrator
privileges. An example of this are files associated with the
delivery of Email. You don't want other users to read your Email,
so make sure such files have all the necessary file permissions
3.9 Encrypt Everything
Additionally, there are files that are private. You may have files
which you do not wish anyone else to have access to. In this case,
it is prudent to encrypt the file. This way, even if your network is
broken into or the systems administrator turns into Mr. Hyde, your
confidential information will not be available. Encryption is also
very important if you share a computer. For example, a home computer
may be shared by room mates who are friends but prefer to keep their
Email and financial information private. Encryption allows for
shared yet private usage.
Before you encrypt files, you should check your site's security
policy. Some employers and countries expressly forbid or restrict
the storing and/or transferring of encrypted files.
Be careful with the passwords or keys you use to encrypt files.
Locking them away safely not only helps to keep them from prying eyes
but it will help you keep them secure too; for if you lose them, you
will lose your ability to decrypt your data as well! It may be wise
to save more than one copy. This may even be required, if your
company has a key escrow policy, for example. This protects against
the possibility that the only person knowing a pass phrase may leave
the company or be struck by lightning.
Whilst encryption programs are readily available, it should be noted
that the quality can vary widely. PGP (which stands for "Pretty Good
Privacy") for example, offers a strong encryption capability. Many
common software applications include the capability to encrypt data.
The encryption facilities in these are typically very weak.
You should not be intimidated by encryption software. Easy-to-use
software is being made available.
3.10 Shred Everything Else
You would be surprised what gets thrown away into the waste-paper
basket: notes from meetings, old schedules, internal phone lists,
computer program listings, correspondence with customers and even
market analyses. All of these would be very valuable to competitors,
recruiters and even an overzealous (hungry?) journalist looking for a
scoop. The threat of dumpster diving is real - take it seriously!
Shred all potentially useful documents before discarding them.
You should also be aware that deleting a file does not erase it in
many cases. The only way to be sure that an old hard disk does not
contain valuable data may be to reformat it.
3.11 What Program is This, Anyway?
Programs have become much more complex in recent years. They are
often extensible in ways which may be dangerous. These extensions
make applications more flexible, powerful and customizable. They
also open the end-user up to all sorts of risks.
- A program may have "plug-in" modules. You should not trust the
plug-ins simply because you are used to trusting the programs
they plug into. For example: Some web pages suggest that the
user download a plug-in to view or use some portion of the web
page's content. Consider: What is this plug-in? Who wrote it?
Is it safe to include it in your web browser?
- Some files are "compound documents". This means that instead of
using one single program, it will be necessary to run several
programs in order to view or edit a document. Again, be careful
of downloading application components. Just because they
integrate with products which are well-known does not mean that
they can be trusted. Say, you receive an Email message which can
only be read if you download a special component. This component
could be a nasty program which wipes out your hard drive!
- Some programs are downloaded automatically when accessing web
pages. While there are some safeguards to make sure that these
programs may be used safely, there have been security flaws
discovered in the past. For this reason, some centrally-
administered sites require that certain web browser capabilities
be turned off.
4. Paranoia is Good
Many people do not realize it, but social engineering is a tool which
many intruders use to gain access to computer systems. The general
impression that people have of computer break-ins is that they are
the result of technical flaws in computer systems which the intruders
have exploited. People also tend to think that break-ins are purely
technical. However, the truth is that social engineering plays a big
part in helping an attacker slip through security barriers. This
often proves to be an easy stepping-stone onto the protected system
if the attacker has no authorized access to the system at all.
Social engineering may be defined, in this context, as the act of
gaining the trust of legitimate computer users to the point where
they reveal system secrets or help someone, unintentionally, to gain
unauthorized access to their system(s). Using social engineering, an
attacker may gain valuable information and/or assistance that could
help break through security barriers with ease. Skillful social
engineers can appear to be genuine but are really full of deceit.
Most of the time, attackers using social enginering work via
telephone. This not only provides a shield for the attacker by
protecting his or her identity, it also makes the job easier because
the attacker can claim to be a particular someone with more chances
of getting away with it.
There are several types of social engineering. Here are a few
examples of the more commonly-used ones:
- An attacker may pretend to be a legitimate end-user who is new to
the system or is simply not very good with computers. This
attacker may approach systems administrators and other end-users
for help. This "user" may have lost his password, or simply can't
get logged into the system and needs to access the system
urgently. Attackers have also been known to identify themselves
as some VIP in the company, screaming at administrators to get
what they want. In such cases, the administrator (or it could be
an end-user) may feel threatened by the caller's authority and
give in to the demands.
- Attackers who operate via telephone calls may never even have seen
the screen display on your system before. In such cases, the
trick attackers use is to make details vague, and get the user to
reveal more information on the system. The attacker may sound
really lost so as to make the user feel that he is helping a
damsel in distress. Often, this makes people go out their way to
help. The user may then reveal secrets when he is off-guard.
- An attacker may also take advantage of system problems that have
come to his attention. Offering help to a user is an effective
way to gain the user's trust. A user who is frustrated with
problems he is facing will be more than happy when someone comes
to offer some help. The attacker may come disguised as the
systems administrator or maintenance technician. This attacker
will often gain valuable information because the user thinks that
it is alright to reveal secrets to technicians. Site visits may
pose a greater risk to the attacker as he may not be able to make
an easy and quick get-away, but the risk may bring fruitful
returns if the attacker is allowed direct access to the system by
the naive user.
- Sometimes, attackers can gain access into a system without prior
knowledge of any system secret nor terminal access. In the same way
that one should not carry someone else's bags through Customs, no user
should key in commands on someone's behalf. Beware of attackers who
use users as their own remotely-controlled fingers to type commands on
the user's keyboard that the user does not understand, commands which
may harm the system. These attackers will exploit system software
bugs and loopholes even without direct access to the system. The
commands keyed in by the end-user may bring harm to the system, open
his own account up for access to the attacker or create a hole to
allow the attacker entry (at some later time) into the system. If you
are not sure of the commands you have been asked to key in, do not
simply follow instructions. You never know what and where these could
To guard against becoming a victim of social engineering, one
important thing to remember is that passwords are secret. A password
for your personal account should be known ONLY to you. The systems
administrators who need to do something to your account will not
require your password. As administrators, the privileges they have
will allow them to carry out work on your account without the need
for you to reveal your password. An administrator should not have to
ask you for your password.
Users should guard the use of their accounts, and keep them for their
own use. Accounts should not be shared, not even temporarily with
systems administrators or systems maintenance techinicians. Most
maintenance work will require special privileges which end-users are
not given. Systems administrators will have their own accounts to
work with and will not need to access computer systems via an
Systems maintenance technicians who come on site should be
accompanied by the local site administrator (who should be known to
you). If the site administrator is not familiar to you, or if the
technician comes alone, it is wise to give a call to your known site
administrator to check if the technician should be there. Yet, many
people will not do this because it makes them look paranoid and it is
embarrassing to show that they have no, or little trust in these
Unless you are very sure that the person you are speaking to is who he
or she claims to be, no secret information should ever be revealed to
such people. Sometimes, attackers may even be good enough to make
themselves sound like someone whose voice you know over the phone. It
is always good to double check the identity of the person. If you are
unable to do so, the wisest thing to do is not to reveal any secrets.
If you are a systems administrator, there should be security
procedures for assignment and reassignment of passwords to users, and
you should follow such procedures. If you are an end-user, there
should not be any need for you to have to reveal system secrets to
anyone else. Some companies assign a common account to multiple
users. If you happen to be in such a group, make sure you know
everyone in that group so you can tell if someone who claims to be in
the group is genuine.
Part Three: End-users self administering a networked computer
The home user or the user who administers his own network has many of
the same concerns as a centrally-administered user. The following is
a summary of additional advice given in Part Three:
- Read manuals to learn how to turn on security features, then turn
- Consider how private your data and Email need to be. Have you
invested in privacy software and learned how to use it yet?
- Prepare for the worst in advance.
- Keep yourself informed about what the newest threats are.
5. Make Your Own Security Policy
You should decide ahead of time what risks are acceptable and then
stick to this decision. It is also wise to review your decision at
regular intervals and whenever the need to do so arises. It may be
wise to simply avoid downloading any software from the network which
comes from an unknown source to a computer storing business records,
other valuable data and data which is potentially damaging if the
information was lost or stolen.
If the system has a mixed purpose, say recreation, correspondence
and some home accounting, perhaps you will hazard some downloading of
software. You unavoidably take some risk of acquiring stuff
which is not exactly what it seems to be.
It may be worthwhile installing privacy software on a computer if it
is shared by multiple users. That way, a friend of a room mate won't
have access to your private data, and so on.
6. Bad Things Happen
If you notice that your files have been modified or ascertain somehow
that your account has been used without your consent, you should
inform your security point-of-contact immediately. When you do
not know who your security point-of-contact is, try calling
your Internet service provider's help desk as a first step.
6.1 How to Prepare for the Worst in Advance
- Read all user documentation carefully. Make sure that it is clear
when services are being run on your computer. If network services
are activated, make sure they are properly configured (set all
permissions so as to prevent anonymous or guest logins, and so
on). Increasingly, many programs have networking capabilities
built in to them. Learn how to properly configure and safely use
- Back up user data. This is always important. Backups are
normally thought of as a way of ensuring you will not lose your
work if a hard disk fails or if you make a mistake and delete a
file. Backing up is also critical to insure that data cannot be
lost due to a computer security incident. One of the most vicious
and unfortunately common threats posed by computer viruses and
Trojan Horse programs is erasing a computer's hard disk.
- Obtain virus checking software or security auditing tools. Learn
how to use them and install them before connecting to a public
network. Many security tools require that they be run on a
"clean" system, so that comparisons can be made between the
present and pristine states. Thus, it is necessary for some work
to be done ahead of time.
- Upgrade networking software regularly. As new versions of
programs come out, it is prudent to upgrade. Security
vulnerabilities will likely have been fixed. The longer you wait
to do this, the greater the risk that security vulnerabilities of
the products will be become known and be exploited by some network
assailant. Keep up to date!
- Find out who to contact if you suspect trouble. Does your
Internet Service Provider have a security contact or Help Desk?
Investigate this before trouble happens so you won't lose time
trying to figure it out should trouble occur. Keep the contact
information both online and offline for easy retrieval.
There are 3 ways to avoid problems with viruses:
1. Don't be promiscuous
If at all possible, be cautious about what software you install on
your system. If you are unaware of or unsure of the origin of a
program, it is wise not to run it. Obtain software from trusted
sources. Do not execute programs or reboot using old diskettes
unless you have reformatted them, especially if the old diskettes
have been used to bring software home from a trade show and other
potentially security-vulnerable places.
Nearly all risk of getting infected by viruses can be eliminated
if you are extremely cautious about what files are stored on your
computer. See "The Dangers of Downloading" for more details.
2. Scan regularly.
Give your system a regular check-up. There are excellent
virus checking and security audit tools for most computer
platforms available today. Use them, and if possible, set them to
run automatically and regularly. Also, install updates of these
tools regularly and keep yourself informed of new virus threats.
3. Notice the unusual.
It's not true that a difference you cannot detect is no difference
at all, but it is a good rule of thumb. You should get used to
the way your system works. If there is an unexplainable change
(for instance, files you believe should exist are gone, or strange
new files are appearing and disk space is 'vanishing'), you should
check for the presense of viruses.
You should take some time to be familiar with computer virus
detection tools available for your type of computer. You should use
an up-to-date tool (i.e. not older than three months). It is very
important to test your computer if you have been using shared
software of dubious origin, someone else's used floppy disks to
transfer files, and so on.
6.2 What To Do if You Suspect Trouble
If you suspect that your home computer has a virus, that a malicious
program has been run, or that a system has been broken into, the
wisest course of action is to first disconnect the system from all
networks. If available, virus detection or system auditing software
should be used.
Checking vital system files for corruption, tampering or malicious
replacement is very tedious work to do by hand. Fortunately there are
many virus detection programs available for PCs and Macintosh
computers. There are security auditing programs available for
UNIX-based computers. If software is downloaded from the network, it
is wise to run virus detection or auditing tools regularly.
If it becomes clear that a home system has been attacked, it is time
to clean up. Ideally, a system should be rebuilt from scratch. This
means erasing everything on the hard disk. Next, install the
operating system and then all additional software the system needs.
It is best to install the operating system and additional software
from the original distribution diskettes or CD-roms, rather than from
backup storage. The reason for this is that a system may have been
broken into some time ago, so the backed up system or program files
may already include some altered files or viruses. Restoring a system
from scratch is tedious but worthwhile. Do not forget to re-install
all security related fixes you had installed before the security
incident. Obtain these from a verified, unsuspicious source.
Remember to be careful with saved Email. Copies of sent or received
Email (or indeed any file at all) placed in storage provided by an
Internet service provider may be vulnerable. The risk is that
someone might break into the account and read the old Email. Keep
your Email files, indeed any sensitive files, on your home machine.
7. Home Alone
A home system can be broken into over the Internet if a home user is
unwary. The files on the home system can be stolen, altered or
destroyed. The system itself, if compromised, could be accessed
again some time in the future. This section describes issues and
makes recommendations relevant to a home user of the Internet.
7.1 Beware of Daemons
A home system which uses PPP to connect directly to the Internet is
increasingly common. These systems are at the greatest risk if they
run certain kinds of programs called "services". If you run a
service, you are in effect making your computer available to others
across the network. Some services include:
- File servers (an NFS server, a PC with 'file sharing' turned on)
- An FTP server
- A Web server
There are, in general, two types of programs which operate on the
Internet: Clients (like web browsers and Email programs) and Servers
(like web servers and mail servers).
Most software which runs on home systems is of the client variety;
but, increasingly, server software is available on traditionally
client platforms (e.g., PCs). Server software which runs in the
background is referred to as a "daemon" (pronounced dee-mon). Many
Internet server software programs that run as daemons have names that
end in `d', like "inetd" (Internet Daemon) and "talkd" (Talk Daemon).
When set to run, these programs wait for clients to request some
particular service from across the network.
There are four very important things to keep in mind as far as the
security implications of running services on a home computer are
- First and most important, if a server is not properly configured,
it is very vulnerable to being attacked over a network. It is
vital, if you run services, to be familiar with the proper
configuration. This is often not easy, and may require training
or technical expertise.
- All software has flaws, and flaws exploited deviously can be used
to breach computer security. If you run a server on your home
machine, you have to stay aware. This requires work: You have to
stay in touch with the supplier of the software to get security
updates. It is highly recommended that you keep up with security
issues through on-line security forums. See [RFC2196] for a list
If security flaws in your server software are discovered, you will
need to either stop using the software or apply "patches" or
"fixes" which eliminate the vulnerability. The supplier of the
software, if it is a decent company or freeware author, will
supply information and updates to correct security flaws. These
"patches" or "fixes" must be installed as soon as possible.
- As a rule of thumb, the older the software, the greater the chance
that it has known vulnerabilities. This is not to say you should
simply trust brand new software either! Often, it takes time to
discover even obvious security flaws in servers.
- Some servers start up without any warning. There are some web
browsers and telnet clients which automatically start FTP servers
if not explicitly configured to not do so. If these servers are
not themselves properly configured, the entire file system of the
home computer can become available to anyone on the Internet.
In general, any software MAY start up a network daemon. The way to
be safe here is to know the products you are using. Read the manual,
and if any questions arise, call the company or mail the author of
free software to find out if you are actually running a service by
using the product.
A home user running a remote login service on his home machine faces
very serious risks. This service allows the home user to log in to
his home machine from other computers on the Internet and can be
quite convenient. But the danger is that someone will secretly
observe the logging in and then be able to masquerade as the user
whenever they choose to do so in the future. See "The Wires Have
Ears" which suggests precautions to take for remote log in.
If possible, activate all "logging" options in your server software
which relate to security. You need to review these logs regularly in
order to gain any benefit from this logging. You should also be
aware that logs often grow very quickly in size, so you need to be
careful they don't fill up your hard disk!
7.2 Going Places
Remote logins allow a user privileged access onto physically remote
systems from the comfort of his own home.
More and more companies are offering their employees the ability to
work from home with access to their computer accounts through dial-up
connections. As the convenience of Internet connectivity has led to
lowered costs and wide-spread availability, companies may allow
remote login to their systems via the Internet. Customers of
companies with Internet access may also be provided with remote login
accounts. These companies include Internet service providers, and
even banks. Users should be very careful when making remote logins.
As discussed in "The Wires have Ears" section, Internet connections
can be eavesdropped on. If you intend to use a remote login service,
check that the connection can be done securely, and make sure that
you use the secure technologies/features.
Connections may be secured using technologies like one-time
passwords, secure shell (SSH) and Secure Sockets Layer (SSL). One-
time passwords make a stolen password useless to steal, while secure
shell encrypts data sent over the connection. Please refer to "Don't
Get Caught in the Web" for a discussion on SSL. Secure services such
as these have to be made available on the systems to which you log in
7.3 Secure It!
Administering your own home computer means you get to choose what
software is run on it. Encryption software provides protection for
data. If you keep business records and other sensitive data on your
computer, encryption will help to keep it safe. For example, if you
ran a network service from your home computer and missed setting
restrictions on a private directory, a remote user (authorized or
not) may gain access to files in this private directory. If the
files are encrypted, the user will not be able to read them. But as
with all forms of encryption running on any system, the keys and
passwords should first be kept safe!
8. A Final Note
This document has provided the reader with an introduction and as
much concise detail as possible. Present security issues go out of
date quickly, and although effort has been made to keep discussions
general, examples given may not be relevant in the future as the
Internet and computer industry continue to grow.
Just as home-owners are now taking increased cautions at the expense
of convenience, to secure their homes in the changing world we live
in, computer network users should not ignore security. It may be
inconvenient, but it is always better to be safe than sorry.
Appendix: Glossary of Security Terms
Acceptable Use Policy (AUP)
A set of rules and guidelines that specify in more or less detail
the expectations in regard to appropriate use of systems or
See (Computer) Account
Anonymous and Guest Log In
Services may be made available without any kind of authentication.
This is commonly done, for instance, with the FTP protocol to
allow anonymous access. Other systems provide a special account
named "guest" to provide access, typically restricting the
privileges of this account.
Tools to analyze computer systems or networks in regard to their
security status or in relation to the set of services provided by
them. COPS (Computer Oracle Password and Security analyzer) and
SATAN (Security Administrator's Tool for Analyzing Networks) are
famous examples of such tools.
Authentication refers to mechanisms which are used to verify the
identity of a user. The process of authentication typically
requires a name and a password to be supplied by the user as proof
of his identity.
A network of systems which is the responsibility of a single group
of administrators who are not distributed but work centrally to
take care of the network.
Certificates are data which is used to verify digital signatures.
A certificate is only as trustworthy as the agency which issued
it. A certificate is used to verify a particular signed item,
such as an Email message or a web page. The digital signature,
the item and the certificate are all processed by a mathematical
program. It is possible to say, if the signature is valid, that
"According to the agency which issued the certificate, the signer
was (some name)".
A computer which has been freshly installed with its operating
system and software obtainied from trusted software distribution
media. As more software and configuration are added to a
computer, it becomes increasingly difficult to determine if the
computer is 'clean' or has been compromised by viruses, trojan
horse or misconfiguration which reduces the security of the
Depending on the point of view, a client might be a computer
system which an end-user uses to access services hosted on another
computer system called a server. 'Client' may also refer to a
program or a part of a system that is used by an end-user to
access services provided by another program (for example, a web
browser is a client that accesses pages provided by a Web Server).
A 'document' is a file containing (a set of) data. Files may
consist of multiple parts: a plain document, an encrypted
document, a digitally-signed documents or a compressed document.
Multi-part files are known as compound documents and may require a
variety of programs to be used in order to interpret and
manipulate it. These programs may be used without the user's
This term describes the authorization to access a specific
computer system or network. Each end-user has to use an account,
which consists most probably of a combination of user name and
password or another means of proving that the end-user is the
person the account is assigned to.
Configuring Network Services
The part of an administrator's task that is related to specifying
the conditions and details of network services that govern the
service provision. In regard to a Web server, this includes which
Web pages are available to whom and what kind of information is
logged for later review purposes.
Cookies register information about a visit to a web site for
future use by the server. A server may receive information of
cookies of other sites as well which create concern in terms of
breach of privacy.
This term is used to describe attackers, intruders or other bad
guys that do not play by the rules and try to circumvent security
mechanisms and/or attack individuals and organisations.
Daemons (inetd, talkd, etc.)
These are processes that run on computer systems to provide
services to other computer systems or processes. Typically,
daemons are considered "servers".
The process of reversing the encryption of a file or message to
recover the original data in order to use or read it.
Some systems and server software come with preconfigured accounts.
These accounts may be set up with a predefined (user name and)
password to allow anyone access and are often put there to make it
convenient for users to login initially. Default accounts should
be turned off or have their predefined passwords changed, to
reduce the risk of abuse to the system.
A way of providing access to computer systems or networks via a
telecommunications network. A computer uses a modem to make a
telephone call to a another modem, which in turn provides 'network
access service'. See also: PPP.
A digital signature is created by a mathematical computer program.
It is not a hand-written signature nor a computer-produced picture
of one. The signature is like a wax seal that requires a special
stamp to produce it, and is attached to an Email message or file.
The origin of the message or file may then be verified by the
digital signature (using special tools).
Software packages retrieved from the Internet (using, for example,
the FTP protocol).
The act of retrieving files from a server on the network.
To communicate via electronic mail, an end-user usually makes use
of an Email client that provides the user-interface to create,
send, retrieve and read Email. Various different Email packages
provide the same set of basic functions but have different user-
interfaces and perhaps, special/extra functions. Some Email
packages provide encryption and digital signature capabilities.
Email Security Software
Software which provides security through digital signatures and
encryption (and decryption) to enable the end-user to protect
messages and documents prior to sending them over a possibly
insecure network. PGP is an example of such software.
Encrypting / Encryption
This is a mathematical process of scambling data for privacy
The software that actually provides the needed functionality for
end users to encrypt messages and files. PGP is one example.
An (human) individual that makes use of computer systems and
Files (programs, data, text and so on)
Files include user data, but also programs, the computer operating
system and the system's configuration data.
A computer system that provides a way of sharing and working on
files stored on the system among users with access to these files
over a network.
The process of transferring files between two computer systems
over a network, using a protocol such as FTP or HTTP.
Fixes, Patches and installing them
Vendors, in response to the discovery of security vulnerabilities,
provide sets of files that have to be installed on computer
systems. These files 'fix' or 'patch' the computer system or
programs and remove the security vulnerability.
FTP (File Transfer Protocol)
A protocol that allows for the transfer of files between an FTP
client and FTP server.
Group of Users
Security software often allow permissions to be set for groups (of
users) as opposed to individuals.
A support entity that can be called upon to get help with a
computer or communication problem.
A collection of interconnected networks that use a common set of
protocols called the TCP/IP stack to enable communication between
the connected computer systems.
Keys are used to encrypt and decrypt files. key escrow is used to
store keys for use by third parties to access the data in
Keys Used to Encrypt and Decrypt Files
To make use of encryption, an end-user has to provide some secret,
in the form of some data, usually called a key.
Log In, Logging into a System
This is an action performed by an end-user, when he authenticates
himself to a computer system.
Log In Prompt
The characters that are displayed when logging into a system to
ask for user name and password.
If an end-user has successfully proven to have legitimate access
to a system, he is considered to be logged in.
Systems and server software often provide the ability to keep
track of events. Events may be configured to be written out to a
file known as a log. The log file can be read later and allows
for system failures and security breaches to be identified.
Masquerade (see Remote Log In)
Anyone who pretends to be someone they are not in order to obtain
access to a computer account is said to be in 'masquerade'. This
may be accomplished by providing a false user name, or stealing
someone else's password and logging in as him.
Network File System (NFS, file sharing with PCs, etc.)
NFS is an application and protocol suite that provides a way of
sharing files between clients and servers. There are other
protocols which provide file access over networks. These provide
similar functionality, but do not interoperate with each other.
Networking Features of Software
Some software has features which make use of the network to
retrieve or share data. It may not be obvious that software has
Services which are not provided on the local computer system the
end-user is working on but on a server located in the network.
One-Time Passwords (OTP)
Instead of using the same password over and over again, a
different password is used on each subsequent log in.
A passphrase is a long password. It is often composed of several
words and symbols to make it harder to guess.
A screen saver obscures the normal display of a monitor. A
password-locked screensaver can only be deactivated if the end-
user's password is supplied. This prevents a logged-in system
from being abused and hides the work currently being done from
See "Fixes, Patches and installing them"
Another word for the access controls that are used to control the
access to files and other resources.
PGP (Pretty Good Privacy)
PGP is an application package that provides tools to encrypt and
digitally sign files on computer systems. It is especially useful
to encrypt and/or sign files and messages before sending them via
Software components that integrate into other software (such as
web browsers) to provide additional features.
In case of security breaches or problems, many organisations
provide a designated point-of-contact which can alert others and
take the appropriate actions.
PPP (Point to Point Protocol)
PPP is the mechanism which most end-users establish a network
connection between their PC and their Internet service provider
with. Once connected, the PC is able to transmit and receive data
to any other system on the network.
Another term for encryption software that highlights the use of
this software to protect the confidentiality and therefore privacy
of the end-users that make use of it.
Remote Access Software
This software allows a computer to use a modem to connect to
another system. It also allows a computer to 'listen' for calls
on a modem (this computer provides 'remote access service'.)
Remote access software may provide access to a single computer or
to a network.
Remote Log In
If an end-user uses a network to log in to a system, this act is
known as remote log in.
These are features which provide protection or enable end-users
and administrators to assess the security of a system, for
example, by auditing it.
A security policy is written by organisations to address security
issues, in the form of "do's" and "don'ts". These guidelines and
rules are for users with respect to physical security, data
security, information security and content (eg. rules stating that
sites with sexual content should not be visited, and that
copyrights should be honoured when downloading software, etc).
A server is a computer system, or a set of processes on a computer
system providing services to clients across a network.
A common account is one which is shared by a group of users as
opposed to a normal account which is available to only one user.
If the account is misused, it is very difficult or impossible to
know which of users was responsible.
Many computer systems allow users to share files over a network.
These systems invariably provide a mechanism for users to use to
control who has permission to read or overwrite these files.
Depending on the context in which this term is used, it might
apply to computer systems that are grouped together by
geographical location, organizational jurisdiction, or network
addresses. A Site typically refers to a network under a common
SSH (Secure Shell)
SSH provides a protocol between a client and server, allowing for
encrypted remote connectivity.
SSL (Secure Sockets Layer)
This protocol provides security services to otherwise insecure
protocols which operate over a network. SSL is typically used by
web browsers to encrypt data sent to and downloaded from a server.
The individual who maintains the system and has system
administrator privileges. In order to avoid errors and mistakes
done by this individual while not acting as an administrator,
he/she should limit the time he/she acts as an administrator (as
known to the system) to a minimum.
System Administrator Privileges
System administrators have more rights (greater permissions) as
their work involve the maintenance of system files.
The set of files on a system that do not belong to end-users,
which govern the functionality of the system. System files have
a great impact on the security of the system.
A protocol that enables remote log in to other computer systems
over the network.
A dumb device that is connected to a computer system in order to
provide (text-based) access to it for users and administrators.
Terms of Service (TOS)
See "Acceptable Use Policy (AUP)".
The potential that an existing vulnerability can be exploited to
compromise the security of systems or networks. Even if a
vulnerability is not known, it represents a threat by this
A program which carries within itself a means to allow the creator
of the program access to the system using it.
A program which replicates itself on computer systems by
incorporating itself (secretly and maliciously) into other
programs. A virus can be transferred onto a computer system in a
variety of ways.
Software that detects and possibly removes computer viruses,
alerting the user appropriately.
A vulnerability is the existence of a weakness, design, or
implementation error that can lead to an unexpected, undesirable
event compromising the security of the system, network,
application, or protocol involved.
Web Browser Cache
This is the part of the file system that is used to store web
pages and related files. It can be utilized to reload recently
accessed files from the cache instead of loading it every time
from the network.
Web Browser Capabilities
The set of functionalities on a web browser for use by the end-
user. This includes the set of plug-ins available.
A server program that provides access to web pages. Some web
servers provide access to other services, such as databases, and
A computer program which replicates itself and is self-
propogating. Worms, as opposed to viruses, are meant to spawn in
The User Security Handbook was a collaborative effort of the Site
Security Handbook Working Group of the IETF. There were also others
who made significant contributions --- Simson Garfinkle and Eric
Luiijf provided very helpful feedback on this document. The Glossary
contribution by Klaus-Peter Kossakowski is much appreciated.
[GLOSSARY] Malkin, G., Ed., "Internet User's Glossary", FYI 18, RFC
1983 August 1996.
[RFC2196] Fraser, B., Ed., "Site Security Handbook", FYI 8, RFC 2196
This document discusses what computer users can do to improve
security on their systems.
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