draft-ietf-ssh-users-05.txt              Erik Guttman / Sun Microsystems
                                             Lorna Leong / COLT Internet
                                                G. Malkin / Bay Networks
                                                          April 30, 1998

                        Users' Security Handbook

Status of this Memo

   This document is an Internet-Draft.  Internet-Drafts are working
   documents of the Internet Engineering Task Force (IETF), its areas,
   and its working groups.  Note that other groups may also distribute
   working documents as Internet-Drafts.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   To view the entire list of current Internet-Drafts, please check
   the "1id-abstracts.txt" listing contained in the Internet-Drafts
   Shadow Directories on ftp.is.co.za (Africa), ftp.nordu.net
   (Northern Europe), ftp.nis.garr.it (Southern Europe), munnari.oz.au
   (Pacific Rim), ftp.ietf.org (US East Coast), or ftp.isi.edu
   (US West Coast).


   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 keep their networks and systems secure.

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   Table of Contents

        Part One: Introduction. . . . . . . . . . . . . . . . . .

   1.   READ.ME . . . . . . . . . . . . . . . . . . . . . . . . .
   2.   The Wires have Ears . . . . . . . . . . . . . . . . . . .

        Part Two: Users in a centrally-administered network . . .

   3.   Watch out! . . . . . . . . . . . . . . . . . . . .  . . .
   3.1.   The Dangers of Downloading  . . . . . . . . . . . . . .
   3.2.   Don't get caught in the Web . . . . . . . . . . . . . .
   3.3.   Email Pitfalls  . . . . . . . . . . . . . . . . . . . .
   3.4.   Passwords . . . . . . . . . . . . . . . . . . . . . . .
   3.5.   Viruses and Other Illnesses . . . . . . . . . . . . . .
   3.6.   Modems  . . . . . . . . . . . . . . . . . . . . . . . .
   3.7.   Abandoned Terminals . . . . . . . . . . . . . . . . . .
   3.8.   File Protections  . . . . . . . . . . . . . . . . . . .
   3.9.   Encrypt Everything  . . . . . . . . . . . . . . . . . .
   3.10.  Shred Everything Else . . . . . . . . . . . . . . . . .
   3.11.  What program is this, anyway? . . . . . . . . . . . . .
   4.   Paranoia is Good  . . . . . . . . . . . . . . . . . . . .

        Part Three: Users self administering a networked computer

   5.   Make your own security policy
   6.   Bad Things Happen . . . . . . . . . . . . . . . . . . . .
   6.1.   What to do if you suspect trouble . . . . . . . . . . .
   6.2.   How to prepare for the worst in advance . . . . . . . .
   7.   Home Alone  . . . . . . . . . . . . . . . . . . . . . . .
   7.1.   Beware of Daemons . . . . . . . . . . . . . . . . . . .
   7.2.   Going Places
   7.3.   Code It!
   8.   A Final Note

   Appendix: Glossary of Security Terms . . . . . . . . . . . . .
   Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . .
   References . . . . . . . . . . . . . . . . . . . . . . . . . .
   Security Considerations  . . . . . . . . . . . . . . . . . . .
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . .

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Part One:  Introduction

   This document is meant to provide guidance to the end users of
   computer systems and networks on what they can do to keep their data
   and communication private, and their systems and networks secure.
   The first part of this documents concerns "corporate users" in small,
   medium and large corporate and campus sites.  The second part of the
   document addresses users who administer their own computers, such as
   home users.

   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 [SSH].

   A glossary of terms is included in an appendix at the end of the
   document introducing computer network security notions to those not
   familiar with them.  Terms in the glossary are in all capital letters
   the first time they are used.


   Before getting connected to the Internet, where available, you should
   get hold of the security policy of the site where you intend to get
   connected, 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.  When using an outside network, you have an obligation
   to follow its Acceptable Use Policy (if it has one).

   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 when problems arise and who to
   contact, and generally informs users what the "rules of the game"

2.  The Wires have Ears

   It is a lot easier to evesdrop 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 evesdropped
   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 "Web Browsing Safely" and
   "Email Pitfalls" sections for specific information on protecting your

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   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
   someone else!

   Many network services involve remote log in.  A user is prompted for
   his or her account ID 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 evesdrop on than internet communications.

   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 name
   and password and then your data over the network without encrypting

   The precaution commonly taken against password evesdropping by larger
   institutions, such as corporations, is to use one-time password
   systems.  Until recently this has been far too complicated and
   expensive for home systems and small businesses.  An increasing
   number of products allow 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.
    - Use a password-locked screensaver or log out when you leave your
    - Don't let just anyone have physical access to your computer or
      your network.
    - Be aware what software you run and very wary of software of

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      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
   greatest risk.

   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 a common program just because they have the
   same name!

   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 file 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
      cracker's machine!

   Many corporate environments explicitly prohibit the downloading and
   running of software from the Internet.

3.2. Web Browsing Safety

   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 was a Trojan Horse program,
   script, etc.)

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   Web browsers may download and execute programs on your behalf.  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
   quite risky to execute on your machine.  (See "What program is this,

   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 messages which are sent between the user's web
   browser to the web server so no one along the way can read it.

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 is not a suitable way to conduct business.  It is
   very easy to forge an Email message, so that it appears to come from

   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 with 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 mail there.  She sets it up so that her
      Email at work is forwarded to her private address.  All the mail
      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

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   read your mail 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 mail messages.  The
   files which come by mail 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."

3.4 Passwords

   Passwords may be easily guessed by an intruder unless precautions are
   taken.  Your password should contain a mix of numbers, punctuation,
   and upper and lower case letters.  Avoid all real words 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 this password!)

   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.

   You should use "one time passwords" if you are logging in over a
   network, unless precautions have been taken to encrypt your password
   when it is sent over the 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 network.

3.5 Viruses and Other Illnesses

   Viruses are essentially unwanted pieces of software that find their
   way into a computer.  What the virus may do once it has entered its
   host depends on several factors:  What the virus has 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

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   particular condition, such as reaching a certain date.  Others remain
   latent in the system unless a particular afflicted program is
   activated.  Still, there are 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 use
   in countering the threat of viruses.  Consult your site policy or ask
   your system administrator to find out 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 technically-
   competent personnel.

   Trojan Horse programs and worms are often categorised with viruses.
   Trojan Horse programs are dealt with in the "What program is this,
   anyway?" section.  Worms should be considered a type of virus for the
   purposes of this section.

3.6 Modems

   You should be careful when attaching anything to your computer, and
   especially something 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 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

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   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 troubles 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.  This means that you should be careful with
   who you allow access to your machine.  If this is impossible, it is
   wise to encrypt your data files kept on your local hard disk.   It is
   wise to lock the door to one's office where the computer is stored,
   if possible.

3.8 File Protections

   Data files and directories on shared systems or networked file
   systems require care and maintenance.  There are two categories
   of such systems:

    - 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 which only you should have access to, but
      which are 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
      set accordingly.

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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.  A home computer may be used
   for preparing taxes and also for playing computer games by children.
   By backing up the data and using encryption, this kind of shared use
   may be done safely.

   Before you encrypt files you should check with your site's security
   policy.  Some employers and countries expressly forbid 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 will help you to keep them secure too; for if you lose
   them, you 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 in the wast paper
   basket:  notes from meetings, old schedules, internal phone lists,
   computer program listings, correspondence with customers, 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.

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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 a mail message which can
      only be read if you download a special component.  This component
      could be a nasty program which reformats 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 request that certain web browser capabilities
      be turned off.

4.  Paranoia is Good

   Many people do not realise 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.  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.

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   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 way 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.  Just like how
      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
      away on the user's keyboard, commands the user does not understand
      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

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      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 lead to...

   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.

   Most maintenance work will require special privileges which end-users
   are not given.  Users should guard the use of their accounts, and
   keep it for their own use.  Accounts should not be shared, not even
   temporarily with a maintenance staff or administrator.  Systems
   administrators will have their own accounts to work with and will not
   need to access a system via an end-user's account.

   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 system user.  The
   following is a summary of additional advice given in Part Three:

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    - Read manuals to learn how to turn on security features, then turn
      them on.
    - 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 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 machine has a mixed purpose, say recreation, correspondence
   and some home accounting, perhaps you will hazard some downloading of
   shareware applications.  You take some risk of acquiring software
   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 roommate 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.  In many cases,
   you will not know who your security point-of-contact is:  Try calling
   your Internet service provider's help desk as a first step.

6.1 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.

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   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 worth while.  Do not forget to
   re-install all security related fixes you had installed before the
   security incident.  Obtain these from a verified, unsuspicious

6.2 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
      these features.

    - 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 become well known and be exploited by some
      network assailant.  Keep up to date!

   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.  Do not run programs which origin you are unaware or
      unsure of.  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

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      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 computer 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.

   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 presence of viruses.

   The best way to avoid problems with viruses is to keep important
   files backed up.  This way, if worse comes to worse, you can always
   restore your system to its state before it was afflicted.

   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 freeware,
   other peoples' used floppy disks to transfer files, and so on.

6.3 Email

   Remember to be careful with saved mail.  Copies of sent or received
   mail (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 mail.  Keep
   your mail 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.

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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
   concerned. 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 [SSH] for a list of

      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

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      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 have been web
      browsers and telnet clients in common use 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 relates 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 a system which is
   far away physically, in 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 sniffed password useless to the intruder, while secure shell
   encrypts data sent over the connection.  Please refer to "Web Browsing
   Safely" for a discussion on SSL.  Secure services such as these have to
   be made available on the systems you make remote logins on.

7.3 Code It!

   Administering your own home computer means you get to choose what
   software is run on it.  Encryption software provides protection on
   data.  If you keep business records and other sensitive data on your
   computer, encryption will help to keep it safe.  Say, if you ran a
   network service from your home computer and missed setting
   restrictions on a private directory, a remote user (authorised 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
   detail as possible while keeping it concise.  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 caution, at the expanse of
   convenience, on securing 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.

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Appendix: Glossary of Security Terms

   Trojan Horse

      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 into other programs which are shared among
      computer systems.


      A computer program which replicates itself and is self-
      propogating.  Worms, as opposed to viruses, are meant to spawn in
      network environments.

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   [GLOSSARY] Malkin, G, ed, "Internet User's Glossary", RFC 1983 (FYI
           18), August, 1996.

   [SSH]   Frasier, Barbara, ed, "Site Security Handbook," RFC 2196 (FYI
           8), June, 1996.

Security Considerations

   This document discusses what computer users can do to improve
   security on their systems.

Authors' Addresses

          Erik Guttman          Lorna Leong         Gary Malkin
          Sun Microsystems      COLT Internet       Bay Networks
          Bahnstr. 2            250 City Road       8 Federal Street
          74915 Waibstadt       City Forum, London  Billerca, MA 01821
          Germany               England             USA

   Phone: +49 7263 911701       +44 171 390 3900    +1 508 916 4237
   Email: erik.guttman@sun.com  lorna@colt.net      gmalkin@baynetworks.com

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