Home Computer Security

8/6/2019 Home Computer Security

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This work was produ ced for FedCIRC an d th e Gen eral ServicesAdm inistration by the CERT® Coordin ation Center, Software

Engineering Institute, Carnegie Mellon University.

Copyright 2002 Carnegie Mellon University



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Contents

Int rod uct ion ..........................................................................1

Thin king About Securin g You r Hom e Com pu ter..................3

Thin gs You Ou gh t To Kno w ..................................................4

Wh at Sh ou ld I Do To Secure My Hom e Com pu ter? .............7

Sum m ary .............................................................. ................33

En d Not es ............................................................................34

Ackn ow ledgem en ts .............................................................34



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Property h as its du ties

as well as its righ ts.Th om as Drumm on d (1797-1840)



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IntroductionYour h om e comp uter is a popular target

for intruders. Why? Because intruders want

wh at you ’ve stored t h ere. Th ey look for

credit card n um bers, bank accoun t

information , and an yth ing else th ey can

fin d. By stealin g that information , intruders

can use your mon ey to buy them selves goods

an d services.

But it’s no t just m on ey-related

information th ey’re after. Intru ders

also want your computer’s resources,

m eanin g your hard disk space, your

fast p rocessor, an d you r Intern et

con n ection . Th ey use these resources

to attack oth er com puters on the

Internet. In fact, th e more comp uters

an intrud er uses, th e harder it is

for law enforcem ent to figure out

wh ere the attack is really com ing

from . If intrud ers can’t be found ,th ey can ’t be stopped, and th ey can’t

be prosecuted.

Wh y are in truders payin g attention

to hom e com puters? Home

com put ers are typically not very

secure an d are easy to break in to.

Wh en com bined with h igh-speed

Internet connections that are always

turned on , intruders can q uickly find an d th en attack hom e computers.

Wh ile intruders also attack hom e compu ters conn ected to the Internet

through dial-in connections, high-speed connections (cable modems

an d DSL m odem s) are a favorite target.

No matter how a ho me com puter is conn ected to th e Internet,

intrud ers’ attacks are often successful. Many h om e com pu ter own ers

don ’t realize that th ey need to pay atten tion t o com pu ter security. In

th e sam e way that you are respon sible for havin g in surance when you

Home Computer Security





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Thinking About Securing Your Home ComputerBefore diving into the tasks you need to

do t o secure your h om e com pu ter, let’s first

th ink about th e problem by relating it to

someth ing you already kn ow h ow to do. In

this way, you can apply your experience to

th is n ew area.

So, thin k of your comp uter as you would your h ouse, your apartm ent,

or your condo. Wh at do you know about how that l iving space works,wh at do you rou tinely do to keep it secure, and wh at have you in stalled

to im prove its security? (We’ll use th is “comp ut er-is-like-a-ho use-and -

th e-th ings-in-it” an alogy th rough out, dep arting on ly a few times to

make a point.)

For examp le, you kn ow th at if you h ave a loud conversation, folks

outside your space can probably h ear you. You also rou tinely lock the

doors and close the wind ows when you leave, an d you do n ’t give the

keys to just anyon e. Som e of you m ay install a security system to

com plemen t you r practices. All of th ese are part of living in you r ho m e.

Let’s no w app ly similar thin king to you r h om e com pu ter. Em ail,

instan t m essagin g, and m ost web traffic go across the Intern et in th e

clear; th at is, anyon e who can captu re th at information can read it.These are th ings you o ugh t to kno w. You shou ld always select an d

use strong passwords and exercise due care when reading all email,

especially the un solicited variety. Th ese are th ings you o ugh t to do.

Finally, you can add a firewall, an an ti-virus program , patch es, and file

encryption to imp rove the level of security on you r hom e comp uter, an d

we’ll call these thin gs you ou ght to in stall.

The rest of this pamp h let describes the th ings you ough t to kn ow, do,

an d install to imp rove the security of your h om e comp uter.




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Things You Ought To KnowOn e starting point for solvin g hom e

computer security problems is being aware

of how th e In ternet an d some of its

techn ologies work. If you know h ow th ey

work, you can evaluate solution s to th e

problems th at com e up. You can also u se

the Internet more safely and responsibly. In

th is section , we’ll talk about two t op ics: trustan d inform ation in th e clear as it crosses the

Internet.

Trust

Hum an bein gs are trusting by nature. We trust m uch o f what we h ear

on th e radio, see on television, an d read in th e newspaper. We trust th e

labels on packages. We trust the mail we receive. We trust our parents,

our p artner or spou se, an d o ur ch ildren. We trust ou r co-workers. In fact,

those who don ’t trust much are thought to be cynical. Th eir opinions

m ay be all too q uickly ignored or d ism issed.

The In ternet was built on trust.1 Back

in th e mid 1960s, com put ers were

very expensive and slow by today’s

stan dard s, but still qu ite useful. To sh are

th e expensive and scarce comp uters

installed aroun d th e coun try, the U.S.

governm ent fun ded a research p roject

to con nect th ese comp uters together so

th at oth er researchers could use them

rem otely. This project was called th e

ARPAn et, nam ed after the govern m ent

research agency – ARPA, the Advanced

Research Projects Agency – th at fun ded

and m anaged the project .Key to th e ARPAn et was t h e level of

trust placed in its users; there was little th ough t given to m alicious

activity. Comp uters com m un icated using a straigh tforward schem e that

relied on everybody p layin g by th e rules. Th e idea was to make sharin g



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ideas an d resources easy and as efficient as the tech n ology of the

day provided. This philosoph y of trust colors man y of the p ractices,

procedures, and techn ologies that are still in place today.

On ly within t h e last few years, when Internet com m erce (kno wn

as e-comm erce) began to spread, it has becom e inadeq uate to rely

prin cipally on t rust. Since th e days of th e ARPAn et, we’ve chan ged

the way we use compu ter networks while oth ers have chan ged th e

un derlying techn ologies, all in an attem pt to im prove th e security of the

Internet an d th e trust we place on i t .

Let’s dig deeper int o two ex am ples of what we t rust in o ur da ily lives.

Wh en you receive mail through th e post office, man y envelopes and th eletters in th em cont ain th e sender’s address. Have you ever wondered

if th ose addresses were valid; th at is, do t h ey match th e address of th e

person or persons wh o really sent th em? Wh ile you could ch eck to see

th at th ose addresses are valid an d refer to th e person th ey nam e, it’s

n ot an easy task.

How would you go about i t? Would you call the ph on e num ber

provided with th e letter? That n um ber could also be in valid, and

the p erson th at answers the ph one could be as misleadin g as the

original address. Perhaps you could call directory assistance or the police

departm ent th at has jurisdiction over th e town wh ere th e letter was

supposedly from . Th ey migh t be h elpful, but th at is likely to take lots of

time. Most people wouldn ’t bot h er.

An d it’s n ot just retu rn addresses eith er. How abo ut advertisemen ts,

n ews stories, or th e information printed on groceries? Supp ose you were

on a low-fat diet. You ’d wan t to b uy food s low in fat. To select the righ t

foods, you’d read th e produ ct label at th e grocery store. How do you

kno w th at th e label in formation is valid? Wh at’s to say it’s no t forged?

And ho w would you kno w?

The Internet h as man y of the same issues, and em ail is on e of the best

examp les. In an email message, an in truder can easily fabricate wh ere the

came from. But th is in formation forging – called spoofin g by intrud ers

and security professionals – is not limited to just email. In fact, the basic

un it of in formation transferred on th e Internet – called a packet – can

also be easily forged o r spoo fed.

Wh at does this m ean an d wh y sho uld you care? It m eans that an y

information you receive from some oth er computer on the Internet

should n ot be trusted autom atically and u ncon dition ally. Wh en you

trust an em ail m essage that tu rns out to h ave a harmful virus attached




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to it, your comp uter can be in fected, your files destroyed, and your work

lost. An d th at’s why you sh ould care.

This is how t h e Internet works. It was built on trust. Over tim e, there

h ave been tech n ological chan ges th at are worthy of a higher level of our

trust th an before. Noneth eless, a true sense of insecurity is better than

a false sense of security. So, think about the information you trust. Be

critical and cautious.

Informat ion in the Clear

Wh en you h ave a conversation with someone in your l iving space,

everybody within earshot can hear th e words and p robably un derstand

th em. If your con versation is especially loud an d your win dows open ,

even p assersby can h ear. If you wan t p rivacy, you an d you r con versation

partner need to go to ano ther room an d close the doors and windo ws.

The Internet works much the sam e way, except th e room is much,

m uch bigger. When you send email, browse a web site, or ch at on lin e

with someone, the con versation between you and that p erson does not

go directly from your com put er to h is or h er comp uter. Instead, it goes

from your comp uter to anoth er com puter to st il l anoth er com puter and

so on , even tually reachin g his or her com put er. Th ink of all of th ese

compu ters as an Internet “room.”

An yon e, or, more accurately, any program , in th at Internet roo m

that can h ear that con versation can also probably und erstand i t . Why?Because just like the con versation at h om e, most Intern et conversations

are in the clear, meaning that the information exchanged between

com put er systems is not con cealed or

hidden in an y way.

Again , this is h ow th e Internet wo rks.

You n eed to kn ow th at th e information

sent across th e Internet m ay be at risk of

oth ers listening in, capturing wh at you

send, and u sin g it for th eir own benefit.

Later in th is pamp h let, we’ll talk abou t

encryption as a way to add ress this

problem. Encryption uses mathematicsto con ceal information . Th ere are m an y

programs you can in stall to encrypt

the information you send across the

Internet.



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What Should I Do To Secure My Home Computer?Securin g your ho m e comp uter is n ot a trivial

task. Th ere are m an y topics to consider and

m an y steps to follow. Th ey take time to

learn an d do . If you can , read th is entire

pam ph let before you begin to secure your

com pu ter. You ’ll h ave a better un derstan din g

of th e effort an d all its facets. Th is ou ght to

h elp you wh en you begin to tackle th e tasksdescribed h ere.

In th e followin g section s we describe two typ es of activities. Som e

you can d o using the programs that came with you r computer: workin g

with passwords and email at tachm ents, runn ing programs, and backing

up you r work. For oth er activities, you m ight n eed to obtain some

specialized programs: app lyin g patch es, and run n ing an ti-virus, firewall,

and file encryption programs. Th ough some vend ors’ produ cts provide

th ese features, we’ll assum e your com put er doesn’t h ave any of th em so

you’ll n eed to add all of them .

Here then is th e list of tasks you n eed to do to secure your h om e

comp uter. Their order is based on h ow intru ders attack comp uters,begin n ing with th e mo st-often used attack m eth ods. By starting with

th e lower nu m bered tasks, you ad dress th e biggest problems you face in

securing your ho m e compu ter. Remem ber that m ost section s end with a

reference to a web site that you can use to find an examp le of h ow to do

th e task on a Microsoft Wind ows 2000 comp uter.

Task 1 - Install and Use Ant i-Virus Programs

If someon e ran g your doorbell and wan ted to com e in to your living

space to sell you som eth ing or to use your telepho n e, you’d need to

m ake a decision wh eth er or n ot to let them in. If they were a neighbor or

someon e you knew, you’d probably let them in. If you didn ’t kno w th em

but b elieved their story and foun d th em t o be oth erwise acceptable, say

th ey were neat an d clean an d n ot th reatening, you’d probab ly also letth em in , but you’d watch t h em closely while they were in your space.

Wh at are you doin g here? You are profiling th is person an d th en

decidin g wh at to do based on th at p rofile. It’s your responsibility to

be con cerned abou t wh o en ters your livin g space. Furth er, if you h ave




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children, you’ve probably also taught th em h ow to deal with strangers

who com e to your door.

An ti-virus programs work m uch th e same way. Th ese program s look

at th e conten ts of each file, search ing for specific pattern s that m atch a

profile – called a virus signatu re – of someth ing kn own to be h armful.

For each file that m atches a sign ature, th e an ti-virus program typically

provides several option s on h ow to respond , such as remo ving th e

offend ing pattern s or destroying th e file.

To un derstand h ow an ti-virus programs work, th ink abou t scam

artists – people who visit your hom e to try to get you to bu y a pho n y

produ ct or service, or to let th em in . Once inside, they m ay try to stealyour valuables or try to h arm you in som e way.

There are a variety of ways you m igh t fin d ou t abou t a specific scam

artist lurking in you r neighbo rho od. Perhaps you see a television report

or read a newspaper article about th em. They m igh t include pictures and

excerpts of th e story th e scam artist uses to scam th eir victims. Th e n ews

report gives you a p rofile of someon e you n eed to be on th e lookout for.

You watch for that p erson un til either th e story fades away or you h ear

that they’ve been caught.

An ti-virus programs work mu ch th e same way. When th e anti-virus

program ven dors learn abou t a new virus, th ey provide an u pdat ed set of

virus signatu res th at include th at n ew one. Th rough features provided by

th e updated an ti-virus program , your hom e com put er also autom atically

learn s of this new virus an d begins ch eckin g each file for it, along with

ch ecking for all the o lder viruses. However, un like scam artists, viruses

n ever com pletely fade away. Th eir signatures remain part of th e m aster

version of all virus signatu res.

Sup pose a scam artist was at your front door. What wou ld you do ?

Perhaps you’d n ot encourage them to come in nor bu y their product but,

at th e same tim e, you’d try n ot to up set them . You’d p olitely listen to

th eir story and th en send th em o n t h eir way. After you closed th e door,

you m ay call the p olice or the telephon e nu m ber given in the report th at

init ially brought them to your attention .

With viruses, you often h ave the chan ce to react to them when th ey’ve

been discovered on your h ome compu ter. Depend ing upon the specific

characteristics of the virus, you m igh t be ab le to clean th e infected file.

Or you m igh t be forced to destroy the file an d load a n ew copy from yo ur

backups or original distribution m edia. Your opt ions depen d up on your

choice of anti-virus program an d th e virus th at’s been d etected.



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In your living space, you look at those who com e to your door an d

you look at wh at you receive in th e mail. Th ese are two of the ways that

items can get in to your living space, so you exam ine th em, sometim es

closely, sometimes not.

Viruses can reach your com put er in m any ways, th rough flo ppy disks,

CD-ROMs, email, web sites, and dow n loaded files. All n eed to b e checked

for viruses each tim e you use th em. In ot h er words, when you in sert a

flopp y disk in to th e drive, ch eck it for viruses. Wh en you receive em ail,

check it for viruses (remem ber to u se the KRESV tests described in Task

3, Use Care W hen Reading Email with At tachm ents). When you download a

file from th e Intern et, check it for viruses before usin g it. You r an ti-virusprogram m ay let you specify all of these as places to ch eck for viruses

each tim e you op erate on th em. Your an ti-virus program m ay also do

th is autom atically. All you n eed to do is to op en o r run th e file to cause

it to be checked.

Just as you w alk aroun d you r living space to see if everythin g is OK, you

also n eed to “walk” aroun d your h om e comp uter to see if th ere are any

viruses lurkin g abou t. Most an ti-virus p rogram s let you sch edule p eriodic

exams of all files on your h om e comp uter on a regular basis, daily for

examp le. If you leave your comp uter turn ed on o ver n ight, thin k about

sch eduling a full-system review durin g th at tim e.




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Som e anti-virus programs have m ore advanced features th at extend

th eir recognition capabilities beyon d virus sign atures. Som etimes a file

won’t m atch an y of the known signatures, but i t may h ave som e of

the characteristics of a virus. This is comparable to getting that “there’s

someth ing n ot qu ite righ t h ere, so I’m n ot going to let them in” feelin g

as you greet som eon e at you r door. Th ese heu ristic tests, as th ey’re called,

h elp you to keep up with n ew viruses th at aren’t yet defined in your

list o f virus signa tu res.

An an ti-virus program is frequen tly an add-on t o your h om e comp uter,

th ough your n ewly purch ased com put er might include a trial version. At

some po int, say after 60 days, you m ust purch ase it to con tinu e usin g it.To decide wh eth er to m ake that purch ase or to look elsewhere, use th ese

steps for evaluating anti-virus programs:

1. The Demand test: Can you ch eck a file on d eman d, for

example, when you want to send an attachm ent as part of the

KRESV tests?

2. The Update test: Can you u pdate th e virus signatures

aut om atically? Daily is best.

3. The Respond test: Wh at are all th e ways th at you can respond

to an infected file? Can th e virus checker clean a file?

4. The Check test: Can you ch eck every file that gets to your

hom e computer, no matter how it gets there, and can th ose

checks be automated?

5. The Heuristics test: Does the virus checker do heuristics tests?

How are th ese defin ed?

These tests – th e DURCH tests – h elp you com pare an ti-virus programs.

Once you’ve made your selection, install it and use all of its capabilities

all of the tim e.

Intrud ers are th e m ost successful in attackin g all com put ers – n ot just

h om e compu ters – wh en th ey use viruses and worms. In stallin g an an ti-

virus program an d keeping it up to date is amon g the best defenses for

your h om e com put er. If your fin an cial resources are lim ited, they are

better spent purchasing a commercial anti-virus program than anything

else.

To see an exam ple th at sho ws how t o op erate a virus checker, see

http://www.fedcirc.gov/homeusers/HomeComputerSecurity/examples.html .



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Task 2 - Keep Your System Patched

If on e of your app liances broke, you’d p robably try to h ave it repaired.

You’d call a repairperson wh om you h ope cou ld do th e job. You’d get an

estimat e an d th en yo u’d eith er get it fixed or replace it. You r goal is to

someh ow restore the function s th at th e applian ce provides.

Wh at do you d o wh en a software “applian ce” – a program – or th e

operating system itself breaks? How do you restore th e function s that

th ey provide? Do you know wh om to call or even wh ere to look to

determin e what to do n ext?

Most vendors provide patches that are supposed to fix bu gs in th eir

produ cts. Frequen tly these patches do wh at th ey’re sup posed to do.However, sometimes a patch fixes one p roblem bu t causes anoth er. For

examp le, did you ever have a repairperson fix an ap pliance but in th e

process, they scratched th e floor or damaged a coun tertop du rin g their

visit? For a com put er, th e repair cycle m ight h ave to be repeated un til a

patch comp letely fixes a problem.

Vend ors often provide free patch es on t h eir web sites. Wh en you

purch ase program s, it’s a good idea to see if and h ow th e vend or supplies

patch es, an d if an d h ow th ey provide a way to ask questions about th eir

produ cts. Just as app lian ce vend ors often sell exten ded warranties for

th eir products, some software vendors m ay also sell supp ort for th eirs.

Have you ever received a recall no tice for your car or an oth er product

you’ve purchased? Vend ors send th ese n otices to prod uct own ers whena safety-related problem has been discovered. Registering your purchase

through the warranty card gives the vend or the information th ey need to

con tact you if th ere is a recall.

Program ven do rs also p rovide a recall-like service. You can receive

patch n otices th rough email by subscribin g to m ailing lists operated

by th e programs’ vend ors. Th rough th is type of service, you can learn

about p roblems with yo ur comp uter even before you discover th em an d,

h opefully, before in truders have the ch an ce to exploit th em. Con sult the

vend or’s web site to see how t o get em ail notices about p atches as soon

as th ey’re available.

Som e vendo rs have gone beyon d m ailing lists. Th ey provide program s

bun dled with t h eir system s that autom atically cont act their web

sites looking for patches specifically for your home computer. These

autom atic upd ates tell you wh en p atches are available, down load them ,

and even install them . You can tailor th e upd ate features to do on ly

want you wan t, such as just telling you someth ing n ew is waiting but

doing noth ing more.




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Wh ile th e patchin g process is getting easier, even to th e point wh ere

it can be com pletely autom ated, it is n ot yet foolproof. In some cases,

installing a patch can cause an oth er seemin gly un related program to

break. The challenge is to do as mu ch h om ework as you can to learn

what a p atch is supposed to do and what p roblems it might cause once

you ’ve installed it.

This is a hard job. Often, th e vendo rs don ’t tell you abo ut p roblems

their patches can cause. Why? Because it is simply impossible to test

all possible program s with all possible patches to discover un expected

side effects. Imagine doing th at job and th en con tinu ing to do th at for

each new p rogram an d p atch th at comes along. Vendors rely on theircustomers to tell them when something un expected happ ens once a

patch is installed. So, if th is h appen s to you, let th em kno w.

Imagine th en th at you’ve eith er foun d a patch on th e vendor’s site or

you’ve received n otice that a p atch is available. What d o you d o n ext?

Follow the steps below to evaluate a patch before you install it:

1. The Affected test: Does this patch affect on e of the p rograms

on your com pu ter? If it doesn’t affect your com put er, you’re

don e. Wh ew!

2. The Break test: Can you t ell from th e vend or’s web site or

th e patch ’s description if in stallin g it breaks som ethin g else th at

you care about? If installation d oes break someth ing, then you

h ave to decide h ow to proceed. Try notifyin g the vend or of theprogram th at m igh t break to learn wh at th eir strategy is for

addressing this problem. Also, use your web browser to learn if

anyon e else has experienced th is problem and what h e or she

did about i t .

3. The Undo test: Can you un do th e patch? That is, can you

restore your com put er to th e way it was before you in stalled

th e patch ? Curren tly, vendors are building m ost patch es with

an un install feature that enables you to remo ve a patch th at

has unwanted consequences. In addition, some computers also

com e with features that h elp you restore them to a previously

kno wn an d working state shou ld th ere be a problem. You n eed

to know wh at your computer provides so th at you can un do a

pat ch if necessary.

Recall from th e Introdu ction t h at int ruders exploit vulnerabilities to

gain access to h ome compu ters. How do intruders find out abou t th ese

vulnerabilities? In m any cases, they read th e same ven dor m ailing lists

and use the same autom atic notification schem es that you use. This

mean s that you n eed to evaluate and install patches on your hom e



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computer as soon as they’re available. The longer a vulnerability is

known , the greater the chan ces are that an intruder will find i t on your

hom e computer and exploit i t . With th e ABU tests, you can quickly

evaluate an d in stall patches to keep intrud ers off your h om e comp uter.

On e last th ing: patches are usually distributed as programs. Th is means

th at you n eed to use th e DCAL steps described in Task 7, Use Care

W hen Down loading and Installing Program s, before loading an d in stallin g

a patch.

Intrud ers often take advan tage of vulnerabilities wherever they m ay be.

In m an y cases, th e vulnerabilities they exploit may h ave patches, but

th ose patches were not in stalled. For your ho m e comp uter, m ake tim eto keep you r programs patch ed wh erever possible. If you can’t patch a

program, shop around for an equivalent program and use it un ti l the

original program is fixed or you’ve aband on ed it in favor of someth ing

more reliable.

You can spend m oney on m ainten ance where you get patches

for programs, but t h at’s usually not n ecessary. Since m ost ven dors

provide free patch es, m ailing lists, and autom atic updates, keepin g your

comp uter patched u sually only costs you time.

To see an exam ple that shows h ow to ch eck for, down load, and in stall

patches, see http://www.fedcirc.gov/homeusers/HomeComputerSecurity/

examples.html.

Task 3 - Use Care When Reading Email with Attachments

We’ve all heard stories about people receiving an item in th e m ail

th at in some way caused them h arm. We’ve heard of letter bomb s

and exploding packages, and in 2001, we learned abo ut Anth rax-laden

letters. Althou gh th eir frequen cy is low, th ey do m ake news.

These unsolicited items are sent to u n suspectin g recipien ts. Th ey may

cont ain a return address, a provocative en velope, or som eth ing else

that encourages its receiver to open it. This technique is called social

engineering. Because we are trusting and curious, social engineering is

often effective.

In th e case of th e An th rax letters addressed to United States sen ators,

th e envelopes con tained a schoo l’s return address as an indu cemen t

to open them . What governm ent official wouldn ’t want to serve their

constituen cy by reading and respon ding to a letter supp osedly sent by a

class at a schoo l, especially an element ary school? By op enin g th e letter

and subsequent ly spreading its lethal conten ts, th e recipien t com plied




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with th e wish es of th e sender, a key foun dation of social engineering.

In th e pre-An th rax letter days, a mail han dler might h ave given little

th ough t to th e conten ts of th e letter or th e validity of the return address.

Th ose days are behind us.

You prob ably receive lots of mail each d ay, m uch of it un solicited

and cont aining u n familiar but plausible return ad dresses. Som e of this

m ail uses social engineering to tell you of a con test that you m ay have

won or th e details of a produ ct th at you m igh t like. Th e sen der is

trying to encou rage you to op en th e letter, read its con ten ts, an d interact

with th em in some way th at is fin ancially beneficial – to them . Even

today, man y of us open letters to learn wh at we’ve won o r what fan tasticdeal awaits us. Since th ere are few con sequen ces, th ere’s no h arm in

opening them.

Em ail-born e viruses and wo rms op erate mu ch t h e same way, except

th ere are con sequences, som etimes significant on es. Malicious email

often con tains a return address of someon e we know and often h as

a provo cative Sub ject lin e. Th is is social en gineerin g at its fin est –

something we want to read from som eone we know.

Em ail viruses an d wo rms are fairly comm on . If you’ve n ot received o n e,

chan ces are you will. Here are steps you can use to h elp you d ecide wh at

to do with every email message with an attachm ent th at you receive. You

shou ld on ly read a m essage that passes all of th ese tests.

1. The Know test: Is th e email from som eon e that you kno w?

2. The Received test: Have you received email from this sender

before?

3. The Expect test: Were you expecting email with an at tachm ent

from th is sender?

4. The Sense test: Does email from th e sen der with th e conten ts as

described in th e Subject line and th e nam e of th e attachm ent(s)

make sense? For example, would you expect the sender – let’s

say your Moth er – to send you an em ail m essage with th e

Subject lin e “Here you h ave, ;o)” that con tains a m essage with

attach m en t – let’s say An n aKou rn ikova.jpg.vbs? A m essage like

th at probably doesn’t m ake sense. In fact, it happ ens to be

an in stance of th e An n a Kourn ikova worm, an d reading it candam age your system .



15

5. The Virus test: Does th is email contain a virus? To d etermine

th is, you n eed to install and u se an an ti-virus program. That

task is described in th e section ent itled Install and Use Anti-Virus

Program s.

You sh ou ld app ly these five tests – KRESV – to every piece of ema il

with an attachm ent th at you receive. If any test fails, toss that em ail.

If th ey all pass, then you still need to exercise care and watch for

un expected results as you read it.

Now, given t h e KRESV tests, im agin e that you wan t to send em ail

with an attachm ent to som eone with wh om you’ve never correspon ded

– what sh ould you do? Here’s a set of steps to follow to begin an emaildialogue with someone.

1. Since th e recipient do esn’t already Know you, you need to

send them an introductory email. It m ust not con tain an

attachment. Basically, you’re introducing yourself and asking

their perm ission to send email with an attachm ent th at they

m ay oth erwise be suspicious of. Tell th em wh o you are, wh at

you’d like to do, an d ask for permission to con tinu e.

2. This introdu ctory email qu alifies as th e m ail Receivedfrom you.

3. Hopefully, th ey’ll respon d; an d if th ey do, h on or their wish es.

If th ey ch oose not t o receive email with an att achm ent from

you, don ’t send on e. If you n ever h ear from t h em, try yourintroductory email one m ore t im e.

4. If th ey accept your offer to receive em ail with an attach m ent,

send it off. They will Know you an d will have Received email

from yo u b efore. Th ey will also Expect th is email with an

attachm ent , so you’ve satisfied th e first th ree requiremen ts of th e

KRESV tests.

5. Wh atever you send sho uld make Sense to th em. Don ’t use a

provocative Subject line or an y oth er social engineering p ractice

to en courage th em to read your email.

6. Check the attachm ents for Viruses. Th is is again based o n

having virus-checking programs, and we’ll discuss that later.

Th e KRESV tests h elp you focus on t h e most imp ortan t issues wh ensendin g and receivin g email with attachm ents. Use it every tim e you

send em ail, but be aware that t h ere is no foolproof sch eme for workin g

with em ail, or security in gen eral. You still need to exercise care.




16

Wh ile an an ti-virus program alerts you to m an y viruses th at m ay find

th eir way to you r ho m e comp uter, there will always be a lag between

wh en a virus is discovered and wh en an ti-virus program ven dors provide

th e n ew virus signatu re. Th is m eans th at you shou ldn’t rely en tirely

on your an ti-virus programs. You m ust con tinu e to exercise care wh en

reading em ail.

Task 4 - Install and Use a Firewall Program

This section describes a firewall, its impo rtance to your h om e com put er

strategy, and a way to th ink abou t th e job you n eed to do . We’re

going to depart from our “comp uter-is-like-a-h ouse-an d-the-thin gs-in-it”

an alogy to u se anoth er that you are probably also fam iliar with : an officebuilding.

Have you ever visited a business where you

first stopped at th e reception desk to interact

with a security guard? That guard ’s job

is to assess everybody who wishes

to en ter or leave the building to

decide if th ey sh ould con tinu e

on or be stopp ed. Th e guard

keeps the unwan ted out and

permits only appropriate

people and o bjects to ent er and

leave th e bu siness’s prem ises.

Let’s dig deeper int o th is an alogy. Wh en

someon e enters a building, the security guard

usually greets them . If they h ave an app ropriate iden tification badge,

th ey show it t o th e guard or swipe it th rough a reader. If all is OK, th ey

pass throu gh th e guard’s checkpoint . However, if som eth ing’s wrong o r

if th ey are a visitor, th ey m ust first stop at t h e guard desk.

The guard asks wh om th ey wish to see. Th e guard m ay also ask for

identification such as a driver’s license or th eir com pan y ID. Th e guard

reviews th e list of expected gu ests to see if this person is approved to

visit th e party in question. If the guard decides everything is all righ t,

th e visitor may p ass. Th e visitor usually signs a logbook with th eir nam e,

the com pan y they represent, whom they are seeing, and th e t im e of day.

On a comp uter, th e firewall acts much like a guard when it looks at

n etwork traffic destin ed for or received from an oth er comp uter. The

firewall determines if th at traffic sh ould con tinu e on to its destination



17

or be stopped . Th e firewall “guard” is im portan t because it keeps th e

un wanted ou t an d p ermits only appropriate traffic to enter an d leave

the com puter.

To d o th is job, th e firewall has to look at every piece of information –

every packet – th at tries to ent er or leave a com put er. Each packet is

labeled with wh ere it came from an d wh ere it wan ts to go. Som e packets

are allowed to go an ywhere (the em ployee with th e ID badge) wh ile

oth ers can on ly go to specific places (visitors for a specific person). If th e

firewall allows th e packet to proceed (bein g acceptable according to th e

rules), it mo ves the packet on its way to the d estin ation. In m ost cases,

th e firewall records wh ere the packet came from , wh ere it’s going, andwhen it was seen. For people entering a building, th is is sim ilar to th e

ID card system keeping track of who ent ers or the visitor sign ing th e

visitor’s log.

The bu ilding’s guard m ay do a few more tasks before deciding th at

th e person can pass. If th e person is a visitor and is n ot on th e visitors

list, the guard calls the em ployee being visited to an n oun ce th e visitor’s

arrival and to ask if they may pass. If the employee accepts the visitor,

th ey may p roceed. The guard m ay also give the visitor a badge th at

identifies them as a visitor. That badge m ay lim it where in th e building

th ey can go an d ind icate if th ey need to be escorted. Finally, no m atter

wheth er the person is a visitor or an em ployee, th e guard may inspect

their briefcase or computer case before they pass.

The firewall can also check wheth er a given p acket sh ould p ass,

allowing th e comp uter’s user to respon d to u n ant icipated n etwork

traffic (just as th e guard d oes with th e un expected visitor). Individual

packets can be allowed to pass, or th e firewall can be ch anged t o

allow all futu re packets of the sam e type to pass. Som e firewalls h ave

advan ced capabilities that m ake it possible to d irect packets to a different

destination an d p erhaps even have th eir contents con cealed in side oth er

packets (sim ilar to th e visitor b eing escorted ). Finally, firewalls can filter

packets based not o n ly on th eir poin t of origin or destin ation, but also

on th eir cont ent (in spectin g the briefcase or comp uter case before being

allowed to p ass).

Back to th e office building, when emp loyees leave th e building, theymay also have to swipe their ID card to show that they’ve left. A visitor

signs out an d return s their tempo rary badge. Both m ay be subject to

h aving th eir possessions in spected before bein g allowed t o leave.




18

Firewalls can also recogn ize an d record wh en a com put er-to-compu ter

conn ection end s. If th e conn ection was temporary (like a visitor), the

firewall rules can chan ge to den y futu re similar conn ection s unt il th e

system’s user aut h orizes th em (just as visitors mu st re-iden tify them selves

and be re-approved by an emp loyee). Finally, outgoing con n ection s

can also be filtered according to con ten t (again, sim ilar to inspecting

possessions at the exit).

Wh at does this all mean ? It m eans th at with a firewall, you can con trol

which p ackets are allowed to en ter your hom e compu ter and which are

allowed to leave. Th at’s th e easy part.

The h ard part is deciding th e details about t h e packets that are allowedto en ter and exit your ho m e comp uter. If your firewall supp orts con tent

filtering, you also n eed to learn wh ich conten t to allow and which n ot

to allow. To h elp you get a h and le on t h is h arder task, let’s return to our

security guard analogy.

Imagin e th at you are th at security guard an d it’s your first day on th e job.

You h ave to decide wh o’s allowed in , who’s allowed out, an d wh at peop le

can bring into an d take out of the building. How do you do th is?

On e strategy is to be very con servative: let no on e in or ou t an d let no

possessions in or out. This is very simple, very easy to achieve, but not

particularly helpful to th e bu sin ess if non e of its emp loyees or visitors

can get in or out. Nor is it helpful if they can ’t bring an yth ing with

th em. With th is type of strategy, your ten ure as a security guard m ay

be short-lived.

If you try th is, you qu ickly learn th at you n eed to ch an ge your strategy

to allow people in and out o n ly if they h ave acceptable identification

and possession s usin g some agreed-to criteria. Add t h e requiremen t th at

if you d on ’t m eet th e precise criteria for adm ittance, you d on ’t get in.

With m ost firewalls, you can d o th e sam e thin g. You can program you r

firewall to let n oth ing in an d n oth ing out. Period. This is a deny-all

firewall strategy and it does work, though it effectively disconnects you

from th e In ternet. It is im practical for m ost ho m e comp uters.

You can d o wh at th e security guard did: review each p acket (emp loyee

or visitor) to see wh ere it’s com ing from an d wh ere it’s going. Som efirewall produ cts let you easily review each p acket so that you can decide

what to do with it. Wh en yo u are shopp ing for a firewall, look for th is

review featu re because it can be q uite h elpful. Practically speakin g, it

isn ’t easy to d ecide wh ich traffic is all right and which is not all righ t.



19

An y feature th at m akes this job easier helps you achieve your goal of

securing your home computer.

Just like the security guard wh o learn s that an ybody with a com pan y

ph oto ID is allowed to pass, you t oo can create firewall rules th at allow

traffic to p ass witho ut reviewin g each p acket each time. For exam ple,

you m ay choo se to allow you r In ternet b rowsers to visit an y web site.

Th is rule would defin e th e source of th at traffic to be you r browsers

(Netscape Navigator and Microsoft Internet Explorer, for example) and

th e destin ation location to be any web server. Th is m eans th at an ybody

using your h om e comp uter could visit any Intern et web site, as long as

th at web server used th e well-kno wn stan dard locations.Now th at you h ave an idea of what your firewall security guard is trying

to do, you n eed a meth od for gathering information and programm ing

you r firewall. Here is a set of steps to use to d o just th at:

1. The Program test: Wh at’s the program th at want s to make

a conn ection to th e Internet? Although man y programs may

need to m ake the same type of conn ection to th e same Internet

destination , you n eed to know th e nam e of each. Avoid gen eral

rules th at allow all program s to m ake a conn ection. This often

results in un wanted an d u nch ecked beh avior.

2. The Location test: Wh at’s the Intern et location of the com put er

system to which your computer wants to connect? Locations consist

of an add ress and a port n um ber. Som etimes a program is allowed toconn ect to any Intern et location, such as a web browser con n ecting

to an y web server. Again, you wan t to limit program s so th at th ey

on ly conn ect to specific locations where possible.

3. The Allowed test: Is th is conn ection allowed o r den ied? Your

firewall rules will contain some of each.

4. The Temporary test: Is th is conn ection tem porary or

perman ent ? For examp le, if you’re goin g to conn ect to th is

specific location m ore than five tim es each tim e you use

the com puter, you probably want to m ake the conn ection

permanen t. This means that you ought to add a rule to your

firewall rules. If you aren’t going t o m ake th is con n ection often,

you sho uld defin e it as temporary.

With each conn ection, apply the PLAT tests to get the inform ation you

n eed to bu ild a firewall rule. Th e an swer to th e PLAT tests tells you if

you n eed to include a n ew firewall rule for th is n ew conn ection. For m ost

firewall programs, you can t emp orarily allow a conn ection but avoid

m aking it perm anen t by no t including it in your rules. Where possible,

allow only temporary connections.




20

As you run each program on your h om e compu ter, you’ll learn h ow

it uses the Internet. Slowly you’ll begin to build th e set of rules that

defin e what t raffic is allowed int o an d ou t of your comp uter. By on ly

lettin g in an d ou t wh at you ap prove and den ying all else, you will strike

a practical balance between allowin g everythin g and allowing not h ing

in or out.

Along th e way, you m ay com e across exception s to you r rules. For

example, you m ight decide that an ybody who u ses your hom e computer

can visit an y web site except a ch osen few web sites. This is an alogou s

to th e security guard lettin g every emp loyee pass except a few who n eed

more attention first .To d o th is with firewall rules, th e exception rules must b e listed b efore

th e general rules. For examp le, th is m eans th at th e web sites whose

conn ection s are not allowed m ust be listed before the rules that allow all

conn ection s to any web site.

Wh y? Most firewall programs search th eir rules starting from th e first

through the last . When the firewall finds a rule that m atches the packet

being examin ed, th e firewall h on ors it, does what th e rule says, and looks

n o furth er. For exam ple, if the firewall find s th e general rule allowing an y

web site conn ection s first, it h on ors this rule and doesn’t look furth er

for rules th at m igh t den y such a con n ection . So, the o rder of firewall

rules is im portan t.

Man y firewalls can b e programm ed to require a password before

changing the rules. This extra level of protection safeguards against

un want ed chan ges n o m atter their source, th at is, you, an in truder, or

another user. Follow the guidance in Task 6, Use Strong Passwords, when

assigning a password to your firewall.

Finally, make a b ackup of you r firewall rules. You ’ve pro bably ta ken a

lot of t ime to build and tun e them to m atch ho w your hom e com puter

is used. These rules are im po rtan t to yo ur com pu ter’s security, so back

th em u p using th e guidan ce in Task 5, Make Backups of Important Files

and Folders.

Firewalls come in two general types: hardware and software (programs).

The software versions also come in two types: free versions and

comm ercial versions (ones th at you p urchase). At a m inimu m , you

shou ld use one of th e free versions on your h om e comp uter. This is

especially im portant if you h ave a laptop th at you con nect to your h om e

n etwork as well as a netwo rk at a h otel, a conferen ce, or your office.



21

If you can afford a h ardware firewall, you sh ould in stall on e of

th ese too. We’ve recomm end ed th is as som eth ing to do later. (Firewall

programs are task 4 on our list of recomm end ed actions, and h ardware

firewalls are task 8.) The same issues apply to the hardware versions that

apply to th e software versions. Many can also be p assword p rotected

again st un want ed chan ges. Search th e In ternet with your browser to see

what’s available and what they cost. The price of hardware firewalls is

coming down as the dem and grows.

A firewall is your security guard th at stand s between your h om e

comp uter and th e In ternet. It lets you con trol which traffic your

comp uter accepts. It also con trols wh ich of your programs can con n ectto th e In ternet. With a firewall, you defin e which con n ection s between

your computer and oth er comp uters on the Internet are allowed an d

which are denied. There are free firewall produ cts that p rovide the

capabilities you n eed to secure your h om e comp uter. Com m ercial

versions h ave even m ore features that can further protect your com put er.

Firewalls are an im portan t part of your h om e com put er’s security

defenses. To see an examp le that shows h ow to operate a firewall, see

http://www.fedcirc.gov/homeusers/HomeComputerSecurity/examples.html .

Task 5 - Make Backups of Im port ant Files and Folders

Wh eth er you kn ow it or n ot, you’ve divided everythin g you own into

two broad categories: those items you can replace and th ose you can ’t.

For th e items you can’t replace, you’ve probably stored th em in a safe

place, either som ewhere in your livin g space or elsewhere, in a lockbox

at a ban k, for exam ple. In either case, you’ve probably also bo ugh t

insurance th at provides the fun ds you’d n eed to buy replacemen ts. Your

insurance po licy covers alm ost everyth ing you o wn.

On your ho m e com put er, have you sim ilarly divided everything into

th e sam e categories? What h ave you don e about th e item s – files in th is

case – that you can ’t replace? Exam ples are th e files th at m ake up your

checking accoun t records, th at n ovel you’ve been writing for the p ast

few years, and th ose pictures you to ok last sum m er with your digital

camera. What h appen s if your com pu ter malfun ctions or is destroyed by

a successful at tacker? Are th ose files gon e forever?

Now th ink abou t your car for a mom ent . Do you h ave a spare tire?

Is it in flated? Wh en was th e last time you used it? Can you im agin e

buying a car witho ut a spare tire? Even if you bou ght a used car withou t

a spare, how soon did you bu y a spare so that you’d have on e when

you n eeded i t?




22

Think back to you r hom e comp uter. Do you h ave a “spare tire,”

mean ing a way to continue compu ting when you h ave a “blowout”

caused by a malfunction or an intrud er? Said ano th er way, can you back

up your files onto some oth er media so th at you can recover them if you

n eed to? If you’d n ever buy a car with out a spare tire, why did you buy a

comp uter withou t a device to back up your files?

Wh en d eciding wh at to do about backing u p files on your computer,

ask these questions:

1. The Files qu estion : What files sh ould you back up? Th e files youselect are tho se that you can n either easily recreate n or reinstall

from somewh ere else, such as the CD-ROMs or th e flopp y disks

that came with your computer.

Be realistic. Th at ch eck register you printed does n ot con stitute

a backup from wh ich you can easily recreate the files needed

by your ch eckin g accoun t program. You’re probably n ot going

to re-ent er all th at dat a if th e files are destroyed. Just as you

protect you r irreplaceable valuables, back up th e files you cann ot

replace, easily or oth erwise.

2. The Often question : How often sh ould you back them up? In

th e best of all cases, you shou ld back up a file every time it

chan ges. If you don ’t, you’ll have to reintrodu ce all th e chan ges

th at h appen ed sin ce your last backup. Just as you store your

precious jewelry in a lockbox at th e local bank lest th e lucky

robber find it in your jewelry box, you n eed to store your files

safely (back th em up ) after every use (ch an ge in th e file) lest an

intrud er destroys the file or t h ere’s a system catastroph e.



23

3. The Media question: Wh ere should you back them u p to; that

is, what m edia sh ould you u se to hold backed up files? The

answer is: whatever you have. It’s a question of how many of that

m edia you have to u se an d h ow con venient it is. For example,

m ost comp uters have a flop py disk drive. You could back up your

irreplaceable files to flop pies. Th at p rocess just t akes lots of time

and m ay not be as convenien t as using anoth er media. Larger

capacity rem ovable disk drives and writable CD-ROMs also work

well, take less time, and are more convenient.

If you don ’t h ave a backup device, th ere are alternatives. Th ere

are In ternet services th at let you back up you r files to anot h er

Internet comp uter. Som e of th ese services provide “tran sparentaccess” to th e backups. Th at is, th ey look like anot h er hard

drive attached to your com put er. You u se the file copy schem e

th at your comp uter provides to back up files an d recover th em

from backed up storage. To fin d t h ese services, do som e Internet

search es using you r browser.

Remem ber that th e in form ation you tran sfer across the In ternet

could be viewed an d captured by oth ers; th at is, the information

is in t h e clear. Be sensitive to th at if you use an Intern et-based

backup com put er. In ad dition, you n eed to be able to trust th e

information wh en you recover a file from th at service.

4. The Store question : Where sho uld you store that m edia once

it contains your backed up files? No matter h ow you back up

your files, you need to be con cern ed about wh ere th ose backedup copies live.

You already know th at intrud ers try to break into you r hom e

com put er to gain access to your files and you r comp uter’s

resources. An oth er way to gain access to t h e same in formation

is by stealing you r backups. It is more d ifficult, tho ugh , sin ce a

robber m ust ph ysically be wh ere your backups are, whereas an

intrud er can access your h om e comp uter from literally anywh ere

in th e world. The key is to know wh ere the m edia is that

cont ains your backed up files.

Just like impo rtant papers stored in a fireproof contain er at your

h ouse, you also n eed to be con cern ed about you r backups being

destroyed if your living space is destroyed or damaged. This

mean s that you ou ght to keep a copy of your backed up files in afireproof con tainer or somewh ere beyon d your livin g space, your

office for example. It is th e eternal com prom ise between security

an d u sability. If you n eed to recover a file and th e backed up

copies are at th e office, th at’s incon venient . However, wh ile

storing them at hom e is more convenien t and more usable, they




24

share th e same risks th at your com pu ter faces shou ld your living

space be destroyed. Be aware of the issues an d m ake a con scious

decision, perh aps keepin g copies in bo th places.

If you have that spare tire for your car or a lockbox for your valuables,

you’ve already plann ed for the worst th at can h appen around your

living space. Contin ue th at good practice by backing up your critical files

on to m edia that you can safely store elsewhere. Do th ose backups often

enough that you can capture the chan ges you’ve made. With the FOMSquestions, you h ave a structured app roach to u se to back up your critical

files. You ’ve no w plan n ed for th e worst.

As you com put erize the routine aspects of your daily life, makin gbackup copies of important files and folders becomes critical. Even if

you can ’t store the backup cop ies in a fireproof contain er or som ewhere

outside your h om e, make backups anyway. An y backup is better th an

none .

Task 6 - Use Strong Passwords

Your livin g space has doo rs an d wind ows, and perh aps mo st of th e time

th ey’re locked. For each lo ck th at u ses a key, chan ces are th at each key is

different. You kn ow to lock up an d n ot to share th e keys with strangers,

and probably no t with m ost of your friends. You sh ould n ot h ide keys

un der the mat or in a flowerpot on your front porch.

Passwords for com put ers are much th e same. For each com put er andservice you u se (on lin e purch asin g, for examp le), you sho uld h ave a

password. Each password sh ould be un ique an d un related to an y of your

oth er passwords. You sh ouldn ’t write th em d own n or should you sh are

them with anyon e, even you r best friend s.

Take a look at you r front do or key. It’s pretty com plicated. There are lots

of n otch es and grooves. If th ere weren’t so m any possible variations, a

th ief could easily make a key for every possible com bination an d th en

try each on your front d oor. This trial-an d-error meth od, (for com put ers,

called b rute force) is likely to be effective even if it ta kes a lon g tim e.

Non eth eless, no m atter how com plicated, if th e th ief gets h old of your

key, he or she can copy it and u se th at copy to open your door.

A password can also be com plicated. Most schem es let you use anycomb ination of letters, both u pper and lower case, an d n um bers; an d

some also let you u se pun ctuation m arks. Length s can vary. You can

create a password to be as com plicated as you wan t. Th e key (n o pu n

inten ded) is to be able to rem emb er this password when ever you n eed it

withou t h aving to write i t down to jog your mem ory.



25

Like the thief at your door, computer intruders also use trial-and-error,

or brute-force techniques, to discover passwords. By bombarding a login

sch eme with all th e words in a diction ary, they m ay “discover” th e

password th at un locks it. If they know som eth ing about you , such as

your spouse’s name, the kind of car you drive, or your interests, clever

intrud ers can n arrow the ran ge of possible passwords an d try th ose first.They are often successful. Even slight variation s, such as addin g a digit

on to th e end of a word or replacin g the letter o (oh) with th e digit 0

(zero), don ’t pro tect pa sswords. In trud ers know we u se tricks like this to

m ake our p asswords m ore difficult to gu ess.

Just like th e fron t d oor key, even a com plicated p assword can be copied

and th e copy reused. Remem ber the earlier discussion abo ut in formation

on th e In ternet bein g in th e clear? Supp ose that really stron g password

you too k a long time to create – th e on e th at’s 14 characters lon g and

cont ains 6 letters, 4 n um bers, and 4 pun ctuation m arks, all in rand om

order – goes across the Internet in t h e clear. An intrud er may be able

to see it, save it, and use it. Th is is called sniffing an d it is a com m on

intrud er practice.

The poin t is th at you n eed to follow the practice of usin g a uniqu e

password with every accoun t you h ave. Below is a set of steps that you

can u se to h elp you create passwords for your accoun ts:




26

1. The Strong test: Is th e password as stron g (m eanin g length andcont ent ) as th e rules allow?

2. The Unique test: Is th e password un ique and u n related to an yof your oth er passwords?

3. The Practical test: Can you rem emb er it with out h aving towrite it down ?

4. The Recent test: Have you ch anged it recent ly?

In spite of th e SUPR tests, you need to b e aware that sn iffin g happ ens,

and even th e best of passwords can be captured an d used by an in truder.

You sho uld use passwords not on ly on your h om e computer butalso for services you use elsewhere on the Internet. All should

h ave the strongest passwords you can use and remem ber, and each

password shou ld be un ique and un related to all oth er passwords. A

strong password is a password that is longer th an it is short, th at

uses com bination s of upp ercase and lowercase letters, nu m bers, and

pun ctuation, an d th at is usually not a word found in a diction ary.

Also remem ber th at n o m atter ho w strong a password is, it can still

be captured if an in truder can see it “in th e clear” somewh ere on th e

Internet. (See the Information in the Clear section.)

Task 7 - Use Care When Downloading and InstallingPrograms

Wh en you buy an appliance, you give li t t le thou ght to i t doing you

or your h ouse an y h arm. Wh y? Because there are organ izations like

Underwriters Laboratories2 th at set stan dards and certify products. When

you see a certifier’s label, you h ave m ore con fiden ce th at a p roduct will

be safer than a comp eting produ ct that d oes not carry the same label.

You ’re willing to accept th e risk because you believe th e prod uct h as met

some stan dards and h as been certified by a respected auth ority.

Unfortun ately, the Intern et is n ot th e sam e. There are neith er stan dards

n or man y certification organization s. An yon e who writes a program can

distribute it through an y mean s available, such as throu gh th e web or by

sending you a copy. Speaking of that, have you ever received a CD-ROM

in th e mail? How do you kn ow th at it con tains what th e label says? Th e

an swer is: you d on ’t kn ow. More im po rtan tly, it’s difficult to kn ow.

No m atter how you acquire a program, i t runs on your computer at th em ercy of the program’s auth or. An yth ing, any op eration , any task that

you can do, th is program can also do . If you’re allowed to rem ove an y

file, th e program can too . If you can sen d em ail, the program can too.

If you can in stall or remove a program, th e program can to o. An yth ing

you can do , the intrud er can d o also, throu gh th e program you ’ve just

installed an d run .



27

Som etimes there’s no explan ation o f what a program is sup posed to d o

or wh at it actually does. Th ere may b e n o u ser’s guide. Th ere may b e

n o way to con tact th e auth or. You’re on your own , tryin g to weigh a

program’s benefits again st the risk of the h arm th at it might cause.

Wh at’s the p roblem you ’re trying to solve h ere? You are trying to

determin e if the program you’ve just foun d satisfies your n eeds (say it

provides a service that you wan t or you ’re just experimen ting) with out

causing harm to your computer and ult imately the inform ation you h ave

on th e com put er. How do yo u d ecide if a program is wh at it says it

is? How do you gauge the r isk to you and your computer by run ning

this program?You address these sam e risk issues when you p urchase an appliance;

you m ay just n ot h ave realized that th at’s what you were doing. Wh en

you m ake that pu rchase, you bu y from eith er a local store you kno w or

a n ational ch ain with an establish ed reputation . If there’s a problem with

your pu rchase, you can take it back to th e store an d exchan ge it or get

your m on ey back. If it causes you h arm, you can seek relief th rough th e

legal system. Th e reputation of the m erch an t, the refund /return policy,

and th e availability of the legal system reduce your risk to a p oint wh ere

you m ake the purchase.

Apply th ese sam e practices when you b uy a p rogram . You sh ould

• Learn as mu ch as you can about th e product and wh at it does

before you pu rch ase it.

• Understand th e refund /return p olicy before you make your

purchase.

• Buy from a local store that you already kno w or a nation al

chain with an established reputation .

Presen tly, it is no t as clear wh at t h e legal system ’s role is for a p rogram

th at causes h arm or does not work as advertised. In th e mean time, the

LUB practices are a good first step.

Today’s In ternet h as a feature th at stand ard produ cts don ’t h ave, or

at least have b ut t o a lesser exten t. This featu re is free program s. Th ere

is a mu ltitud e of free program s available for all types of system s, with

m ore available each day. Th e challenge is to decide which programsdeserve your con fiden ce and are, therefore, worth th e risk of installing

and runn ing on your home computer.

So th en, h ow do you d ecide if a program is worth it? To decide if

you should install and run a program on your hom e computer, follow

th ese steps:




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1. The Do test: Wh at does th e program do ? You sh ould be able

to read a clear description of what th e program d oes. Th is

description cou ld be on th e web site where you can down load it

or on th e CD-ROM you use to in stall it. You n eed to realize th at

if th e program was written with m alicious inten t, the auth or/

intrud er isn’t going to tell you th at th e program will harm you r

system. They will probably try to mislead you. So, learn what

you can, but con sider the source an d con sider wheth er you can

trust th at information.

2. The Changes test: Wh at files are in stalled and wh at oth er

chan ges are made on your system wh en you in stall and run th e

program? Again , to do th is test, you m ay have to ask the auth or/ intrud er h ow th eir program chan ges your system . Consider th e

source.

3. The Author test: Wh o is th e auth or? (Can you u se em ail,

telephon e, letter, or some oth er means to con tact them ?) On ce

you get this in formation , use it to try to contact th em to verify

th at th e contact information works. Your interactions with th em

m ay give you m ore clues about th e program an d its poten tial

effects on your comp uter and you .

4. The Learn test: Has anybody else used th is program, an d wh at

can you learn from h im or h er? Try some Intern et searches usin g

your web b rowser. Som ebody h as probably used th is program

before you, so learn what you can before you in stall it.

If you can ’t determ ine th ese th ings – th e DCAL tests for sho rt – about

th e program you’d like to install, th en strongly consider wheth er it’s

worth th e risk. On ly you can d ecide wh at’s best. Wh atever you do, be

prepared to rebuild your comp uter from scratch in case th e program goes

awry and destroys it. The section on backups (Task 5) tells you h ow to

make a copy of your imp ortant information should you need i t .

Your an ti-virus program prevents som e of th e problems caused by

down loading and in stallin g program s. However, you n eed to rem emb er

th at th ere’s a lag between recognizing a virus and when your com put er

also kn ows about it. Even if that n ifty program yo u’ve just down loaded

doesn’t con tain a virus, it m ay beh ave in an un expected way. You sh ould

continue to exercise care and do your h om ework when downloading,

installing, and running new programs.

Task 8 - Install and Use a Hardware Firewall

Com plemen t your firewall program by in stallin g a h ardware firewall.

Together, th ese two firewalls stand between your h om e comp uter and

th e In ternet. This is ano th er place where your m on ey is well spen t.



29

Please go to Task 4, Install and Use a Firewall Program , to learn m ore

about firewalls. That section concen trates primarily on firewall programs,

but m uch of the inform ation ap plies to h ardware firewalls as well.

To fin d ou t wh at h ardware firewall produ cts are available, search th e

Internet with your web browser.

Task 9 - Install and Use a File Encryption Program andAccess Cont rols

Let’s return to yo ur livin g space and our o riginal an alogy. Think ab out

your ch eckbook, your insuran ce policies, perhap s your birth certificate or

passport , and other important d ocumen ts you h ave at h ome. Wh ere are

th ey? Th ey’re probab ly stored in a filing cabin et or a safe, eith er of whichth at can be or is routinely locked. Why do yo u store these im portan t

items in a locked contain er?

Withou t realizing it, you are satisfying on e of the th ree com pon ent s

of in formation security – con fiden tiality. Con fiden tiality mean s keeping

secrets secret. On ly those wh o are supposed to see that in form ation

shou ld h ave access to it. You are keepin g information sensitive to you

and oth ers away from th ose who sho uld n ot be able to get to it, for

examp le a fam ily mem ber or an in truder. By th e way, the oth er two

comp on ent s of information security are integrity (Has m y information

chan ged?) an d availability (Can I get to m y information wh enever I

need it?).

You furth er protect in formation con fiden tiality when you en force it by

using an access con trol device, nam ely the lock on your filing cabinet

or safe. Th is device stands between t h e information an d th ose seekin g

access, and it gran ts access to all who h ave the com bination , the key,

or wh atever tool un locks th e con tainer. Wh en several layers of access

cont rol devices are used (called “defense in d epth ”) – you m igh t also

find th at th ese con tainers are them selves in locked room s. Would-be

intrud ers mu st pass throu gh several levels of protection before fin ally

gain ing access to th e information th ey seek.

Now, th ink back to your ho m e comp uter. Th e problem is to con trol

access to files an d folders. Th e access con trol device h ere is th e access

cont rol list or ACL. ACLs define who can perform actions on a file or

folder: readin g an d writin g, for exam ple. ACLs are equivalent to a lockedfilin g cabinet for paper docum ent s.

Different com pu ter system s provide differen t typ es of ACLs. Som e h ave

fine-grained con trols wh ile oth ers h ave virtually non e. Th e key is to use

all th e controls that are available on you r comp uter.




30

Frequ en tly, vend ors defin e ACLs th at are overly perm issive. This

satisfies th eir n eed to en sure that access lim itation s don ’t get in th e way

of using th eir system s. You r challen ge is to tigh ten th ose ACLs so th at

th ey properly restrict access to on ly tho se who n eed access. Th is mean s

th at you n eed to m odify the ACLs from th e settings set by th e vendo r.

We’ll talk m ore about h ow to do t h is shortly.

Returning to the h ome en vironm ent, do you remember a t ime wh en

adults in your hou se wanted to say someth ing to one an other in front of

their children but in such a way that th e children couldn’t un derstand

wh at was bein g said? Perhap s th ey spelled th eir message or used Pig Latin

(ig-pay Atin-lay) to conceal the meaning. This worked for a while, untilth e children learned to spell or could oth erwise und erstan d wh at was

being said. What’s really hap pen ing h ere?

Very simply, th e adults could no t con trol who cou ld h ear their

conversation . It was in con venient or perhap s im possible for th em t o go

to an other room where they couldn’t be h eard. They h ad to talk in a

way that on ly those who knew th e concealing schem e could u nd erstand

what was bein g said.

On a comp uter, wh en access to inform ation can ’t be lim ited, such

for an e-comm erce transaction over the In ternet, th at information is

concealed th rough a m ath ematical process called encryption . En cryption

transforms in formation from on e form (readable text) to anoth er

(en crypted text). Its inten t is to h ide in formation from t h ose who h aveneither th e transformation m ethod nor the particulars (the decryption

keys) to transform th e encrypted text in to readable text. Th e encrypted

text appears to be gibberish and remains so for people wh o do n ’t h ave

the scheme an d th e keys.

Back on t h e hom e front , the children eventu ally learned h ow to spell

and perhap s also learned th e trick to u sin g Pig Latin. They can n ow

un derstand th e conversations th e adults are h aving. While th ey could

also un derstand th e conversation s held weeks, m on th s, or even years

before, th e in formation in t h ose con versations is n o longer im portan t.

Th e encryption schem e – spellin g or Pig Latin – is stron g eno ugh to

guard th e information durin g its useful lifetime.

Computer-based encryption schemes must also withstand the test of time. For examp le, if a credit card encryption schem e needs six m on th s

of com put er time to break, the resulting clear text credit card nu m ber

is probably still valid and, therefore, useful to an intruder. In this case,

the en cryption schem e isn’t strong enou gh to guard th e information for

its ent ire useful lifetime.



31

So, to guard pap er or compu ter files, you n eed to limit who h as access

to th em b y using th e access cont rol devices, wheth er filing cabinets and

safes for pap er or access con trol lists for inform ation on a com pu ter

system. For assets whose access cannot be sufficiently limited, you need

to encrypt th em strongly enou gh so th at the t im e i t takes to decrypt

th em is longer th an th eir useful life.

Now, what can you do?

First, if m ore than on e person uses your com put er, you can adjust th e

ACLs th at con trol access to sen sitive files an d folders. You r goal is to

allow th e correct type of access to th e files an d folders that each user

needs, and no thing m ore. The steps below h elp you to decide h ow toadjust th e ACLs for files an d folders:

1. The Who test: Wh o – which users – n eed access to files besides

you?

2. The Access test: Wh at type of access do th ey n eed? Read?

Write?

3. The Files/ Folders test: Which files and folders need special

access? Just like your firewall rules, your gen eral policy sh ou ld be

to limit access to on ly you first, and th en gran t access beyond

that wh ere needed.

By applying th e WAF tests, you can limit access to sensitive files on

your computer to on ly those who need i t .Settin g prop er ACLs is not a trivial task. Be prep ared t o rep eat it a few

times un til you get it righ t for th e way your com put er is used. It’s worth

the t ime spent, but kno w that i t m ay take lon ger than you expect.

For very sensitive files an d for files that are on a lapto p, do n ’t rely solely

on file and folder ACLs. You n eed to go furth er and use encryption .

Som e vendors provide encryption with th eir system s righ t from th e

start. This means th at all you h ave to do is follow th e vend or’s

instructions on h ow to use th ose features, but be certain t o use them .

On systems wh ere encryption is n ot included, you n eed to install

addition al encryption programs. For encryption p rogram s that you

down load from th e In ternet, be sure to follow the instruction s in Task 7, Use Care W hen Downloading and Installing Programs. Also follow t h e

instructions in Task 6, Use Strong Passwords, for addition al guidan ce on

passwords required by encryption programs.




32

There are free an d com m ercial encryption programs, and in m ost

cases, th e free version s suffice. However, com m ercial program s may

provide more features and m ay keep up better with n ewer and , therefore,

stronger en cryption m eth ods. If you rely on a laptop com pu ter, you

shou ld consider purchasin g a com m ercial file encryption programs.

Wh eth er paper files aroun d you r living space or files and folders on

your com put er, limit access where you can . On you r comp uter, use

encryption programs either when you can ’t restrict access to th e extent

th at you’d like or when you wan t even m ore security protecting your

comp uter files and folders.

To see examp les th at show h ow to use an en cryption programand how to adjust ACLs, see

http://www.fedcirc.gov/homeusers/HomeComputerSecurity/examples.html



33

SummaryGrowing up, you learn m any of the th ings

you need to know about h ow to operate and

care for a car by sittin g in th e back seat

wh ile adu lts drive an d care for th eir veh icles.

Similarly, you learn many of the things you

need to kn ow about h ow to care for and

maintain a h om e by watching what is don e

to t h e on e wh ere you live. It is a slow, gradu al

process, so slow in fact you are proba blyun aware that you are learn ing th e skills you

need to do these same jobs yourself.

You d on ’t h ave that sam e luxury of tim e to learn h ow to care for and

operate your h ome com puter. When you attach i t to th e Internet for

th e first tim e, it instantly becom es a target for intrud ers. You n eed to be

ready right from th e start.

As you grow up, you also learn that you n eed to spend tim e and mo ney to

repair and replace th ose things aroun d your livin g space and your car th at

need your attention . You learn th at you have to spend m ore tim e and m ore

mo ney to tailor them to m eet your n eeds and to keep you and oth ers safe

durin g th eir use. You accept th ese respon sibilities and th eir costs as part of

th e total cost of own ersh ip of that car and living space.

Your h om e comp uter is m uch th e sam e. Th ere is th e initial m on ey

th at you pay to p urchase that system. Th en th ere are addition al costs to

tailor it and t o keep you an d th e oth ers wh o use your system safe. Th ese

addition al costs are also your respon sibility, and th ey are part of th e total

cost of own ersh ip of your ho m e com put er.

Th is pam ph let h elps you thin k about th e problem s you face when you

have a h om e compu ter and gives you advice on h ow to address th ese

problem s. On th e web, th ere are checklists and worksheets th at help you

keep t rack of important inform ation abou t th e steps you take to secure your

com pu ter, and a list of additional resources if you wan t to kn ow m ore.

Checklists:http://www.fedcirc.gov/homeusers/HomeComputerSecurity/checklists/checklist1-9.pdf

Additional Resources:

htt p:/ / www.fedcirc.gov/homeusers/ HomeComput erSecurity/ index.html#resources




34

By takin g the t ime to read th is pamp hlet , you kn ow m ore about

securing your h om e compu ter and th e extra costs required to do th is job.

Do th e tasks described here an d share th is pam ph let with your friends.

We all benefit from a m ore secure In ternet.

End Notes1 W here W izards Stay Up Late: The Origins of th e Internet by Katie

Hafner and Matthew Lyon. ISBN: 0684832674. Read a review at http://

www.mantex.co.uk/reviews/hafner.htm.

2 The Underwriters Laboratories web site is http://www.ul.com/

Acknowledgments:This pam ph let was designed and written by Lawrence R. Rogers

([email protected] u.edu ). It was ed ited b y Lind a Hu tz Pesante

(lh [email protected] u.edu ). The graph ic design an d illustration s were created

by David Biber ([email protected] u.edu ). All work in th e Netwo rked

System s Survivability Program at C arn egie Mellon Un iversity’s Software

Engineering Institute in Pittsburgh, PA.

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