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Original publication date: June 22, 2001

HTML
<br>This document gives home users an overview of the security risks
and countermeasures associated with Internet connectivity, especially
in the context of “always-on” or broadband access services (such
as cable modems and DSL). However, much of the content is also
relevant to traditional dial-up users (users who connect to the
Internet using a modem).  <br> 



<ol TYPE="I">
<p>
<li>
<a href="#I">Computer security</a></li>

<ol type="A">
<li>
<a href="#I-A">What is computer security?</a></li>

<li>
<a href="#I-B">Why should I care about computer security?</a></li>

<li>
<a href="#I-C">Who would want to break into my computer at home?</a></li>

<li>
<a href="#I-D">How easy is it to break into my computer?</a></li>
</ol>

<li>
<a href="#II">Technology</a></li>

<ol type="A">

<li>
<a href="#II-A">What does "broadband" mean?</a></li>

<li>
<a href="#II-B">What is cable modem access?</a></li>

<li>
<a href="#II-C">What is DSL access?</a></li>

<li>
<a href="#II-D">How are broadband services different from traditional dial-up
services?</a></li>

<li>
<a href="#II-E">How is broadband access different from the network I
use at work?</a></li>

<li>
<a href="#II-F">What is a protocol?</a></li>

<li>
<a href="#II-G">What is IP?</a></li>

<li>
<a href="#II-H">What is an IP address?</a></li>

<li>
<a href="#II-I">What are static and dynamic addressing?</a></li>

<li>
<a href="#II-J">What is NAT?</a></li>

<li>
<a href="#II-K">What are TCP and UDP ports?</a></li>

<li>
<a href="#II-L">What is a firewall?</a></li>

<li>
<a href="#II-M">What does antivirus software do?</a></li>
</ol>

<li>
<a href="#III">Computer security risks to home users </a></li>

<ol type="A">
<li>
<a href="#III-A">What is at risk?</a></li>

<li>
<a href="#III-B">Intentional misuse of your computer</a></li>

<ol>
<li>
<a href="#III-B-1">Trojan horse programs</a></li>

<li>
<a href="#III-B-2">Back door and remote administration programs</a></li>

<li>
<a href="#III-B-3">Denial of service</a></li>

<li>
<a href="#III-B-4">Being an intermediary for another attack</a></li>

<li>
<a href="#III-B-5">Unprotected Windows shares</a></li>

<li>
<a href="#III-B-6">Mobile code (Java, JavaScript, and ActiveX)</a></li>

<li>
<a href="#III-B-7">Cross-site scripting</a></li>

<li>
<a href="#III-B-8">Email spoofing</a></li>

<li>
<a href="#III-B-9">Email-borne viruses</a></li>

<li>
<a href="#III-B-10">Hidden file extensions</a></li>

<li>
<a href="#III-B-11">Chat clients</a></li>

<li>
<a href="#III-B-12">Packet sniffing</a></li>
</ol>

<li>
<a href="#III-C">Accidents and other risks</a></li>

<ol>
<li>
<a href="#III-C-1">Disk failure</a></li>

<li>
<a href="#III-C-2">Power failure and surges</a></li>

<li>
<a href="#III-C-3">Physical theft</a></li>
</ol>
</ol>

<li>
<a href="#IV">Actions home users can take to protect their computer systems</a></li>

<ol>
<li>
<a href="#IV-A-1">Consult your system support personnel if you work from home</a></li>

<li>
<a href="#IV-A-2">Use virus protection software</a></li>

<li>
<a href="#IV-A-3">Use a firewall</a></li>

<li>
<a href="#IV-A-4">Don’t open unknown email attachments</a></li>

<li>
<a href="#IV-A-5">Don’t run programs of unknown origin</a></li>

<li>
<a href="#IV-A-6">Disable hidden filename extensions</a></li>

<li>
<a href="#IV-A-7">Keep all applications (including your operating system) patched</a></li>

<li>
<a href="#IV-A-8">Turn off your computer or disconnect from the network when not in use</a></li>

<li>
<a href="#IV-A-9">Disable Java, JavaScript, and ActiveX if possible</a></li>

<li>
<a href="#IV-A-10">Disable scripting features in email programs</a></li>

<li>
<a href="#IV-A-11">Make regular backups of critical data</a></li>

<li>
<a href="#IV-A-12">Make a boot disk in case your computer is damaged or compromised</a></li>

</ol>

<p><a href="#appendix">Appendix: References and additional information</a></p>

</ol>



<a href="#history">Document Revision History</a>
<br>
<hr SIZE=2 NOSHADE ALIGN=LEFT>
<ol type="I">
<h3>
<a NAME="I"></a><li>Computer security</li></h3>


<ol type="A">
<h4>
<a NAME="I-A"></a><li>What is computer security?</li></h4> 

<p>Computer security is the process of preventing and detecting
unauthorized use of your computer. Prevention measures help you to
stop unauthorized users (also known as "intruders") from accessing any
part of your computer system. Detection helps you to determine whether
or not someone attempted to break into your system, if they were
successful, and what they may have done.</p>

<h4>
<a NAME="I-B"></a><li>Why should I care about computer security?</li></h4>

<p>We use computers for everything from banking and investing to shopping
and communicating with others through email or chat programs.  Although
you may not consider your communications "top secret," you probably do
not want strangers reading your email, using your computer to attack other
systems, sending forged email from your computer, or examining personal
information stored on your computer (such as financial statements).</p>

<h4>
<a NAME="I-C"></a><li>Who would want to break into my computer at home?</li></h4>


<p>Intruders (also referred to as hackers, attackers, or crackers) may
not care about your identity. Often they want to gain control of your
computer so they can use it to launch attacks on other computer
systems.</p>

<p>Having control of your computer gives them the ability to hide their
true location as they launch attacks, often against high-profile computer
systems such as government or financial systems. Even if you have a computer
connected to the Internet only to play the latest games or to send email
to friends and family, your computer may be a target.

<p>Intruders may be able to watch all your actions on the computer, or
cause damage to your computer by reformatting your hard drive or
changing your data.


<h4>
<a NAME="I-D"></a><li>How easy is it to break into my computer?</li></h4>

<p>Unfortunately, intruders are always discovering new vulnerabilities
(informally called "holes") to exploit in computer software. The
complexity of software makes it increasingly difficult to thoroughly
test the security of computer systems. </p>

<p>When holes are discovered, computer vendors will usually develop
patches to address the problem(s).  However, it is up to you, the user, to
obtain and install the patches, or correctly configure the software to
operate more securely. Most of the incident reports of computer
break-ins received at the CERT/CC could have been prevented if system
administrators and users kept their computers up-to-date with patches
and security fixes.</p>

<p>Also, some software applications have default settings that allow other
users to access your computer unless you change the settings to be more secure.
Examples include chat programs that let outsiders execute commands on your
computer or web browsers that could allow someone to place harmful programs
on your computer that run when you click on them.</p>
</ol>
<h3>

<a NAME="II"></a><li>Technology</li></h3>

<p>This section provides a basic introduction to the technologies that
underlie the Internet.  It was written with the novice end-user in
mind and is not intended to be a comprehensive survey of all
Internet-based technologies. Subsections provide a short overview of
each topic. This section is a basic primer on the relevant
technologies.  For those who desire a deeper understanding of the
concepts covered here, we include links to additional information.</p>

<ol type="A">

<h4>
<a NAME="II-A"></a><li>What does broadband mean?</li></h4>

<p>"Broadband" is the general term used to refer to high-speed network
connections.  In this context, Internet connections via cable
modem and Digital Subscriber Line (DSL) are frequently referred to as
broadband Internet connections. "Bandwidth" is the term used to
describe the relative speed of a network connection -- for example,
most current dial-up modems can support a bandwidth of 56 kbps
(thousand bits per second).  There is no set bandwidth threshold
required for a connection to be referred to as "broadband", but it is
typical for connections in excess of 1 Megabit per second (Mbps) to be
so named.</p>

<h4>
<a NAME="II-B"></a><li>What is cable modem access?</li></h4>

<p>A cable modem allows a single computer (or network of computers) to
connect to the Internet via the cable TV network. The cable modem
usually has an Ethernet LAN (Local Area Network) connection to the
computer, and is capable of speeds in excess of 5 Mbps.</p>

<p>Typical speeds tend to be lower than the maximum, however, since
cable providers turn entire neighborhoods into LANs which share the
same bandwidth.  Because of this "shared-medium" topology, cable
modem users may experience somewhat slower network access during
periods of peak demand, and may be more susceptible to risks such as
packet sniffing and unprotected windows shares than users with other
types of connectivity. (See the "<a href="#III">Computer security
risks to home users</a>" section of this document.)</p>

<h4>
<a NAME="II-C"></a><li>What is DSL access?</li></h4>

<p>Digital Subscriber Line (DSL) Internet connectivity, unlike cable
modem-based service, provides the user with dedicated
bandwidth. However, the maximum bandwidth available to DSL users is
usually lower than the maximum cable modem rate because of differences
in their respective network technologies.  Also, the "dedicated
bandwidth" is only dedicated between your home and the DSL provider's
central office -- the providers offer little or no guarantee of
bandwidth all the way across the Internet.</p>

<p>DSL access is not as susceptible to packet sniffing as cable modem
access, but many of the other security risks we'll cover apply to both
DSL and cable modem access.  (See the "<a href="#III">Computer
security risks to home users</a>" section of this document.)</p>

<h4>
<a NAME="II-D"></a><li>How are broadband services different from traditional
dial-up services?</li></h4>

<p>Traditional dial-up Internet services are sometimes referred to as
"dial-on-demand" services.  That is, your computer only connects to
the Internet when it has something to send, such as email or a
request to load a web page.  Once there is no more data to be sent, or
after a certain amount of idle time, the computer disconnects the
call.  Also, in most cases each call connects to a pool of modems at
the ISP, and since the modem IP addresses are dynamically assigned,
your computer is usually assigned a different IP address on each call.  As a
result, it is more difficult (not impossible, just difficult) for an
attacker to take advantage of vulnerable network services to take
control of your computer.</p>

<p>Broadband services are referred to as "always-on" services
because there is no call setup when your computer has something to
send.  The computer is always on the network, ready to send or receive
data through its network interface card (NIC).  Since the connection
is always up, your computer’s IP address will change less
frequently (if at all), thus making it more of a fixed target for
attack.</p>

<p>What’s more, many broadband service providers use well-known IP
addresses for home users.  So while an attacker may not be able to
single out your specific computer as belonging to you, they may at
least be able to know that your service providers’ broadband
customers are within a certain address range, thereby making your
computer a more likely target than it might have been otherwise.</p>

<p>The table below shows a brief comparison of traditional dial-up and broadband
services.</p>
<br> 
<center><table BORDER COLS=3 WIDTH="75%" >
<tr VALIGN=CENTER>
<td VALIGN=CENTER></td>

<td><b>Dial-up</b></td>

<td><b>Broadband</b></td>
</tr>

<tr>
<td><b>Connection type</b></td>

<td>Dial on demand</td>

<td>Always on</td>
</tr>

<tr>
<td><b>IP address</b></td>

<td>Changes on each call</td>

<td>Static or infrequently changing</td>
</tr>

<tr>
<td><b>Relative connection speed</b></td>

<td>Low</td>

<td>High</td>
</tr>

<tr>
<td><b>Remote control potential</b></td>

<td>Computer must be dialed in to control remotely </td>

<td>Computer is always connected, so remote control can occur anytime</td>
</tr>

<tr>
<td><b>ISP-provided security</b></td>

<td>Little or none</td>

<td>Little or none</td>
</tr>

<tr>
<td COLSPAN="3">
<center><i><font size=-1>Table 1: Comparison of Dial-up and Broadband Services</font></i></center>
</td>
</tr>
</table></center>

<br> 
<h4>
<a NAME="II-E"></a><li>How is broadband access different from the
network I use at work?</li></h4>

<p>Corporate and government networks are typically protected by many
layers of security, ranging from network firewalls to encryption.  In
addition, they usually have support staff who maintain the security
and availability of these network connections.</p>

<p>Although your ISP is responsible for maintaining the services they
provide to you, you probably won’t have dedicated staff on hand to
manage and operate your home network.  You are ultimately responsible
for your own computers.  As a result, it is up to you to take
reasonable precautions to secure your computers from accidental or
intentional misuse.</p>

<h4>
<a NAME="II-F"></a><li>What is a protocol?</li></h4>

<p>A protocol is a well-defined specification that allows computers to
communicate across a network.  In a way, protocols define the
"grammar" that computers can use to "talk" to each other.</p>

<h4>
<a NAME="II-G"></a><li>What is IP?</li></h4>

<p>IP stands for "Internet Protocol".  It can be thought of as the common
language of computers on the Internet.  There are a number of
detailed descriptions of IP given elsewhere, so we won't cover it in
detail in this document.  However, it is important to know a few
things about IP in order to understand how to secure your computer.
Here we’ll cover IP addresses, static vs. dynamic addressing, NAT,
and TCP and UDP Ports.</p>

<p>An overview of TCP/IP can be found in the TCP/IP Frequently Asked
Questions (FAQ) at</p>

<dl>
<dd><a
href="http://www.faqs.org/faqs/internet/tcp-ip/tcp-ip-faq/part1/">http://www.faqs.org/faqs/internet/tcp-ip/tcp-ip-faq/part1/</a></dd>
</dl>

<p>and</p>

<dl>
<dd><a
href="http://www.faqs.org/faqs/internet/tcp-ip/tcp-ip-faq/part2/">http://www.faqs.org/faqs/internet/tcp-ip/tcp-ip-faq/part2/</a></dd>
</dl>

<h4>
<a NAME="II-H"></a><li>What is an IP address?</li></h4> 

<p>IP addresses are analogous to telephone numbers – when you want
to call someone on the telephone, you must first know their telephone
number.  Similarly, when a computer on the Internet needs to send
data to another computer, it must first know its IP address.  IP
addresses are typically shown as four numbers separated by decimal
points, or “dots”.  For example, 10.24.254.3 and 192.168.62.231
are IP addresses.</p>

<p>If you need to make a telephone call but you only know the
person’s name, you can look them up in the telephone directory (or
call directory services) to get their telephone number.  On the
Internet, that directory is called the Domain Name System, or DNS
for short.  If you know the name of a server, say www.cert.org, and
you type this into your web browser, your computer will then go ask
its DNS server what the numeric IP address is that is associated with
that name.  </p>

<p>Every computer on the Internet has an IP address associated with it
that uniquely identifies it.  However, that address may change over
time, especially if the computer is </p>
<ul>
<li>dialing into an Internet Service
Provider (ISP)</li>
<li>connected behind a network firewall</li>
<li>connected to a broadband service using dynamic IP addressing.</li>
</ul>

<h4>
<a NAME="II-I"></a><li>What are static and dynamic addressing?</li></h4>

<p>Static IP addressing occurs when an ISP permanently assigns one or
more IP addresses for each user.  These addresses do not change over
time.  However, if a static address is assigned but not in use, it is
effectively wasted.  Since ISPs have a limited number of addresses
allocated to them, they sometimes need to make more efficient use of
their addresses.</p>

<p>Dynamic IP addressing allows the ISP to efficiently utilize their
address space.  Using dynamic IP addressing, the IP addresses of
individual user computers may change over time.  If a dynamic address is
not in use, it can be automatically reassigned to another computer as
needed.</p>

<h4>
<a NAME="II-J"></a><li>What is NAT?</li></h4>

<p>Network Address Translation (NAT) provides a way to hide the IP
addresses of a private network from the Internet while still allowing
computers on that network to access the Internet.  NAT can be used in
many different ways, but one method frequently used by home users is
called "masquerading".</p>

<p>Using NAT masquerading, one or more devices on a LAN can be made to
appear as a single IP address to the outside Internet.  This allows
for multiple computers in a home network to use a single cable modem or
DSL connection without requiring the ISP to provide more than one IP
address to the user.  Using this method, the ISP-assigned IP address
can be either static or dynamic.  Most network firewalls support NAT
masquerading.</p>


<h4>
<a NAME="II-K"></a><li>What are TCP and UDP Ports?</li></h4>

<p>TCP (Transmission Control Protocol) and UDP (User Datagram
Protocol) are both protocols that use IP.  Whereas IP
allows two computers to talk to each other across the Internet, TCP and
UDP allow individual applications (also known as "services") on those
computers to talk to each other.</p>

<p>In the same way that a telephone number or physical mail box might be
associated with more than one person, a computer might have multiple
applications (e.g.  email, file services, web services) running
on the same IP address.  Ports allow a computer to differentiate
services such as email data from web data.  A port is simply a
number associated with each application that uniquely identifies that
service on that computer. Both TCP and UDP use ports to identify
services.  Some common port numbers are 80 for web (HTTP), 25 for
email (SMTP), and 53 for Dmain Name System (DNS).</p>

<h4>
<a NAME="II-L"></a><li>What is a firewall?</li></h4>
<p>The Firewalls FAQ (<a href="http://www.faqs.org/faqs/firewalls-faq/">http://www.faqs.org/faqs/firewalls-faq/</a>)
defines a firewall as "a system or group of systems that enforces an access
control policy between two networks."  In the context of home networks,
a firewall typically takes one of two forms:</p>
<ul>
<li>
<i>Software firewall</i> - specialized software running on an individual
computer, or</li>

<li>
<i>Network firewall</i> - a dedicated device designed to protect one or
more computers.</li>
</ul>
<p>Both types of firewall allow the user to define access policies for
inbound connections to the computers they are protecting.  Many also
provide the ability to control what services (ports) the protected
computers are able to access on the Internet (outbound access).  Most
firewalls intended for home use come with pre-configured security
policies from which the user chooses, and some allow the user to
customize these policies for their specific needs.</p>

<p>More information on firewalls can be found in the <a href="#appendix-b">Additional
resources</a> section of this document.</p>


<h4>
<a NAME="II-M"></a><li>What does antivirus software do?</li></h4>

<p>There are a variety of antivirus software packages that operate in
many different ways, depending on how the vendor chose to implement
their software.  What they have in common, though, is that they all
look for patterns in the files or memory of your computer that indicate
the possible presence of a known virus.  Antivirus packages know what
to look for through the use of virus profiles (sometimes called
"signatures") provided by the vendor.</p>


<p>New viruses are discovered daily. The effectiveness of
antivirus software is dependent on having the latest virus profiles
installed on your computer so that it can look for recently discovered
viruses.  It is important to keep these profiles up to date.</p>
</ol>
<h3>
<a NAME="III"></a><li>Computer security risks to home users </li></h3>
<ol type="A">
<h4>
<a NAME="III-A"></a><li>What is at risk?</li></h4>
<p>Information security is concerned with three main areas:</p>
<ul>
<li>
Confidentiality - information should be available only to those who rightfully
have access to it</li>

<li>
Integrity -- information should be modified only by those who are authorized
to do so</li>

<li>
Availability -- information should be accessible to those who need it when
they need it</li>
</ul>

<p>These concepts apply to home Internet users just as much as they
would to any corporate or government network. You probably wouldn't
let a stranger look through your important documents. In the same way,
you may want to keep the tasks you perform on your computer
confidential, whether it's tracking your investments or sending email
messages to family and friends.  Also, you should have some assurance
that the information you enter into your computer remains intact and
is available when you need it.</p>

<p>Some security risks arise from the possibility of intentional
misuse of your computer by intruders via the Internet.  Others are
risks that you would face even if you weren't connected to the
Internet (e.g. hard disk failures, theft, power outages).  The bad
news is that you probably cannot plan for every possible risk.  The
good news is that you can take some simple steps to reduce the chance
that you'll be affected by the most common threats -- and some of
those steps help with both the intentional and accidental risks you're
likely to face.</p> 

<p>Before we get to what you can do to protect
your computer or home network, let’s take a closer look at some of
these risks.</p>

<h4>
<a NAME="III-B"></a><li>Intentional misuse of your computer</li></h4>

<p>The most common methods used by intruders to gain control of home
computers are briefly described below.  More detailed information is
available by reviewing the URLs listed in the <a
href="#appendix-A">References</a> section below.</p>

<ol>
<li>
<a href="#III-B-1">Trojan horse programs</a></li>

<li>
<a href="#III-B-2">Back door and remote administration programs</a></li>

<li>
<a href="#III-B-3">Denial of service</a></li>

<li>
<a href="#III-B-4">Being an intermediary for another attack</a></li>

<li>
<a href="#III-B-5">Unprotected Windows shares</a></li>

<li>
<a href="#III-B-6">Mobile code (Java, JavaScript, and ActiveX)</a></li>

<li>
<a href="#III-B-7">Cross-site scripting</a></li>

<li>
<a href="#III-B-8">Email spoofing</a></li>

<li>
<a href="#III-B-9">Email-borne viruses</a></li>

<li>
<a href="#III-B-10">Hidden file extensions</a></li>

<li>
<a href="#III-B-11">Chat clients</a></li>

<li>
<a href="#III-B-12">Packet sniffing</a></li>
</ol>

<ol type="1">
<h5>
<a NAME="III-B-1"></a><li>Trojan horse programs</li></h5>

<p>Trojan horse programs are a common way for intruders to trick you
(sometimes referred to as "social engineering") into installing "back
door" programs.  These can allow intruders easy access to your computer without
your knowledge, change your system configurations, or infect your computer
with a computer virus.  More information about Trojan horses can be
found in the following document.  </p>

<dl>
<dd>
<a
href="http://www.cert.org/advisories/CA-1999-02.html">http://www.cert.org/advisories/CA-1999-02.html</a></dd>
</dl>

<h5>
<a NAME="III-B-2"></a><li>Back door and remote administration programs</li></h5>

<p>On Windows computers, three tools commonly used by intruders to
gain remote access to your computer are BackOrifice, Netbus, and
SubSeven. These back door or remote administration programs, once
installed, allow other people to access and control your computer. 
<dl>
</dl>

<h5>
<a NAME="III-B-3"></a><li>Denial of service</li></h5>

<p>Another form of attack is called a denial-of-service (DoS)
attack. This type of attack causes your computer to crash or to become
so busy processing data that you are unable to use it. In most cases,
the latest patches will prevent the attack. The following documents
describe denial-of-service attacks in greater detail.</p>

<dl><dd><a href="http://www.cert.org/advisories/CA-2000-01.html">http://www.cert.org/advisories/CA-2000-01.html</a></dd>
<dd><a href="http://www.cert.org/archive/pdf/DoS_trends.pdf">http://www.cert.org/archive/pdf/DoS_trends.pdf</a></dd>
</dl>

<p>It is important to note that in addition to being the target of a
DoS attack, it is possible for your computer to be used as a
participant in a denial-of-service attack on another system.</p>

<h5>
<a NAME="III-B-4"></a><li>Being an intermediary for another attack</li></h5>

<p>Intruders will frequently use compromised computers as launching pads for
attacking other systems.  An example of this is how distributed denial-of-service 
(DDoS) tools are used.  The intruders install an "agent"
(frequently through a Trojan horse program) that runs on the compromised
computer awaiting further instructions.  Then, when a number of agents
are running on different computers, a single "handler" can instruct all
of them to launch a denial-of-service attack on another system.  Thus,
the end target of the attack is not your own computer, but someone else’s
-- your computer is just a convenient tool in a larger attack.</p>

<h5>
<a NAME="III-B-5"></a><li>Unprotected Windows shares</li></h5>

<p>Unprotected Windows networking shares can be exploited by intruders
in an automated way to place tools on large numbers of Windows-based
computers attached to the Internet. Because site security on the
Internet is interdependent, a compromised computer not only creates
problems for the computer's owner, but it is also a threat to other sites
on the Internet. The greater immediate risk to the Internet community
is the potentially large number of computers attached to the Internet
with unprotected Windows networking shares combined with distributed
attack tools such as those described in</p>

<dl>
<dd>
 <a href="http://www.cert.org/incident_notes/IN-2000-01.html">http://www.cert.org/incident_notes/IN-2000-01.html</a></dd>
</dl>

<p>Another threat includes malicious and destructive code, such as viruses
or worms, which leverage unprotected Windows networking shares to propagate.
One such example is the 911 worm described in</p>

<dl>
<dd>
<a href="http://www.cert.org/incident_notes/IN-2000-03.html">http://www.cert.org/incident_notes/IN-2000-03.html</a></dd></dl>

<p>There is great potential for the emergence of other intruder tools
that leverage unprotected Windows networking shares on a widespread
basis.</p>

<h5>
<a NAME="III-B-6"></a><li>Mobile code (Java/JavaScript/ActiveX)</li></h5>

<p>There have been reports of problems with "mobile code" (e.g. Java,
JavaScript, and ActiveX). These are programming languages that let web
developers write code that is executed by your web browser. Although
the code is generally useful, it can be used by intruders to gather
information (such as which web sites you visit) or to run malicious code
on your computer.  It is possible to disable Java, JavaScript, and
ActiveX in your web browser.  We recommend that you do so if you are
browsing web sites that you are not familiar with or do not trust.</p>

<p>Also be aware of the risks involved in the use of mobile code
within email programs. Many email programs use the same code as web
browsers to display HTML. Thus, vulnerabilities that affect Java,
JavaScript, and ActiveX are often applicable to email as
well as web pages.</p>

<p>More information on ActiveX security is available in <a
href="http://www.cert.org/archive/pdf/activeX_report.pdf">http://www.cert.org/archive/pdf/activeX_report.pdf</a></p>


<h5>
<a NAME="III-B-7"></a><li>Cross-site scripting</li></h5>

<p>A malicious web developer may attach a script to something sent to
a web site, such as a URL, an element in a form, or a database
inquiry.  Later, when the web site responds to you, the malicious
script is transferred to your browser.</p>

<p>You can potentially expose your web browser to malicious
scripts by</p>

<ul>
<li>
following links in web pages, email messages, or newsgroup
postings without knowing what they link to</li>

<li>
using interactive forms on an untrustworthy site</li>

<li>
viewing online discussion groups, forums, or other dynamically
generated pages where users can post text containing HTML tags</li>
</ul>

<p>More information regarding the risks posed by malicious code in web
links can be found in <a
href="http://www.cert.org/advisories/CA-2000-02.html">CA-2000-02
Malicious HTML Tags Embedded in Client Web Requests</a>. </p>


<h5>
<a NAME="III-B-8"></a><li>Email spoofing</li></h5>

<p>Email “spoofing” is when an email message appears to have originated
from one source when it actually was sent from another source.  Email
spoofing is often an attempt to trick the user into making a damaging statement
or releasing sensitive information (such as passwords).</p>

<p>Spoofed email can range from harmless pranks to social engineering
ploys.  Examples of the latter include</p>
<ul>
<li>
email claiming to be from a system administrator requesting users to change
their passwords to a specified string and threatening to suspend their
account if they do not comply</li>
<li>
email claiming to be from a person in authority requesting users to send
them a copy of a password file or other sensitive information</li>
</ul>

<p>Note that while service providers may occasionally request that you change
your password, they usually will <b>not</b> specify what you should change it
to.  Also, most legitimate service providers would <b>never</b> ask you to
send them any password information via email.  If you suspect that
you may have received a spoofed email from someone with malicious intent,
you should contact your service provider's support personnel immediately.</p>
<h5>
<a NAME="III-B-9"></a><li>Email borne viruses</li></h5>

<p>Viruses and other types of malicious code are often spread as
attachments to email messages. Before opening any
attachments, be sure you know the source of the attachment. It is not
enough that the mail originated from an address you recognize. The
Melissa virus (see <a href="#appendix-A">References</a>) spread precisely
because it originated from a familiar address. Also, malicious code
might be distributed in amusing or enticing programs.</p>

<p>Many recent viruses use these social engineering techniques to
spread.  Examples include</p>
<ul>
<li>W32/Sircam -- <a href="http://www.cert.org/advisories/CA-2001-22.html">http://www.cert.org/advisories/CA-2001-22.html
</a></li>
<li>W32/Goner -- <a href="http://www.cert.org/incident_notes/IN-2001-15.html">http://www.cert.org/incident_notes/IN-2001-15.html
</a></li>

</ul>

<p>Never run a program unless you know it to be authored by a person
or company that you trust.  Also, don't send programs of unknown
origin to your friends or coworkers simply because they are amusing --
they might contain a Trojan horse program.</p>

<h5>
<a NAME="III-B-10"></a><li>Hidden file extensions</li></h5>

<p>Windows operating systems contain an option to "Hide file
extensions for known file types". The option is enabled by default,
but a user may choose to disable this option in order to have file
extensions displayed by Windows.  Multiple email-borne viruses are
known to exploit hidden file extensions. The first major attack that
took advantage of a hidden file extension was the VBS/LoveLetter worm
which contained an email attachment named
"LOVE-LETTER-FOR-YOU.TXT.vbs". Other malicious programs have since
incorporated similar naming schemes.  Examples include</p>
<ul>
<li>
Downloader (MySis.avi.exe or QuickFlick.mpg.exe)</li>

<li>
VBS/Timofonica (TIMOFONICA.TXT.vbs)</li>

<li>
VBS/CoolNote (COOL_NOTEPAD_DEMO.TXT.vbs)</li>

<li>
VBS/OnTheFly (AnnaKournikova.jpg.vbs)</li>
</ul>

<p>The files attached to the email messages sent by these viruses may
appear to be harmless text (.txt), MPEG (.mpg), AVI (.avi) or other
file types when in fact the file is a malicious script or executable
(.vbs or .exe, for example).</p> 

<h5>
<a NAME="III-B-11"></a><li>Chat clients</li></h5>

<p>Internet chat applications, such as instant messaging applications
and Internet Relay Chat (IRC) networks, provide a mechanism for
information to be transmitted bi-directionally between computers on the
Internet.  Chat clients provide groups of individuals with the means
to exchange dialog, web URLs, and in many cases, files of any
type.</p>

<p>Because many chat clients allow for the exchange of executable
code, they present risks similar to those of email clients.  As with
email clients, care should be taken to limit the chat client’s
ability to execute downloaded files.  As always, you should be wary
of exchanging files with unknown parties.</p>

<h5>
<a NAME="III-B-12"></a><li>Packet sniffing</li></h5>

<p>A packet sniffer is a program that captures data from information packets
as they travel over the network. That data may include user names, passwords,
and proprietary information that travels over the network in clear text.
With perhaps hundreds or thousands of passwords captured by the packet sniffer,
intruders can launch widespread attacks on systems. Installing a packet
sniffer does not necessarily require administrator-level access.</p>

<p>Relative to DSL and traditional dial-up users, cable modem users
have a higher risk of exposure to packet sniffers since entire
neighborhoods of cable modem users are effectively part of the same
LAN.  A packet sniffer installed on any cable modem user's computer in a
neighborhood may be able to capture data transmitted by any other
cable modem in the same neighborhood.</p>

</ol>
<h4>
<a NAME="III-C"></a><li>Accidents and other risks</li></h4>

<p>In addition to the risks associated with connecting your computer to the
Internet, there are a number of risks that apply even if the computer has
no network connections at all.  Most of these risks are well-known,
so we won’t go into much detail in this document, but it is important to
note that the common practices associated with reducing these risks may
also help reduce susceptibility to the network-based risks discussed above.</p>

<ol type="1">
<h5>
<a NAME="III-C-1"></a><li>Disk failure</li></h5>

<p>Recall that availability is one of the three key elements of
information security.  Although all stored data can become unavailable
-- if the media it’s stored on is physically damaged, destroyed, or
lost -- data stored on hard disks is at higher risk due to the
mechanical nature of the device.  Hard disk crashes are a common cause
of data loss on personal computers.  Regular system backups are the
only effective remedy.</p>

<h5>
<a NAME="III-C-2"></a><li>Power failure and surges</li></h5>

<p>Power problems (surges, blackouts, and brown-outs) can cause
physical damage to a computer, inducing a hard disk crash or otherwise
harming the electronic components of the computer.  Common mitigation
methods include using surge suppressors and uninterruptible power
supplies (UPS).</p>

<h5>
<a NAME="III-C-3"></a><li>Physical Theft</li></h5> 

<p>Physical theft of a computer, of course, results in the loss of
confidentiality and availability, and (assuming the computer is ever
recovered) makes the integrity of the data stored on the disk suspect.
Regular system backups (with the backups stored somewhere away from
the computer) allow for recovery of the data, but backups alone cannot
address confidentiality.  Cryptographic tools are available that can
encrypt data stored on a computer’s hard disk.  The CERT/CC encourages the
use of these tools if the computer contains sensitive data or is at
high risk of theft (e.g. laptops or other portable computers).</p>

</ol>
</ol>
<h3>
<a NAME="IV"></a><li>Actions home users can take to protect their
computer systems</li></h3>

<p>The CERT/CC recommends the following
practices to home users:</p>
<ol>

<li>
<a href="#IV-A-1">Consult your system support personnel if you work
from home</a></li>

<li>
<a href="#IV-A-2">Use virus protection software</a></li>

<li>
<a href="#IV-A-3">Use a firewall</a></li>

<li>
<a href="#IV-A-4">Don’t open unknown email attachments</a></li>

<li>
<a href="#IV-A-5">Don’t run programs of unknown origin</a></li>

<li>
<a href="#IV-A-6">Disable hidden filename extensions</a></li>

<li>
<a href="#IV-A-7">Keep all applications (including your operating system) patched</a></li>

<li>
<a href="#IV-A-8">Turn off your computer or disconnect from the network when not in use</a></li>

<li>
<a href="#IV-A-9">Disable Java, JavaScript, and ActiveX if possible</a></li>

<li>
<a href="#IV-A-10">Disable scripting features in email programs</a></li>

<li>
<a href="#IV-A-11">Make regular backups of critical data</a></li>

<li>
<a href="#IV-A-12">Make a boot disk in case your computer is damaged or compromised</a></li>


</ol>
<p>Further discussion on each of these points is given below.</p>
<h4>
Recommendations</h4>
<ol>

<h5>
<a NAME="IV-A-1"></a><li>Consult your system support personnel if you work
from home</li></h5> 

<p>If you use your broadband access to connect to your employer's
network via a Virtual Private Network (VPN) or other means, your
employer may have policies or procedures relating to the security of
your home network.  Be sure to consult with your employer's support
personnel, as appropriate, before following any of the steps outlined
in this document.</p>


<h5>
<a NAME="IV-A-2"></a><li>Use virus protection software</li></h5>

<p>The CERT/CC recommends the use of anti-virus software on all
Internet-connected computers.  Be sure to keep your anti-virus
software up-to-date. Many anti-virus packages support automatic
updates of virus definitions.  We recommend the use of these automatic
updates when available.</p>


<h5>
<a NAME="IV-A-3"></a><li>Use a firewall</li></h5> 

<p>We strongly recommend the use of some type of firewall product,
such as a network appliance or a personal firewall software package.
Intruders are constantly scanning home user systems for known
vulnerabilities.  Network firewalls (whether software or
hardware-based) can provide some degree of protection against these
attacks.  However, no firewall can detect or stop all attacks, so
it’s not sufficient to install a firewall and then ignore all other
security measures.</p>

<h5>
<a NAME="IV-A-4"></a><li>Don't open unknown email attachments</li></h5>

<p>Before opening any email attachments, be sure you know the source
of the attachment. It is not enough that the mail originated from an
address you recognize. The Melissa virus spread precisely because it
originated from a familiar address. Malicious code might be
distributed in amusing or enticing programs. </p>

<p>If you must open an attachment before you can verify the source, we
suggest the following procedure:</p>

<ol>

<li>be sure your virus definitions are up-to-date (see <a
href="#IV-A-2">"Use virus protection software"</a> above)</li>

<li>save the file to your hard disk</li>

<li>scan the file using your antivirus software</li>

<li>open the file</li>
</ol>

<p>For additional protection, you can disconnect your computer's network
connection before opening the file.</p>

<p>Following these steps will reduce, but not wholly eliminate, the
chance that any malicious code contained in the attachment might
spread from your computer to others.</p>


<h5>
<a NAME="IV-A-5"></a><li>Don't run programs of unknown origin</li></h5>

<p>Never run a program unless you know it to be authored by a person
or company that you trust.  Also, don't send programs of unknown
origin to your friends or coworkers simply because they are amusing --
they might contain a Trojan horse program.</p>

<h5>
<a NAME="IV-A-6"></a><li>Disable hidden filename extensions</li></h5>

<p>Windows operating systems contain an option to "Hide file
extensions for known file types". The option is enabled by default,
but you can disable this option in order to have file extensions
displayed by Windows.  After disabling this option, there are still
some file extensions that, by default, will continue to remain hidden.</p>

<p>There is a registry value which, if set, will cause Windows to hide
certain file extensions regardless of user configuration choices elsewhere
in the operating system. The "NeverShowExt" registry value is used to 
hide the extensions for basic Windows file types. For example, the ".LNK"
extension associated with Windows shortcuts remains hidden even after a
user has turned off the option to hide extensions.</p>

<p>Specific instructions for disabling hidden file name extensions are
given in <a href="http://www.cert.org/incident_notes/IN-2000-07.html">http://www.cert.org/incident_notes/IN-2000-07.html</a>

<h5>
<a NAME="IV-A-7"></a><li>Keep all applications, including your
operating system, patched</li></h5>

<p>Vendors will usually release patches for their software when a
vulnerability has been discovered.  Most product documentation offers
a method to get updates and patches. You should be able to obtain
updates from the vendor's web site. Read the manuals or browse the
vendor's web site for more information.</p>

<p>Some applications will automatically check for available updates,
and many vendors offer automatic notification of updates via a
mailing list. Look on your vendor's web site for information about
automatic notification. If no mailing list or other automated
notification mechanism is offered you may need to check periodically
for updates.</p>

<h5>
<a NAME="IV-A-8"></a><li>Turn off your computer or disconnect from the
network when not in use</li></h5>

<p>Turn off your computer or disconnect its Ethernet interface when you are not
using it.  An intruder cannot attack your computer if it is powered off
or otherwise completely disconnected from the network.</p>

<h5>
<a NAME="IV-A-9"></a><li>Disable Java, JavaScript, and ActiveX if possible</li></h5>

<p>Be aware of the risks involved in the use of "mobile code" such as
ActiveX, Java, and JavaScript. A malicious web developer may attach a
script to something sent to a web site, such as a URL, an element in a
form, or a database inquiry.  Later, when the web site responds to
you, the malicious script is transferred to your browser.</p>

<p>The most significant impact of this vulnerability can be avoided by
disabling all scripting languages.  Turning off these options will
keep you from being vulnerable to malicious scripts. However, it will
limit the interaction you can have with some web sites. </p>

<p>Many legitimate sites use scripts running within the browser to add
useful features.  Disabling scripting may degrade the functionality of
these sites.</p>

<p>More information on ActiveX security, including recommendations for
users who administer their own computers, is available in <a
href="http://www.cert.org/archive/pdf/activeX_report.pdf">http://www.cert.org/archive/pdf/activeX_report.pdf</a></p>

<p>More information regarding the risks posed by malicious code in web
links can be found in <a
href="http://www.cert.org/advisories/CA-2000-02.html">CA-2000-02
Malicious HTML Tags Embedded in Client Web Requests</a>. </p>

<h5>
<a NAME="IV-A-10"></a><li>Disable scripting features in email programs</li></h5>

<p>Because many email programs use the same code as web browsers to
display HTML, vulnerabilities that affect ActiveX, Java, and
JavaScript are often applicable to email as well as web pages.
Therefore, in addition to disabling scripting features in web browsers
(see <a href="#IV-A-9">"Disable Java, JavaScript, and ActiveX if
possible"</a>, above), we recommend that users also disable these
features in their email programs.</p>

<h5>
<a NAME="IV-A-11"></a><li>Make regular backups of critical data</li></h5>

<p>Keep a copy of important files on removable media such as ZIP disks
or recordable CD-ROM disks (CD-R or CD-RW disks).  Use software backup
tools if available, and store the backup disks somewhere away from the
computer.</p>

<h5>
<a NAME="IV-A-12"></a><li>Make a boot disk in case your computer is damaged
or compromised</li></h5>

<p>To aid in recovering from a security breach or hard disk failure,
create a boot disk on a floppy disk which will help when recovering a
computer after such an event has occurred.  Remember, however, you
must create this disk <b>before</b> you have a security event.</p>

<br>
</ol>
</ol>
<HR size=2 noshade align=left>

<h3>
<a NAME="appendix"></a>Appendix</h3>
<h4>
References and additional information</h4>

<dl>This section contains links to <a href="#appendix-A">references</a> and <a href="#appendix-b">additional
resources</a> related to this document.
<h4>
<a NAME="appendix-A"></a>References</h4>

<dt>
The following documents were used in compiling portions of this document:</dt>

<ul>
<li>
<a href="#appendix-A-1">CERT Advisories</a></li>

<li>
<a href="#appendix-A-2">CERT Incident Notes</a></li>

<li>
<a href="#appendix-A-4">CERT Tech Tips</a></li>

<li>
<a href="#appendix-A-5">Other CERT documents</a></li>
</ul>

<h5>
<a NAME="appendix-A-1"></a>CERT Advisories</h5>

<dt>CA-1999-02: Trojan Horses</dt>

<dd>
<a href="http://www.cert.org/advisories/CA-1999-02.html">http://www.cert.org/advisories/CA-1999-02.html</a></dd>

<br>
<dt>CA-1999-04: Melissa Macro Virus</dt>

<dd>
<a
href="http://www.cert.org/advisories/CA-1999-04.html">http://www.cert.org/advisories/CA-1999-04.html</a></dd>

<br>
<dt>CA-2000-01: Denial-of-Service Developments</dt>

<dd>
<a href="http://www.cert.org/advisories/CA-2000-01.html">http://www.cert.org/advisories/CA-2000-01.html</a></dd>

 
<br>
<dt>
CA-2000-02: Malicious HTML Tags Embedded in Client Web Requests</dt>

<dd>
<a href="http://www.cert.org/advisories/CA-2000-02.html">http://www.cert.org/advisories/CA-2000-02.html</a></dd>

<br>
<dt>
CA-2001-22: W32/Sircam Malicious Code</dt>
<dd><a href="http://www.cert.org/advisories/CA-2001-22.html">http://www.cert.org/advisories/CA-2001-22.html</a></dd>

</dl>

<h5>
<a NAME="appendix-A-2"></a>CERT Incident Notes</h5>

<dl>
<dt>
IN-2000-01: Windows Based DDOS Agents</dt>

<dd>
<a href="http://www.cert.org/incident_notes/IN-2000-01.html">http://www.cert.org/incident_notes/IN-2000-01.html</a></dd>

<br> 
<dt>
IN-2000-02: Exploitation of Unprotected Windows Networking Shares</dt>

<dd>
<a href="http://www.cert.org/incident_notes/IN-2000-02.html">http://www.cert.org/incident_notes/IN-2000-02.html</a></dd>

<br> 
<dt>
IN-2000-03: 911 Worm</dt>

<dd>
<a href="http://www.cert.org/incident_notes/IN-2000-03.html">http://www.cert.org/incident_notes/IN-2000-03.html</a></dd>

<br> 
<dt>
IN-2000-07: Exploitation of Hidden File Extensions</dt>

<dd>
<a href="http://www.cert.org/incident_notes/IN-2000-07.html">http://www.cert.org/incident_notes/IN-2000-07.html</a></dd>

<br> 
<dt>
IN-2000-08: Chat Clients and Network Security</dt>

<dd>
<a href="http://www.cert.org/incident_notes/IN-2000-08.html">http://www.cert.org/incident_notes/IN-2000-08.html</a></dd>


<br>
<dt>
IN-2001-15: W32/Goner Worm</dt>

<dd><a href="http://www.cert.org/incident_notes/IN-2001-15.html">http://www.cert.org/incident_notes/IN-2001-15.html</a></dd>
</dl>

<h5>
<a NAME="appendix-A-4"></a>CERT Tech Tips</h5>

<dl>

<dt>
Spoofed/Forged Email</dt>

<dd>
<a href="http://www.cert.org/tech_tips/email_spoofing.html">http://www.cert.org/tech_tips/email_spoofing.html</a></dd>


<h5>
<a NAME="appendix-A-5"></a>Other CERT documents</h5>

<dl>
<dt>
Results of the Security in ActiveX Workshop</dt>
<dd>
<a href="http://www.cert.org/archive/pdf/activeX_report.pdf">http://www.cert.org/archive/pdf/activeX_report.pdf</a></dd>

<br> 
<dt>
Security of the Internet</dt>

<dd>
<a href="http://www.cert.org/encyc_article/tocencyc.html#PackSnif">http://www.cert.org/encyc_article/tocencyc.html#PackSnif</a></dd>


<br>
<dt>
Trends in Denial of Service Attack Technology
<dd>
<a href="http://www.cert.org/archive/pdf/DoS_trends.pdf">http://www.cert.org/archive/pdf/DoS_trends.pdf</a></dd>
</dl>

<h4>
<a NAME="appendix-b"></a>Additional resources</h4>
Additional information is available from the following sources.
<dl>

<dt>TCP/IP Frequently Asked Questions</dt>

<dd><a
href="http://www.faqs.org/faqs/internet/tcp-ip/tcp-ip-faq/part1/">http://www.faqs.org/faqs/internet/tcp-ip/tcp-ip-faq/part1/</a></dd>

<dd><a
href="http://www.faqs.org/faqs/internet/tcp-ip/tcp-ip-faq/part2/">http://www.faqs.org/faqs/internet/tcp-ip/tcp-ip-faq/part2/</a></dd>

<dt>
Computer Virus Frequently Asked Questions for New Users</dt>

<dd>
<a href="http://www.faqs.org/faqs/computer-virus/new-users/">http://www.faqs.org/faqs/computer-virus/new-users/</a></dd>

<br> 
<dt>
alt.comp.virus Frequently Asked Questions</dt>

<dd>
<a href="http://www.faqs.org/faqs/computer-virus/alt-faq/part1/index.html">http://www.faqs.org/faqs/computer-virus/alt-faq/part1/</a></dd>

<dd>
<a href="http://www.faqs.org/faqs/computer-virus/alt-faq/part1/index.html">http://www.faqs.org/faqs/computer-virus/alt-faq/part2/</a></dd>

<dd>
<a href="http://www.faqs.org/faqs/computer-virus/alt-faq/part1/index.html">http://www.faqs.org/faqs/computer-virus/alt-faq/part3/</a></dd>

<dd>
<a href="http://www.faqs.org/faqs/computer-virus/alt-faq/part1/index.html">http://www.faqs.org/faqs/computer-virus/alt-faq/part4/</a></dd>

<br> 
<dt>
VIRUS-L/comp.virus Frequently Asked Questions</dt>

<dd>
<a href="http://www.faqs.org/faqs/computer-virus/faq/">http://www.faqs.org/faqs/computer-virus/faq/</a></dd>

<br> 
<dt>
Firewalls Frequently Asked Questions</dt>

<dd>
<a href="http://www.faqs.org/faqs/firewalls-faq/">http://www.faqs.org/faqs/firewalls-faq/</a></dd>

</dl>

<!-- END CONTENT -->
<p><!--#include virtual="/include/footer_nocopyright.html" --></p>
<p>Copyright 2001 Carnegie Mellon University.</p>

<HR SIZE=2 NOSHADE ALIGN=LEFT>

<TABLE>
<A NAME="history">

<TR>
<TD>
<FONT SIZE=3 FACE="Verdana">
Revision History
</TD>
</TR>

<TR>
<TD VALIGN=TOP WIDTH=30%>
<FONT SIZE=2 FACE="Verdana">
June 22, 2001<BR>
</TD>
<TD VALIGN=TOP WIDTH=70%>
<FONT SIZE=2 FACE="Verdana">
Initial Release<BR>
</TD>
</TR>

<TR>
<TD VALIGN=TOP WIDTH=30%>
<FONT SIZE=2 FACE="Verdana">
June 26, 2001<BR>
</TD>
<TD VALIGN=TOP WIDTH=70%>
<FONT SIZE=2 FACE="Verdana">
Added SubSeven to Remote Administration Programs section<BR>
</TD>
</TR>

<TR>
<TD VALIGN=TOP WIDTH=30%>
<FONT SIZE=2 FACE="Verdana">
August 6, 2001<BR>
</TD>
<TD VALIGN=TOP WIDTH=70%>
<FONT SIZE=2 FACE="Verdana">
Clarification of IP addressing for ISP dial-up modem pools<BR>
</TD>
</TR>

<TR>
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December 5, 2001<BR>
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Fixed broken link to CA-1999-02, added links for Sircam, Goner, and DDoS Trends<BR>
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February 27, 2006<BR>
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Removed link to defunct directory that was on cert.org previously.
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