| Dial-up | Broadband | |
| Connection type | Dial-on-demand | Always-on |
| IP address | Changes on each call | Static or infrequently changing |
| Relative connection speed | Low | High |
| Remote control potential | Computer must be dialed-in to control remotely | Computer is always connected, so remote control can occur anytime |
| ISP-provided security | Little or none | Little or none |
| Table 1: Comparison of Dial-up and Broadband Services | ||
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.
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.
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.
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.
An overview of TCP/IP can be found in the TCP/IP Frequently Asked Questions (FAQ) at
and
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.
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 Naming Service, 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.
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
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.
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.
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".
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.
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.
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. e-mail, file services, web services) running on the same IP address. Ports allow a computer to differentiate services such as e-mail 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 e-mail (SMTP), and 53 for domain name services (DNS).
The Firewalls FAQ (http://www.faqs.org/faqs/firewalls-faq/) 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:
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.
More information on firewalls can be found in the Additional resources section of this document
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.
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.
More information about viruses and antivirus software can be found on the CERT Computer Virus Resource page
Information security is concerned with three main areas:
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 e-mail 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.
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.
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.
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 References section below.
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.
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. We recommend that you review the CERT vulnerability note about Back Orifice. This document describes how it works, how to detect it, and how to protect your computers from it:
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 document describes Denial of Service attacks in greater detail.
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.
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.
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
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
There is great potential for the emergence of other intruder tools that leverage unprotected Windows networking shares on a widespread basis.
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.
Also be aware of the risks involved in the use of mobile code within e-mail programs. Many e-mail programs use the same code as web browsers to display HTML. Thus, vulnerabilities that affect Java, JavaScript, and ActiveX are often applicable to e-mail as well as web pages.
More information on malicious code is available in http://www.cert.org/tech_tips/malicious_code_FAQ.html
More information on ActiveX security is available in http://www.cert.org/archive/pdf/activeX_report.pdf
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.
You can potentially expose your web browser to malicious scripts by
More information regarding the risks posed by malicious code in web links can be found in CA-2000-02 Malicious HTML Tags Embedded in Client Web Requests.
E-mail “spoofing” is when an e-mail message appears to have originated from one source when it actually was sent from another source. E-mail spoofing is often an attempt to trick the user into making a damaging statement or releasing sensitive information (such as passwords).
Spoofed e-mail can range from harmless pranks to social engineering ploys. Examples of the latter include
Note that while service providers may occasionally request that you change your password, they usually will not specify what you should change it to. Also, most legitimate service providers would never ask you to send them any password information via e-mail. If you suspect that you may have received a spoofed e-mail from someone with malicious intent, you should contact your service provider's support personnel immediately.
Viruses and other types of malicious code are often spread as attachments to e-mail 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 References) spread precisely because it originated from a familiar address. Also, malicious code might be distributed in amusing or enticing programs.
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.
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 e-mail-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 e-mail attachment named "LOVE-LETTER-FOR-YOU.TXT.vbs". Other malicious programs have since incorporated similar naming schemes. Examples include
The files attached to the e-mail 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). For further information about these and other viruses, please visit the sites listed on our Computer Virus Resource page:
Internet chat applications, such as instant messaging applications and Internet Relay Chat (IRC) networks, provide a mechanism for information to be transmitted bidirectionally 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.
Because many chat clients allow for the exchange of executable code, they present risks similar to those of e-mail clients. As with e-mail 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.
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.
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 computer in a neighborhood may be able to capture data transmitted by any other cable modem in the same neighborhood.
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.
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.
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).
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).
The CERT/CC recommends the following practices to home users:
Further discussion on each of these points is given below.
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.
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.
See http://www.cert.org/other_sources/viruses.html#VI for more information.
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.
Before opening any e-mail 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.
If you must open an attachment before you can verify the source, we suggest the following procedure:
For additional protection, you can disconnect your computer's network connection before opening the file.
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.
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.
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.
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.
Specific instructions for disabling hidden file name extensions are given in http://www.cert.org/incident_notes/IN-2000-07.html
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.
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.
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.
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.
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.
Many legitimate sites use scripts running within the browser to add useful features. Disabling scripting may degrade the functionality of these sites.
Detailed instructions for disabling browser scripting languages are available in http://www.cert.org/tech_tips/malicious_code_FAQ.html
More information on ActiveX security, including recommendations for users who administer their own computers, is available in http://www.cert.org/archive/pdf/activeX_report.pdf
More information regarding the risks posed by malicious code in web links can be found in CA-2000-02 Malicious HTML Tags Embedded in Client Web Requests.
Because many e-mail programs use the same code as web browsers to display HTML, vulnerabilities that affect ActiveX, Java, and JavaScript are often applicable to e-mail as well as web pages. Therefore, in addition to disabling scripting features in web browsers (see "Disable Java, JavaScript, and ActiveX if possible", above), we recommend that users also disable these features in their e-mail programs.
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.
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 before you have a security event.
CERT/CC personnel answer the hotline 08:00-17:00 EST(GMT-5) / EDT(GMT-4) Monday through Friday; they are on call for emergencies during other hours, on U.S. holidays, and on weekends.
We strongly urge you to encrypt sensitive information sent by email. Our public PGP key is available from
If you prefer to use DES, please call the CERT hotline for more information.subscribe cert-advisory
* "CERT" and "CERT Coordination Center" are registered in the U.S. Patent and Trademark Office.
Copyright 2001 Carnegie Mellon University.
| Revision History | |
|
June 22, 2001 |
Initial Release |
|
June 26, 2001 |
Added SubSeven to Remote Administration Programs section |
|
August 6, 2001 |
Clarification of IP addressing for ISP dial-up modem pools |
|
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