What is computer security?
Computer security prevents and detects unauthorized use of your computer. Prevention measures help you stop unauthorized users (“intruders”) from accessing any part of your computer system. Detection lets you determine whether someone attempted to break into your system, if they were successful, and what they may have done.
Why should I care about computer security?
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 emails from your computer, or examining personal information stored on your computer (such as financial statements).
Who would want to break into my computer at home?
Intruders (also referred to as hackers, attackers, or crackers) may not care about your identity. Often, they want to gain control of your Computer to use it to launch attacks on other computer systems. Having control of your Computer allows them 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 emails to friends and family, your Computer may be a target. 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.
How easy is it to break into my Computer?
Unfortunately, intruders always discover new vulnerabilities (called “holes”) to exploit computer software. The complexity of software makes it increasingly difficult to test the security of computer systems thoroughly. Computer vendors will usually develop patches to address the problem(s) when holes are discovered. However, it is up to you, the user, to install the patches or correctly configure the software to operate more securely. Most 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.
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.
Technology
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. We include links to additional information for those who want a deeper understanding of the concepts.
What does broadband mean?
“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 called 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). No set bandwidth threshold is required for a connection to be referred to as “broadband,” but it is typical for connections over 1 Megabit per second (Mbps) to be so named.
What is cable modem access?
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 can speed up to more than 5 Mbps.
However, typical speeds tend to be lower than the maximum since cable providers turn entire neighborhoods into LANs with the same bandwidth. Because of this “shared-medium” topology, cable modem users may experience slower network access during periods of peak demand. They may be more susceptible to risks such as packet sniffing and unprotected Windows shares than users with other types of connectivity. (See this document’s “Computer security risks to home users” section.)
What is DSL access?
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 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 bandwidth guarantee across the Internet.
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 this document’s “Computer security risks to home users” section.)
How are broadband services different from traditional dial-up services?
Traditional dial-up Internet services are sometimes referred to as “dial-on-demand” services. Your Computer only connects to the Internet when it has something to send, such as an email or a request to load a web page. Once no more data is 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. Since the modem IP addresses are dynamically assigned, your Computer is usually given 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.
Broadband services are called “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.
Moreover, 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.
The table below shows a brief comparison of traditional dial-up and broadband services.
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 The Computer must be dialed in to control remotely
The Computer is always connected so that remote control can occur anytime
ISP-provided security Little or none Little or none
Table 1: Comparison of Dial-up and Broadband Services
How is broadband access different from the network I use at work?
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 computers. As a result, it is up to you to take reasonable precautions to secure your computers from accidental or intentional misuse.
What is a protocol?
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.
What is IP?
IP stands for “Internet Protocol.” It can be considered the common language of computers on the Internet. Several detailed descriptions of IP are given elsewhere so that we won’t cover it in detail in this document. However, knowing few things about IP is important 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
What is an IP address?
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 understand 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 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, and type this into your web browser, your Computer will then ask its DNS server what the numeric IP address is associated with that name.
Every Computer on the Internet has an IP address that uniquely identifies it. However, that address may change over time, especially if the Computer is
dialing into an Internet Service Provider (ISP)
connected behind a network firewall
connected to a broadband service using dynamic IP addressing.
What is static and dynamic addressing?
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, it is effectively wasted if a static address is assigned but not in use. Since ISPs have limited addresses allocated to them, they sometimes need to make more efficient use of their lessons.
Dynamic IP addressing allows the ISP to utilize their address space efficiently. Using dynamic IP addressing, the IP addresses of individual user computers may change over time. If an active address is not in use, it can be automatically reassigned to another computer as needed.
What is NAT?
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 multiple home network computers 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 static or dynamic. Most network firewalls support NAT masquerading.
What are TCP and UDP Ports?
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 mailbox 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 like 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 the web (HTTP), 25 for email (SMTP), and 53 for Domain Name System (DNS).
What is a firewall?
The 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:
Software firewall – specialized software running on an individual computer or
Network firewall – a dedicated device designed to protect one or more computers.
Both types of firewalls allow users to define access policies for inbound connections to the computers they are protecting. Many also can control what services (ports) the protected computers can access the Internet (outbound access). Most firewalls intended for home use come with pre-configured security policies from which the user chooses; 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.
What does antivirus software do?
Various antivirus software packages operate in many different ways, depending on how the vendor implements 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 vendor’s virus profiles (sometimes called “signatures”).