Confidentiality - information should be available only to those who rightfully have access to it
Integrity -- information should be modified only by those who are authorized to do so
Availability -- information should be accessible to those who need it when they need it
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.
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.
Intentional misuse of your computer
The most common methods used by intruders to gain control of home computers are briefly described below.
Trojan horse programs: 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.
Back door and remote administration programs: 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.
Denial of service: 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.
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.
Being an intermediary for another attack: 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 shares
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.
Another threat includes malicious and destructive code, such as viruses or worms, which leverage unprotected Windows networking shares to propagate.
There is great potential for the emergence of other intruder tools that leverage unprotected Windows networking shares on a widespread basis.
Cross-site scripting: 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 following links in web pages, email messages, or newsgroup postings without knowing what they link to using interactive forms on an untrustworthy site viewing online discussion groups, forums, or other dynamically generated pages where users can post text containing HTML tags.
Email spoofing: 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).
Spoofed email can range from harmless pranks to social engineering ploys. Examples of the latter include 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 email claiming to be from a person in authority requesting users to send them a copy of a password file or other sensitive information.
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 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.
Email borne viruses: 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 References) spread precisely because it originated from a familiar address. Also, malicious code might be distributed in amusing or enticing programs.
Many recent viruses use these social engineering techniques to spread. Examples include
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.
Hidden file extensions: 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
- Downloader (MySis.avi.exe or QuickFlick.mpg.exe)
- VBS/Timofonica (TIMOFONICA.TXT.vbs)
- VBS/CoolNote (COOL_NOTEPAD_DEMO.TXT.vbs)
- VBS/OnTheFly (AnnaKournikova.jpg.vbs)
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).
Chat clients: 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.
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.
Packet sniffing: 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's computer in a neighborhood may be able to capture data transmitted by any other cable modem in the same neighborhood.
Accidents and other risks
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 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.
Disk failure: 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 failure and surges: 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: 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).