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Montgomery County, Maryland
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Computer Security
Computer Security is the procedure of preventing and detecting unauthorized use of your computer. Prevention helps by stopping "intruders" from accessing your computer system and detection helps by determining if someone has attempted to break into your computer system.

Computer Security is important because you probably do not want strangers reading your e-mail, using your computer to attack other computer systems, or prying into the personal information stored on your computer (such as financial information).

If you would like to learn a little bit about computer security risks to home users and/or what actions home users can take to protect their computer systems, then please read the excerpt below from CERT® Coordination Center's Home Network Security document.



  1. Computer security risks to home users
    1. What is at risk?
    2. Intentional misuse of your computer
      1. Trojan horse programs
      2. Back door and remote administration programs
      3. Denial of Service
      4. Being an intermediary for another attack
      5. Unprotected Windows shares
      6. Mobile code (Java, JavaScript, and ActiveX)
      7. Cross-site scripting
      8. E-mail spoofing
      9. E-mail-borne viruses
      10. Hidden file extensions
      11. Chat clients
      12. Packet sniffing
    3. Accidents and other risks
      1. Disk failure
      2. Power failure and surges
      3. Physical theft
  2. Actions home users can take to protect their computer systems
    1. Consult your system support personnel if you work from home
    2. Use virus protection software
    3. Use a firewall
    4. Donít open unknown e-mail attachments
    5. Donít run programs of unknown origin
    6. Disable hidden filename extensions
    7. Keep all applications (including your operating system) patched
    8. Turn off your computer or disconnect from the network when not in use
    9. Disable Java, JavaScript, and ActiveX if possible
    10. Disable scripting features in e-mail programs
    11. Make regular backups of critical data
    12. Make a boot disk in case your computer is damaged or compromised
    13. Montgomery County Department of Police Computer Crime Unit


  1. Computer security risks to home users
    1. What is at risk?
    2. Information security is concerned with three main areas:

      • Confidentiality -- information should only be available to those who rightfully have access to it
      • Integrity -- information should only be modified 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 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, letís take a closer look at some of these risks.

    3. Intentional misuse of your computer
    4. The most common methods used by intruders to gain control of home computers are briefly described below.

      1. Trojan horse programs
      2. Back door and remote administration programs
      3. Denial of Service
      4. Being an intermediary for another attack
      5. Unprotected Windows shares
      6. Mobile code (Java, JavaScript, and ActiveX)
      7. Cross-site scripting
      8. E-mail spoofing
      9. E-mail-borne viruses
      10. Hidden file extensions
      11. Chat clients
      12. Packet sniffing
      1. Trojan horse programs
      2. 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.

      3. Back door and remote administration programs
      4. 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.

      5. Denial of Service
      6. 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.

      7. Being an intermediary for another attack
      8. 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 elseís -- your computer is just a convenient tool in a larger attack.

      9. Unprotected Windows shares
      10. 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.

      11. Mobile code (Java/JavaScript/ActiveX)
      12. 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.

      13. Cross-site scripting
      14. 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, e-mail 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
      15. E-mail spoofing
      16. E-mail ďspoofingĒ 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

        • e-mail 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
        • e-mail 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 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.

      17. E-mail borne viruses
      18. 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. 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.

      19. Hidden file extensions
      20. 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

        • 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 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).

      21. Chat clients
      22. 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 clientís ability to execute downloaded files. As always, you should be wary of exchanging files with unknown parties.

      23. Packet sniffing
      24. 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.

    5. Accidents and other risks
    6. 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 wonít 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.

      1. Disk failure
      2. Recall that availability is one of the three key elements of information security. Although all stored data can become unavailable -- if the media itís 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.

      3. Power failure and surges
      4. 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).

      5. Physical Theft
      6. 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 computerís hard disk. MontgomeryCountyMd.gov 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).

  2. Actions home users can take to protect their computer systems
  3. eMontgomery recommends the following practices to home users:

    1. Consult your system support personnel if you work from home
    2. Use virus protection software
    3. Use a firewall
    4. Donít open unknown e-mail attachments
    5. Donít run programs of unknown origin
    6. Disable hidden filename extensions
    7. Keep all applications (including your operating system) patched
    8. Turn off your computer or disconnect from the network when not in use
    9. Disable Java, JavaScript, and ActiveX if possible
    10. Disable scripting features in e-mail programs
    11. Make regular backups of critical data
    12. Make a boot disk in case your computer is damaged or compromised

    Further discussion on each of these points is given below.

    Recommendations

    1. Consult your system support personnel if you work from home
    2. 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.

    3. Use virus protection software
    4. MontgomeryCountyMd.gov 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.

    5. Use a firewall
    6. 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 itís not sufficient to install a firewall and then ignore all other security measures.

    7. Don't open unknown e-mail attachments
    8. 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:

      1. be sure your virus definitions are up-to-date
      2. save the file to your hard disk
      3. scan the file using your antivirus software
      4. open the file

      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.

    9. Don't run programs of unknown origin
    10. 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.

    11. Disable hidden filename extensions
    12. 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.

    13. Keep all applications, including your operating system, patched
    14. 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.

    15. Turn off your computer or disconnect from the network when not in use
    16. 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.

    17. Disable Java, JavaScript, and ActiveX if possible
    18. 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.

    19. Disable scripting features in e-mail programs
    20. 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.

    21. Make regular backups of critical data
    22. 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.

    23. Make a boot disk in case your computer is damaged or compromised
    24. 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.



If you are interested in learning more, please look at CERT® Coordination Center's Home Network Security document. It gives home users an overview of security risks and countermeasures associated with Internet connectivity. Traditional dial-up users (i.e. modem) and "always-on" or broadband access (i.e. cable modems and DSL) services are discussed. The document is written with the novice user in mind and is not intended to be a comprehensive discussion of all Internet-based technologies.
CERT® Coordination Center's Home Network Security
 
Last edited: 4/24/2008