This document gives 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).

 

Recommendations
 

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

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.

2. Make regular backups of critical data

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.

 

3. Use virus protection software
 

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.

4. Use a firewall
 

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.

5. Don't open unknown email attachments

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.

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

 

• be sure your virus definitions are up-to-date.
• save the file to your hard disk.
• scan the file using your antivirus software.
• 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.

6. Don't run programs of unknown origin

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.

7. Disable hidden filename extensions

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

8. Keep all applications, including your operating system, patched

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.

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

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.

10. Disable Java, JavaScript, and ActiveX if possible

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.

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.

11. Disable scripting features in email programs

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 "Disable Java, JavaScript, and ActiveX if possible", above), we recommend that users also disable these features in their email programs.

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

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.