CERT® Advisory CA-2004-02 Email-borne VirusesOriginal release date: January 27, 2004
Last revised: --
A complete revision history can be found at the end of this file.
In recent weeks there have been several mass-mailing viruses released on the Internet. It is important for users to understand the risks posed by these pieces of malicious code and the steps necessary to protect their systems from virus infection.
Over the past week, we have seen two more mass-mailing viruses, W32/Bagle and W32/Novarg, impact a significant number of home users and sites. The technology used in these viruses is not significantly different from prior mass-mailing viruses such as W32/Sobig and W32/Mimail. Unsolicited email messages containing attachments are sent to unsuspecting recipients. They may contain a return address, a provocative envelope, or something else that encourages its receiver to open it. This technique is called social engineering. Because we are trusting and curious, social engineering is often effective. The widespread impact of these latest viruses, which rely on human intervention to spread, demonstrates the effectiveness of social engineering.
It continues to be important to ensure that anti-virus software is used and updated regularly, that attachments are examined on mail servers, and that firewalls filter unneeded ports and protocols. It also remains necessary that users be educated about the dangers of opening attachments, especially executable attachments.
A virus infection can have significant consquences on your computer system. These consequences include, but are not limited to:
Run and maintain an anti-virus product
While an up-to-date antivirus software package cannot protect against all malicious code, for most users it remains the best first line of defense against malicious code attacks. Users may wish to read IN-2003-01 for more information on anti-virus software and security issues.
Most antivirus software vendors release frequently updated information, tools, or virus databases to help detect and recover from malicious code. Therefore, it is important that users keep their antivirus software up to date. The CERT/CC maintains a partial list of antivirus vendors.
Many antivirus packages support automatic updates of virus definitions. The CERT/CC recommends using these automatic updates when available.
Do not run programs of unknown origin
Do not download, install, or run a program unless you know it to be authored by a person or company that you trust.
Email users should be wary of unexpected attachments. Be sure you know the source of an attachment before opening it. Also remember that it is not enough that the mail originated from an email address you recognize. The Melissa virus spread precisely because it originated from a familiar email address.
Users should also be wary of URLs in email messages. URLs can link to malicious content that in some cases may be executed without user intervention. A common social engineering technique known as "phishing" uses misleading URLs to entice users to visit malicious web sites. These sites spoof legitimate web sites to solicit sensitive information such as passwords or account numbers.
In addition, users of Internet Relay Chat (IRC), Instant Messaging (IM), and file-sharing services should be particularly careful of following links or running software sent to them by other users. These are commonly used methods among intruders attempting to build networks of distributed denial-of-service (DDoS) agents.
Use a personal firewall
A personal firewall will not necessarily protect your system from an email-borne virus, but a properly configured personal firewall may prevent the virus from downloading additional components or launching attacks against other systems. Unfortunately, once on a system, a virus may be able to disable a software firewall, thus eliminating its protection.
Email gateway filtering
Depending on your business requirements, it is advisable to configure filtering of specific file extensions of email attachments at the email gateway. This filtering should be configured carefully, as this may affect legitimate attachments as well. It is recommended that attachments are quarantined for later examination and/or possible retrieval.
Recovering from a system compromise
If you believe a system under your administrative control has been compromised, please follow the steps outlined in
This document is available from: http://www.cert.org/advisories/CA-2004-02.html
CERT/CC Contact Information
Phone: +1 412-268-7090 (24-hour hotline)
Fax: +1 412-268-6989
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.
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Copyright 2004 Carnegie Mellon University.