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  • CERT Advisory CA-2004-02 Email-borne Viruses

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Original release date: January 27, 2004
Last revised: --
Source: CERT/CC

A complete revision history can be found at the end of this file.

Systems Affected

  • Any system running Microsoft Windows (all versions from Windows 95 and up) and used for reading email or accessing peer-to-peer file sharing services.


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.

I. Description

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.

II. Impact

A virus infection can have significant consquences on your computer system. These consequences include, but are not limited to:

  • Information disclosure - Mass-mailing viruses typically harvest email addresses from the addressbooks or files found on an infected system. Some viruses will also attempt to send files from an infected host to other potential victims or even back to the virus author. These files may contain sensitive information.

  • Add/Modify/Delete files - Once a system is compromised, a virus could potentially add, modify or delete arbitrary files on the system. These files may contain personal information or be required for the proper operation of the computer system.

  • Affect system stability - Viruses can consume significant amounts of computer resources causing a system to run slowly or be rendered unusable.

  • Install a backdoor - Many viruses will install a backdoor on an infected system. This backdoor may be used by a remote attacker to gain access to the system, or view/add/modify/delete files on the system. These backdoors may also be leveraged to download and control additional tools for use in distributed denial-of-service (DDoS) attacks against other sites.

  • Attack other systems - Systems infected by viruses are frequently used to attack other systems. These attacks frequently involve attempts to exploit vulnerabilities on the remote systems or denial-of-service attacks that utilize a high volume of network traffic.

  • Send unsolicited bulk email (spam) to other users - There have been numerous reports of spammers leveraging compromised systems to send unsolicited bulk email. Frequently these compromised systems are poorly protected end user computers (e.g., home and small business systems).

III. Solution

In addition to following the steps outlined in this section, the CERT/CC encourages home users to review the "Home Network Security" documents.

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

Steps for Recovering from a UNIX or NT System Compromise

Authors: Jeff Carpenter, Chad Dougherty, Jeff Havrilla, Allen Householder, Brian King, Marty Lindner, Art Manion, Damon Morda, Rob Murawski

Copyright 2004 Carnegie Mellon University.

Revision History

January 27, 2004:  Initial release

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