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Copyright: Dr. Nikolai Bezroukov 1994-2013. Unpublished notes. Version 0.80.October, 2013
Contents : Foreword : Ch01 : Ch02 : Ch03 : Ch04 : Ch05 : Ch06 : Ch07 : Ch08 : Ch09 : Ch10 : Ch11 : Ch12 : Ch13
Chapter 6: Mail Worms
Original release date: September 18, 2001
Revised: September 25, 2001
A complete revision history is at the end of this file.
The CERT/CC has received reports of new malicious code known as the "W32/Nimda worm" or the "Concept Virus (CV) v.5." This new worm appears to spread by multiple mechanisms:
The worm modifies web documents (e.g., .htm, .html, and .asp files) and certain executable files found on the systems it infects, and creates numerous copies of itself under various file names.
We have also received reports of denial of service as a result of network scanning and email propagation.
The Nimda worm has the potential to affect both user workstations (clients) running Windows 95, 98, ME, NT, or 2000 and servers running Windows NT and 2000.
This worm propagates through email arriving as a MIME "multipart/alternative" message consisting of two sections. The first section is defined as MIME type "text/html", but it contains no text, so the email appears to have no content. The second section is defined as MIME type "audio/x-wav", but it contains a base64-encoded attachment named "readme.exe", which is a binary executable.
Due to a vulnerability described in CA-2001-06 (Automatic Execution of Embedded MIME Types), any mail software running on an x86 platform that uses Microsoft Internet Explorer 5.5 SP1 or earlier (except IE 5.01 SP2) to render the HTML mail automatically runs the enclosed attachment and, as result, infects the machine with the worm. Thus, in vulnerable configurations, the worm payload will automatically be triggered by simply opening (or previewing) this mail message. As an executable binary, the payload can also be triggered by simply running the attachment.
The email message delivering the Nimda worm appears to also have the following characteristics:
The worm also contains code that will attempt to resend the infected email messages every 10 days.
The email addresses targeted for receiving the worm are harvested from two sources
These files are passed through a simple pattern matcher which collects strings that look like email addresses. These addresses then receive a copy of the worm as a MIME-encoded email attachment. Nimda stores the time the last batch of emails were sent in the Windows registry, and every 10 days will repeat the process of harvesting addresses and sending the worm via email.
Likewise, the client machines begin scanning for vulnerable IIS servers. Nimda looks for backdoors left by previous IIS worms: Code Red II [IN-2001-09] and sadmind/IIS worm [CA-2001-11]. It also attempts to exploit various IIS Directory Traversal vulnerabilities (VU#111677 and CA-2001-12). The selection of potential target IP addresses follows these rough probabilities:
The infected client machine attempts to transfer a copy of the Nimda code via tftp (69/UDP) to any IIS server that it scans and finds to be vulnerable.
This modification of web content allows further propagation of the worm to new clients through a web browser or through the browsing of a network file system.
In order to further expose the machine, the worm
Furthermore, the Nimda worm infects existing binaries on the system by creating Trojan horse copies of legitimate applications. These Trojan horse versions of the applications will first execute the Nimda code (further infecting the system and potentially propagating the worm), and then complete their intended function.
As part of the infection process, the Nimda worm modifies all web content files it finds (including, but not limited to, files with .htm, .html, and .asp extensions). As a result, any user browsing web content on the system, whether via the file system or via a web server, may download a copy of the worm. Some browsers may automatically execute the downloaded copy, thereby infecting the browsing system.
The Nimda worm creates numerous MIME-encoded copies of itself (using file names with .eml and .nws extensions) in all writable directories (including those found on a network share) to which the user has access. If a user on another system subsequently selects the copy of the worm file on the shared network drive in Windows Explorer with the preview option enabled, the worm may be able to compromise that system.
Additionally, by creating Trojan horse versions of legitimate applications already installed on the system, users may unknowingly trigger the worm when attempting to make use of these programs.
The scanning activity of the Nimda worm produces the following log entries for any web server listing on port 80/tcp:
GET /scripts/root.exe?/c+dir GET /MSADC/root.exe?/c+dir GET /c/winnt/system32/cmd.exe?/c+dir GET /d/winnt/system32/cmd.exe?/c+dir GET /scripts/..%5c../winnt/system32/cmd.exe?/c+dir GET /_vti_bin/..%5c../..%5c../..%5c../winnt/system32/cmd.exe?/c+dir GET /_mem_bin/..%5c../..%5c../..%5c../winnt/system32/cmd.exe?/c+dir GET /msadc/..%5c../..%5c../..%5c/..\xc1\x1c../..\xc1\x1c../..\xc1\x1c../winnt/system32/cmd.exe?/c+dir GET /scripts/..\xc1\x1c../winnt/system32/cmd.exe?/c+dir GET /scripts/..\xc0/../winnt/system32/cmd.exe?/c+dir GET /scripts/..\xc0\xaf../winnt/system32/cmd.exe?/c+dir GET /scripts/..\xc1\x9c../winnt/system32/cmd.exe?/c+dir GET /scripts/..%35c../winnt/system32/cmd.exe?/c+dir GET /scripts/..%35c../winnt/system32/cmd.exe?/c+dir GET /scripts/..%5c../winnt/system32/cmd.exe?/c+dir GET /scripts/..%2f../winnt/system32/cmd.exe?/c+dir
Note: The first four entries in these sample logs denote attempts to connect to the backdoor left by Code Red II, while the remaining log entries are examples of exploit attempts for the Directory Traversal vulnerability.
Intruders can execute arbitrary commands within the LocalSystem security context on machines running the unpatched versions of IIS. In the case where a client is compromised, the worm will be run with the same privileges as the user who triggered it. Hosts that have been compromised are also at high risk for being party to attacks on other Internet sites.
The high scanning rate of the Nimda worm may also cause bandwidth denial-of-service conditions on networks with infected machines.
To determine if your system has been compromised, look for the following:
The only safe way to recover from the system compromise is to format the system drive(s) and reinstall the system software from trusted media (such as vendor-supplied CD-ROM). Additionally, after the software is reinstalled, all vendor-supplied security patches must be applied. The recommended time to do this is while the system is not connected to any network. However, if sufficient care is taken to disable all server network services, then the patches can be downloaded from the Internet.
Detailed instructions for recovering your system can be found in the CERT/CC tech tip:
A cumulative patch which addresses all of the IIS-related vulnerabilities exploited by the Nimda worm is available from Microsoft at
Ingress filtering manages the flow of traffic as it enters a network under your administrative control. Servers are typically the only machines that need to accept inbound connections from the public Internet. In the network usage policy of many sites, there are few reasons for external hosts to initiate inbound connections to machines that provide no public services. Thus, ingress filtering should be performed at the border to prohibit externally initiated inbound connections to non-authortized services. With Nimda, ingress filtering of port 80/tcp could prevent instances of the worm outside of your network from scanning or infecting vulnerable IIS servers in the local network that are not explicitly authorized to provide public web services. Filtering of port 69/udp will also prevent the downloading of the worm to IIS via tftp.
Cisco has published a tech tip specifically addressing filtering guidelines to mitigate the impact of the Nimda worm at
Egress filtering manages the flow of traffic as it leaves a network under your administrative control. There is typically limited need for machines providing public services to initiate outbound connections to the Internet. In the case of Nimda, employing egress filtering on port 69/udp at your network border will prevent certain aspects of the worms propogation both to and from your network.
If you are running a vulnerable version of Internet Explorer (IE), the CERT/CC recommends upgrading to at least version
5.0 since older versions are no longer officially maintained by Microsoft. Users of IE 5.0 and above are encourage to apply
patch for the "Automatic Execution of Embedded MIME Types" vulnerability available from Microsoft at
Note: IE 5.5 SP1 users should apply the patches discussed in MS01-027
It is important for users to update their anti-virus software. Most anti-virus software vendors have released updated information, tools, or virus databases to help detect and partially recover from this malicious code. A list of vendor-specific anti-virus information can be found in Appendix A.
Many anti-virus packages support automatic updates of virus definitions. We recommend using these automatic updates when available.
The Nimda worm may arrive as an email attachment named "readme.exe". Users should not open this attachment.
You may wish to visit the CERT/CC's computer virus resources page located at
Feedback on this document may be directed to the authors, Roman Danyliw, Chad Dougherty, Allen Householder, Robin Ruefle
This document is available from: http://www.cert.org/advisories/CA-2001-26.html
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.
CERT publications and other security information are available from our web site
To subscribe to the CERT mailing list for advisories and bulletins, send email to
email@example.com. Please include in the body of your message
* "CERT" and "CERT Coordination Center" are registered in the U.S. Patent and Trademark Office.
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September 18, 2001: Initial Release September 19, 2001: Updated link to MS advisory MS01-027 September 19, 2001: Updated antivirus vendor information, updated e-mail propagation description, added reference to second related IIS vul September 20, 2001: Added link to Computer Associates in vendor information, Updated overview, payload, file system propagation, and recommendations for system administrator sections September 20, 2001: Fix link to CA-2001-12 in payload section September 21, 2001: Added recommendations for network administrators, updated payload section, updated vendor information clarified recommendations for end user systems September 25, 2001: Qualified note concerning MS01-027
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