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AUSCERT External Security Bulletin Redistribution
ESB-2001.401 -- CERT Advisory CA-2001-26
19 September 2001
AusCERT Security Bulletin Summary
Operating System: Microsoft Windows 95
Microsoft Windows 98
Microsoft Windows ME
Microsoft Windows NT
Microsoft Windows 2000
Impact: Denial of Service
Execute Arbitrary Code/Commands
Modify Arbitrary Files
Access Required: Remote
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CERT Advisory CA-2001-26 Nimda Worm
Original release date: September 18, 2001
A complete revision history is at the end of this file.
* 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
* from client to web server via scanning for the back doors left
behind by the "Code Red II" (IN-2001-09), and "sadmind/IIS"
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.
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 email message delivering the Nimda worm appears to also have the
* 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
* 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
window.open("readme.eml", null, "resizable=no,top=6000,left=6000")
This modification of web content allows further propagation of the
worm to new clients through a browser or browsing of a network file
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
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.
The scanning activity of the Nimda worm produces the following log
entries for any web server listing on port 80/tcp:
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
Intruders can execute arbitrary commands within the LocalSystem
security context on machines running the unpatched versions of IIS.
Host 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.
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
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.
Nimda worm by browsing web sites hosted by infected servers. This
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
You may wish to visit the CERT/CC's computer virus resources page
Authors: Roman Danyliw, Chad Dougherty, Allen Householder, Robin
This document is available from:
CERT/CC Contact Information
Phone: +1 412-268-7090 (24-hour hotline)
Fax: +1 412-268-6989
CERT Coordination Center
Software Engineering Institute
Carnegie Mellon University
Pittsburgh PA 15213-3890
CERT personnel answer the hotline 08:00-20:00 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
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Copyright 2001 Carnegie Mellon University.
September 18, 2001: Initial Release
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