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CA-2001-26 Nimda Worm
release date: September 18, 2001
revision history is at the end of this file.
running Microsoft Windows 95, 98, ME, NT, and 2000
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:
client to client via email
client to client via open network shares
web server to client via browsing of compromised web sites
client to web server via active scanning for and exploitation
of the "Microsoft IIS 4.0 / 5.0 directory traversal" vulnerability
client to web server via scanning for the back doors left
behind by the "Code Red II" (IN-2001-09), and "sadmind/IIS"
analysis indicates that the worm contains no destructive
payload beyond modification of web content to facilitate its own
also receiving reports of denial of service as a result of
network scanning and email propagation.
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.
propagates through email arriving as a MIME
"multipart/alternative" message consisting of two sections.
section is defined as MIME type "text/html", but it contains
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.
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,
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
message delivering the Nimda worm appears to also have the
text in the subject line of the mail message appears to be
variable, but those seen to date have been over 80 characters
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
client machines attempt to send copies of the Nimda worm via
email to all addresses found in the Windows address book.
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
of the time, an address with the same first two octets will be
of the time, an address with the same first octet will be
of the time, a random address will be chosen
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
containing web content (e.g., HTML or ASP files) is found, the
window.open("readme.eml", null, "resizable=no,top=6000,left=6000")
of web content allows further propagation of the
worm to new clients through a browser or browsing of a network file
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
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.
activity of the Nimda worm produces the following log
entries for any web server listing on port 80/tcp:
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
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.
scanning rate of the Nimda worm may also cause bandwidth
denial-of-service conditions on networks with infected machines.
for System Administrators of IIS machines
if your system has been compromised, look for the
artifact (indicates a compromise by Code Red II or
sadmind/IIS worms making the system vulnerable to the Nimda worm)
artifact or unexpected .eml files in the directories
with web content (indicates compromise by the Nimda worm)
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.
instructions for recovering your system can be found in the
CERT/CC tech tip:
for Recovering from a UNIX or NT System Compromise
the appropriate patch from your vendor
patch which addresses all of the IIS-related
vulnerabilities exploited by the Nimda worm is available from
for End User Systems
the appropriate patch from your vendor
are running a vulnerable version of Internet Explorer (IE), the
CERT/CC recommends applying patch for the "Automatic Execution
Embedded MIME Types" vulnerability available from Microsoft at
Maintain an Anti-Virus Product
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.
packages support automatic updates of virus
definitions. We recommend using these automatic updates when
open e-mail attachments
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
A. Vendor Information
wish to visit the CERT/CC's computer virus resources page
Roman Danyliw, Chad Dougherty, Allen Householder, Robin
is available from:
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
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.
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Our public PGP key is available from
prefer to use DES, please call the CERT hotline for more
and other security information are available from
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added to our mailing list for advisories and bulletins, send
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2001 Carnegie Mellon University.
18, 2001: Initial Release
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