Threat Description



Aliases:Net-Worm:​W32/Nimda, W32/Nimda@mm, W32/Nimda.A@mm


A type of worm that replicates by sending complete, independent copies of itself over a network.


Automatic action

Once detected, the F-Secure security product will automatically disinfect the suspect file by either deleting it or renaming it.


You may wish to refer to the Support Community for further assistance. You also may also refer to General Removal Instructions for a general guide on alternative disinfection actions.

For more general information on disinfection, please see Removal Instructions .

Network Disinfection

For general instructions on disinfecting a local network infection, please see Eliminating A Local Network Outbreak.

Manual Disinfection

Caution: Manual disinfection is a risky process; it is recommended only for advanced users.

If you're running Windows ME, you need to turn off the Autorestore functionality before starting any disinfection. Do this by clicking My Computer on desktop, then Performance->File System ->Troubleshooting->Disable System Restore. Turn it back on when done.

To disinfect the worm and restore security of affected workstations, please follow these instructions:

  • Disable all network sharing or temporarily kill the network. This is a must as the worm uses the network to spread itself.
  • Scan _all_ files (not just files with selected extensions) on all local hard drives and clean all infected EXE files using F-Secure Anti-Virus and the latest updates. It is recommended that you use one of the latest FSAV versions to remove infection.
  • Delete or rename (if not possible to delete instantly) all non-disinfectable or locked files including worm droppers (typically 57kB in size):
    • MMC.EXE (in Windows directory)
    • LOAD.EXE (in Windows' system directory)
    • ADMIN.DLL (in root folder of all local hard drives)
    • RICHED20.DLL (in all folders on all local hard drives)
    • All *.EML and *.NWS files (typically 79kB in size) that are detected as infected with Nimda should be deleted. Note that you might have clean EML files as well, for example if you've saved e-mails to file from Outlook Express, so only delete files that FSAV detects as infected.
  • If an infected file is locked by Windows, complete disinfection, exit to pure DOS or boot your system with a clean system diskette and rename/delete the file manually. In case of NT/2000 based system the locked file(s) should be renamed with a non-executable extension to ensure that it doesn't start when Windows is booted next time.
  • Restart a system. Do not connect it to the network yet. It is advised to scan all files on all local drives with FSAV again to ensure that there are no more infected files in a system.
  • Locate SYSTEM.INI file in your Windows directory and open it with Wordpad or Notepad. Replace the string "shell=explorer.exe load.exe -donotloadold" with "shell=explorer.exe" string.
  • Delete all files with .TMP extensions from your local temporary directories - typically \Temp\ or \Windows\Temp\ or \documents and settings\username\local settings\temp.
  • Copy a clean RICHED20.DLL file to \Windows\System\ or \WinNT\System32\ folders. This DLL file is used by many applications and they won't run if this DLL is missing. You can locate a clean RICHED20.DLL file from a clean Windows machine, or extract it from Office 2000 CD with this command:
    • EXTRACT /A r:\ riched20.dll /L c:\windows\system
  • Remove all shares from all local hard drives and renew these shares with correct access rights if needed. This needs to be done because the worm affects shares security. Check especially the \\localhost\c$ share rights.
  • Remove 'Guest' account and renew it with correct access rights and group placement ('Guest' account should not be in 'Administrators' group).
  • Check all *.HTML, *.ASP, and *.HTM as well as files that have 'DEFAULT', 'INDEX', 'MAIN' and 'README' words in their filenames for the small JavaScript code referring to README.EML file and remove it or restore the affected files from a backup. This JavaScript code is located in the very end of affected files.
  • When cleaning a webserver from Nimda, the CodeRed II backdoor infections should be removed as well. Please refer to 'CodeRed' description and cleaning instructions:
  • Correct Windows Explorer's settings concerning displaying of hidden files and certain extensions if necessary as the worm makes Explorer to hide certain files and extensions.
  • Restore network connections only after all workstations are disinfected or the worm will re-infected already clean computers!


A web site can get infected in two ways:

  • Infected htmls are copied the secure site. This can happen even if you're using a patched version of IIS or something else entirely (such as Apache or Netscape). If there are infected computers in your organization, their local html files get infected. Users might then later copy or upload such infected pages to your www server. Alternatively, if your www files are accessible via file sharing the worm might infect them directly from a workstation. To clean your site, locate all html pages which refer to "README.EML" and remove the extra Javascript code from the end of the pages.
  • Direct web worm infection. If your web site is running an unsafe version of IIS, the worm can infect your site by accessing it through http. After this it will restart spreading from your server. In this case, it is not enough to just clean the virus - your web server is unsafe and has been so for a while. It's likely there have been previous illegimate accesses to your site as well and it should be considered compromised. We recommend rebuilding the web server and applying latest patches before restoring clean copies of the html pages.

Remember, F-Secure Management Server 4.x uses IIS as a web server platform. Keep them patched. F-Secure Policy Manager Server 5.0 and higher do NOT use IIS.

Technical Details

The first variant in the Net-Worm:W32/Nimda family was found on September 18th, 2001, and quickly spread around the world.

Nimda is a complex virus with a mass mailing worm component which spreads itself in e-mail attachments named README.EXE.It affects Windows 95, Windows 98, Windows Me, Windows NT 4 and Windows 2000 users.

Nimda also uses the Unicode exploit to infect IIS web servers. This hole can be closed with a Microsoft patch, downloadable from: The MIME exploit used by the worm can be fixed with this patch:

Nimda is the first worm to modify existing web sites to start offering infected files for download. Also it is the first worm to use normal end user machines to scan for vulnerable web sites. This technique enables Nimda to easily reach intranet web sites located behind firewalls - something worms such as Code Red couldn't directly do.

The worm has a copyright text string that is never displayed:

  • Concept Virus(CV) V.5, Copyright(C)2001 R.P.China

It should be said that the worm has bugs that cause crashes or inability to spread itself in certain conditions.

The details below refer to the Net-Worm:W32/Nimda.A variant. For details of other variants in the Nimda family, please see:


This worm is especially relevant to F-Secure as around 15:00 GMT on 11th of October, 2001, hundreds of e-mails infected with Nimda.A was sent to various addresses around the world. These e-mails looked like they were sent by "" (do note that F-Secure used to be called; company name and domain was changed in early 2000). Mr. Mikko Hypponen is our Manager of Anti-Virus Research. He naturally had nothing to do with this incident.

These e-mails were apparently sent from an infected machine located somewhere in Canada. For more information about the worm's spread, see


The actual lifecycle of Nimda can be split to four parts:

  • Infecting files
  • Mass mailing
  • Web worm
  • LAN propagation

1) File infection

Nimda locates EXE files from the local machine and infects them by putting the file inside its body as a resource, thus 'assimilating' that file.These files then spread the infection when people exchange programs such as games.

2) Mass mailer

Nimda locates e-mail addresses via MAPI from your e-mail client as well as searching local HTML files for additional addresses. Then it sends one e-mail to each address. These mails contain an attachment called README.EXE, which might be executed automatically on some systems.

3) Web worm

Nimda starts to scan the internet, trying to locate www servers. Once a web server is found, the worm tries to infect it by using several known security holes. If this succeeds, the worm will modify random web pages on the site. End result of this modification is that web surfers browsing the site will get automatically infected by the worm.

4) LAN propagation

The worm will search for file shares in the local network, either from file servers or from end user machines. Once found, it will drop a hidden file called RICHED20.DLL to any directory which has DOC and EML files. When other users try to open DOC or EML files from these directories, Word, Wordpad or Outlook will execute RICHED20.DLL causing an infection of the PC. The worm will also infect remote files if it was started on a server.


First, it should be noted that the worm behaves differently when started from files with different file names and with different command lines.

Starting on a server:

If the name of worm's file is ADMIN.DLL, the worm creates a mutex with 'fsdhqherwqi2001' name, copies itself as MMC.EXE into \Windows\ directory and starts this file with '-qusery9bnow' command line. Usually the worm is started as ADMIN.DLL on infected webservers. In this case the worm starts to scan and infect files on all available drives including removable and network ones. The EXE files (except WINZIP32.EXE) on these drives will get infected with the worm. The infection technique the worm uses is new - the worm puts an infected file inside its body as a resource. When the infected file is run, the worm extracts the embedded original EXE file, runs it and tries to delete it afterwards. If instant deletion is not possible, the worm creates WININIT.INI file that will delete the extracted file on next Windows startup.

The worm accesses the following key:

  • SOFTWARE\Microsoft\Windows\CurrentVersion\App Paths]

It reads subkeys from there and infects all files listed in the subkeys. The worm also reads user's personal folders from the following key:

  • [Software\Microsoft\Windows\CurrentVersion\Explorer\Shell Folders]

And infects files in these folders as well. The worm doesn't infect WinZip32.exe files.

Then the worm starts to search local hard drives for *.HTML, .ASP, and .HTM files and if such files are found, the worm creates README.EML file (which is the multi-partite message with MIME-encoded worm) in the same directory and adds a small JavaScript code to the end of found files. That JavaScript code would open README.EML file when the infected HTML file is loaded by a web browser. As a result the MIME-encoded worm will get activated because of a security hole and a system will get infected.

The worm's file runs from a minimized window when downloaded from an infected webserver. This technique affects users who are browsing the web with Internet Explorer 5.0 or 5.01.The worm will also put *.EML and *.NWS files in almost all folders of computers it accesses. The RICHED20.DLL file with hidden and system attribute will be put in all folders where DOC or EML files are located. The worm will also try to replace Windows' original RICHED20.DLL file with its own copy.

Starting on a workstation:

If the worm is started from README.EXE file (or a file that has more than 5 symbols in its name and EXE extension), it copies itself to temporary folder with a random name that has 'MEP*.TMP' name and runs itself there with '-dontrunold' command line option.

When started, the worm loads itself as a DLL library, looks for a specific resource there and checks its size. If the resource size is less than 100, the worm unloads itself, otherwise it extracts its resource to a file and launches it. Checking the resource size is done to be able to detect if a worm runs from infected EXE files.

Then the worm gets current time and generates a random number. After performing a few arithmetic operations with this number the worm checks the result. If a result is bigger than worm's counter, the worm starts to search and delete README*.EXE files from temporary folder.

After that the worm prepares its MIME-encoded copy by extrating a pre-defined multi-partite MIME message from its body and appending its MIME-encoded copy to it. The file with a random name is created in a temporary folder.

The worm then looks for EXPLORER process, opens it and assigns its process as remote thread of Explorer. On some platforms the worm fails to run as Explorer's thread. The worm gets API creates a mutex with 'fsdhqherwqi2001' name, startups Winsock services, gets an infected computer (host) info and sleeps for some time. When resumed, the worm checks what platform it is running. If it is running on NT-based system, it compacts its memory blocks to occupy less space in memory and copies itself as LOAD.EXE to Windows system directory.

Then it modifies SYSTEM.INI file by adding the following string after SHELL= variable in [Boot] section:

  • explorer.exe load.exe -dontrunold

This will start the worm's copy every time Windows starts. The worm also copies itself as RICHED20.DLL file to system folder and sets hidden and system attributes to this file as well as to LOAD.EXE file. Then the worm enumerates shared network resources and starts to recursively scan files on remote systems.

When searching for files on remote systems the worm looks for .DOC and .EML files and then copies its binary image with RICHED20.DLL name to the folders where DOC and EML files are located. The copied DLL file has system and hidden attributes. This is done to increase the chances of worm activation on remote systems as Windows' original RICHED20.DLL component is used to open OLE files. But instead the worm's RICHED20.DLL file from current directory will be launched.

Also when the worm browsing the remote computers' directories it creates .EML and .NWS (rarely) files that have the names of document or webpage files that the worm could find on a remote system. These .EML and .NWS files are worm's multi-partite messages with a worm MIME-encoded in them. When scanning the worm can also delete the .EML and .NWS files it previously created.

The worm doesn't try to infect local or remote EXE files when started from a workstation.


The worm adjusts the properties of Windows Explorer, it accesses the following key:

  • [Software\Microsoft\Windows\CurrentVersion\Explorer\Advanced]

and adjusts 'Hidden', 'ShowSuperHidden' and 'HideFileExt' keys. This affects Windows' (especially ME and 2000) ability to show hidden files - worm's files will not be seen in Explorer any more.

After that the worm adds a 'guest' account to infected system account list, activates this account, adds it to 'Administrator' and 'Guests' groups and shares C:\ drive with full access priviledges. The worm also deletes all subkeys from the following key:

  • [SYSTEM\CurrentControlSet\Services\lanmanserver\Shares\Security]

to disable sharing security.


E-Mail spreading:

The worm searches trough all the '.htm' and '.html' file in the Temporary Internet Files folder for e-mail addresses. It reads trough user's inbox and collects the sender addresses. When the address list is ready it uses it's own SMTP engine to send the infected messages.

IIS spreading:

The worm uses backdoors on IIS servers such as the one CodeRed II installs. It scans random IP addresses for these backdoors. When a host is found to have one the worm instructs the machine to download the worm code (Admin.dll) from the host used for scanning. After this it executes the worm on the target machine this way infecting it.


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