F-Secure Virus Descriptions

NAME: Nimda
ALIAS: W32/Nimda.A@mm
ALIAS: W32/Nimda@mm, I-Worm.Nimda, Readme, Readme.exe
SIZE: 57344

This worm was found on September 18th, 2001. It quickly spread around the world.

Also see http://www.F-Secure.com/news/2001/news_2001091900.shtml

F-Secure Anti-Virus detects the worm with updates released on September 18th, 2001 19:20 EET. Disinfection was added in the updates from September 19th, 2001 17:12 EET.


For removal instructions, see the bottom of the page.

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

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.

Nimda uses the Unicode exploit to infect IIS web servers. This hole can be closed with a Microsoft patch, downloadable from: http://www.microsoft.com/technet/security/bulletin/ms00-078.asp

The MIME exploit used by the worm can be fixed with this patch: http://www.microsoft.com/technet/security/bulletin/MS01-020.asp

The actual lifecycle of Nimda can be split to four parts: 1) Infecting files, 2) Mass mailing, 3) Web worm and 4) 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 also accesses [SOFTWARE\Microsoft\Windows\CurrentVersion\App Paths] key reads subkeys from there and infects all files listed in the subkeys. The worm doesn't infect WinZip32.exe file. Also the worm reads user's personal folders from [Software\Microsoft\Windows\CurrentVersion\Explorer\Shell Folders] key and infects files in these folders as well.

Then the worm starts to search local hard drives for HTML, .ASP, and .HTM files and also for files with 'DEFAULT', 'INDEX', 'MAIN' and 'README' words in their filenames 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. It should be noted that the worm will not always do the above described operation, it depends on a random number the worm generates prior to this action.

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 numerous *.EML and *.NWS files in almost all folders. 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. 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.

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.

Affecting the security:
The worm adjusts the properties of Windows Explorer, it accesses [Software\Microsoft\Windows\CurrentVersion\Explorer\Advanced] key 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 [SYSTEM\CurrentControlSet\Services\lanmanserver\Shares\Security] key to disable sharing security.

Additional information:
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.

F-Secure Anti-Virus with the latest updates can detect and disinfect Nimda infections. But full disinfection of the worm will require some additional manual actions. To disinfect the worm and restore security of affected workstations, please follow these instructions:

1. Disable all network sharing or temporarily kill the network. This is a _must_ as the worm uses the network to spread itself.

2. 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.

3. 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 folder 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.

4. 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.

5. 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.

6. Delete all files with .TMP extensions from your local temporary directories - typically \Temp\ or \Windows\Temp\ or \documents and settings\username\local settings\temp.

7. 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.

8. 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.

9. Remove 'Guest' account and renew it with correct access rights and group placement ('Guest' account should not be in 'Administrators' group).

10. 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.

11. When cleaning a webserver from Nimda, the CodeRed II backdoor infections should be removed as well. Please refer to 'CodeRed' description and cleaning instructions.


12. 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.

13. Restore network connections only after all workstations are disinfected or the worm will re-infected already clean computers!

F-Secure Anti-Virus detects the worm with updates released on September 18th, 2001 19:20 EET. Disinfection was added in the updates from September 19th, 2001 17:12 EET.


[Analysis: K. Tocheva, G. Erdelyi, A. Podrezov, S. Rautiainen and M. Hypponen; F-Secure Corp.; September 18-19th, 2001]

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