Discovered: September 18, 2001
Updated: February 13, 2007 11:37:18 AM
Also Known As: W32/Nimda@MM [McAfee], PE_NIMDA.A [Trend], I-Worm.Nimda [Kaspersky], W32/Nimda-A [Sophos], Win32.Nimda.A [Computer Associ
Type: Worm, Virus
Systems Affected: Microsoft IIS, Windows
CVE References: CVE-2000-0884 CVE-2001-0154
NOTE: As of January 15, 2003, due to a decreased rate of submissions, Symantec Security Response has downgraded this threat from a Category 4 to a Category 2.
Symantec has not seen any significant increase in activity due to the re-activation of the emailing routine after its initial 10-day sleep period.
W32.Nimda.A@mm is a mass-mailing worm that uses multiple methods to spread itself. The name of the virus came from the reversed spelling of "admin."
- Sends itself by email
- Searches for open network shares
- Attempts to copy itself to unpatched or already vulnerable Microsoft IIS web servers
- Is a virus infecting both local files and files on remote network shares.
The worm uses the Unicode Web Traversal exploit. A patch for computers running Windows NT 4.0 Service Packs 5 and 6a or Windows 2000 Gold or Service Pack 1, as well as information regarding this exploit can be found at http://www.microsoft.com/technet/security/bulletin/ms00-078.asp .
When the worm arrives by email, it uses a MIME exploit allowing the threat to be executed by reading or previewing the file. Information and a patch for this exploit can be found at http://www.microsoft.com/technet/security/bulletin/MS01-020.asp
If you visit a compromised Web server, you will be prompted to download a .eml (Outlook Express) email file, which contains the worm as an attachment. You can disable "File Download" in your Internet Explorer Internet Security Zones to prevent this compromise.
Also, the worm will create open network shares on the infected computer, allowing access to the system. During this process, the worm creates the guest account with Administrator privileges.
Virus Definitions may be downloaded using LiveUpdate or from the Symantec Security Response Web site.
Symantec offers a host of solutions to defend and protect against W32.Nimda.A@mm. Click here to review Symantec's recommendations on how to address W32.Nimda.A@mm and similar "blended threats."
Information for Macintosh users
Although this worm does not infect Macintosh computers, the worm can be passed through Macintosh email to Windows computers. Also, if you share a network with Windows computers, files could be placed on your hard drive. For additional information, read the document, "Are Macintoshes affected by the Nimda virus? "
Information for Novell users
Novell servers are not directly vulnerable, but a Novell client running under Windows can access the Novell server and execute the file from there (using a login script or other means), which can further spread the virus.
NOTE: Microsoft has released a cumulative roll up for IIS 4.0 on NT 4.0 SP5 and later, as well as all security patches released to date for IIS 5.0. This information can be found at http://www.microsoft.com/technet/security/bulletin/MS01-044.asp .
Microsoft has provided information regarding this threat at: http://www.microsoft.com/technet/treeview/default.asp?url=/technet/security/topics/nimda.asp .
For information on .enc detections, read the article, What is an .enc detection?
For additional information, read the Microsoft TechNet article, Information on the "Nimda" Worm .
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Symantec Enterprise Security Manager (ESM)
Symantec Enterprise Security Manager is a scalable security policy compliance and host-based vulnerability assessment tool. Using this tool you can detect systems that are running IIS server, detect systems that have the web Directory Traversal Vulnerability and can also detect modified files, new files and deleted files through its snapshot technology. It can also detect other modifications in the registry, useful in forensic analysis. If you have not already deployed ESM within your enterprise it is of limited use in recovering from a widespread compromise like W32.Nimda.A@mm. However, it has tremendous strength in mitigating the risk of the next W32.Nimda.A@mm type worm since it enforces best practices, e.g., identifying inadequate patch levels, unneeded services, and weak passwords. Click here to review the Enterprise Security Manager Security Response Policy for Nimda on Windows NT and Windows 2000.
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Antivirus Protection Dates
- Initial Rapid Release version September 18, 2001
- Latest Rapid Release version May 07, 2019 revision 006
- Initial Daily Certified version September 18, 2001
- Latest Daily Certified version May 07, 2019 revision 008
- Initial Weekly Certified release date September 18, 2001
Click here for a more detailed description of Rapid Release and Daily Certified virus definitions.
Infection by a Web Server
W32.Nimda.A@mm attempts to infect unpatched Microsoft IIS Web servers. On Microsoft IIS 4.0 and 5.0, it is possible to construct a URL that would cause IIS to navigate to any desired folder on the logical drive that contains the Web folder structure, and then access files in it. A patch and information regarding this exploit can be found at http://www.microsoft.com/technet/security/bulletin/ms00-078.asp .
Successful exploitation of the Directory Traversal Vulnerability gives the attacker the ability to install and run code, as well as add, change, or delete files or Web pages on the compromised server. The limitations of the original vulnerability include:
- The server configuration. The vulnerability only allows files to be accessed if they reside on the same logical drive as the Web folders.
For example, if a Web administrator had configured the server so that the operating system files were installed on the C drive and the Web folders were installed on the D drive, the attacker would be unable to use the vulnerability to access the operating system files.
- The attacker must be logged onto the server interactively.
- The gained privileges would be only those of a locally logged-on user. The vulnerability would only allow the malicious user to take actions in the context of the IUSR_machinename account.
However, by using the W32.Nimda.A@mm worm as a delivery mechanism, the attacker can remotely compromise a vulnerable IIS server, and once compromised, create a local account on the targeted server with administrator privileges, regardless of the drive on which the IIS server is installed. The worm uses directory traversal techniques to access cmd.exe on unpatched IIS servers. The worm also attempts to use the IIS servers that CodeRed II had previously compromised to propagate and to access root.exe from the inetpub/scripts directory.
NOTE: If Norton AntiVirus RealTime protection detects files such as "TFTP34%4.txt" as being infected with W32.Nimda.A@mm in your inetpub/scripts folder, you may have been previously exposed to CodeRed II. We recommend that you download and execute the CodeRed removal tool to make sure that your system has been cleaned of the CodeRed II threat.
The worm searches for Web servers using randomly generated IP addresses. Using the Unicode Web Traversal exploit, the worm copies itself to the Web server as admin.dll via TFTP. Infected machines create a listening TFTP server (port 69/UDP) to transfer the copy of the worm.
Then, this file is executed on the Web server and copied to multiple locations. In addition to this exploit, the worm attempts to exploit already compromised Web servers using the files root.exe or cmd.exe, which are located in remotely executable Web directories.
Readme.eml is an Outlook Express email file, with the worm as an attachment. The email messages use the MIME exploit. Thus, a computer may be infected by browsing the infected Web page.
When executed, the worm determines where it is being executed from. The worm overwrites Mmc.exe in the \Windows folder, or creates a copy of itself in the Windows Temporary folder.
Then, the worm infects executables, creates itself as .eml and .nws files, and copies itself as Riched20.dll in folders that contain .doc files on the local drive. The worm searches for files in the paths listed in the registry keys:
The worm hooks the system by modifying the System.ini file as follows:
Shell = explorer.exe load.exe -dontrunold
The worm also replaces the file, Riched20.dll, which is a legitimate Windows .dll file that programs, such as Microsoft Word, use. By replacing this file, the worm is executed each time programs (such as Microsoft Word) are executed.
The worm also registers itself as a service process or adds itself as a remote thread to the Explorer process. This allows the worm to continue to execute even when a user is not actively logged on.
The worm copies itself as the file, %Windows\System%\load.exe
NOTE: %Windows\System% is a variable. The worm locates the \Windows\System folder (by default this is C:\Windows\System) and copies itself to that location.
Next, the worm creates open network shares for all the drives on the computer by modifying the registry key:
CurrentVersion\Network\LanMan\[C$ -> Z$]
A reboot of the computer is required for these settings to take effect.
The worm searches for all the open shares on the network by iterating through Network Neighborhood and by using randomly generated IP addresses. All the files on any open network shares are examined for possible infection. The worm infects all the .exe files except Winzip32.exe.
Next, the .eml and .nws files are copied to the open network shares, and the worm copies itself over as Riched20.dll to any folder that contains .doc files.
The worm changes the Explorer settings to not show Hidden files and known file extensions.
The worm adds the user, Guest, under the groups Guests and Administrators. This gives the guest account Administrative privileges. In addition, the worm actively shares C$ = C:\ No reboot is required.
Nimda contains a mass-mailing routine that is executed every 10 days. The worm begins this routine by first searching for email addresses. The worm searches for email addresses in the .htm and .html files on the local system. The worm also uses MAPI to iterate through all the messages contained in any MAPI-compliant email clients. Any MAPI-supporting email clients may be affected, including Microsoft Outlook and Outlook Express. The worm uses these email address for the To: and the From: addresses. Thus, mail sent from the infected computer will appear to have been sent by the people whose addresses that Nimda found, not by the person whose computer is infected.
The worm uses its own SMTP server to send email messages using the configured DNS entry to obtain a mail server record (MX record).
When the worm is received by email, the worm uses an old known MIME exploit to auto-execute itself. The worm will be unable to execute using Microsoft Outlook or Outlook Express if the system has been patched against this exploit.
Information regarding this exploit can be found at http://www.microsoft.com/technet/security/bulletin/MS01-020.asp .
The worm also attempts to infect .EXE files. First, the worm checks to see whether the file is already infected. If the file is not infected, the worm makes a copy of itself in the Temporary directory. The victim file is embedded inside the copy. This new file is then copied over the victim file, replacing the originally clean file with an infected version. Infected executables will be approximately 57344 bytes larger. When an infected file is executed, the worm will extract the original clean file to a temporary file and execute it along with itself. Thus, one may not notice that their executable has become infected.
During execution, the worm may attempt to delete copies of itself. If the file is in use or locked, the worm will create the file, Wininit.ini, with an entry to delete itself upon reboot.
When infecting files, the worm may create two temporary files in the Windows Temporary folder as:
Both files will be Hidden and have the system attribute set.
Ports that the worm uses are listed below. NOTE : These are all standard ports.
- TCP 25 (SMTP): Used to send email to targets with addresses taken from the compromised client.
- TCP 69 (TFTP): Opens port 69/udp for the TFTP transfer of admin.dll for the IIS infection. As part of this protocol, it makes outgoing connections to transfer the files.
- TCP 80 (HTTP): Uses this port to target vulnerable IIS servers.
- TCP 137-139, 445 (NETBIOS): Used in the transmission of the worm.
The worm contains bugs and can be resource-intensive. Thus, all the actions may not occur and you may notice system instability.
Symantec Security Response encourages all users and administrators to adhere to the following basic security "best practices":
- Use a firewall to block all incoming connections from the Internet to services that should not be publicly available. By default, you should deny all incoming connections and only allow services you explicitly want to offer to the outside world.
- Enforce a password policy. Complex passwords make it difficult to crack password files on compromised computers. This helps to prevent or limit damage when a computer is compromised.
- Ensure that programs and users of the computer use the lowest level of privileges necessary to complete a task. When prompted for a root or UAC password, ensure that the program asking for administration-level access is a legitimate application.
- Disable AutoPlay to prevent the automatic launching of executable files on network and removable drives, and disconnect the drives when not required. If write access is not required, enable read-only mode if the option is available.
- Turn off file sharing if not needed. If file sharing is required, use ACLs and password protection to limit access. Disable anonymous access to shared folders. Grant access only to user accounts with strong passwords to folders that must be shared.
- Turn off and remove unnecessary services. By default, many operating systems install auxiliary services that are not critical. These services are avenues of attack. If they are removed, threats have less avenues of attack.
- If a threat exploits one or more network services, disable, or block access to, those services until a patch is applied.
- Always keep your patch levels up-to-date, especially on computers that host public services and are accessible through the firewall, such as HTTP, FTP, mail, and DNS services.
- Configure your email server to block or remove email that contains file attachments that are commonly used to spread threats, such as .vbs, .bat, .exe, .pif and .scr files.
- Isolate compromised computers quickly to prevent threats from spreading further. Perform a forensic analysis and restore the computers using trusted media.
- Train employees not to open attachments unless they are expecting them. Also, do not execute software that is downloaded from the Internet unless it has been scanned for viruses. Simply visiting a compromised Web site can cause infection if certain browser vulnerabilities are not patched.
- If Bluetooth is not required for mobile devices, it should be turned off. If you require its use, ensure that the device's visibility is set to "Hidden" so that it cannot be scanned by other Bluetooth devices. If device pairing must be used, ensure that all devices are set to "Unauthorized", requiring authorization for each connection request. Do not accept applications that are unsigned or sent from unknown sources.
- For further information on the terms used in this document, please refer to the Security Response glossary.
Symantec Security Response has posted a tool to remove the infections that W32.Nimda.A@mm caused.
NOTE: Once W32.Nimda.A@mma has attacked a computer, an unauthorized user may remotely access your system. For this reason, it is impossible to guarantee the integrity of a system that has had such an infection. The remote user could have made changes to your system, including but not limited to the following:
- Stealing or changing passwords or password files
- Installing remote-connectivity host software, also known as backdoors
- Installing keystroke logging software
- Configuring of firewall rules
- Stealing of credit card numbers, banking information, personal data, and so on
- Deleting or modifying files
- Sending of inappropriate or even incriminating material from a customer's email account
- Modifying access rights on user accounts or files
- Deleting information from log files to hide such activities
If you need to be certain that your organization is secure, re-install the operating system and restore the files from a backup that was made before the infection took place, and then change all the passwords that may have been on the infected computers or that were accessible from it. This is the only way to ensure that your systems are safe. For more information regarding security in your organization, contact your system administrator.
Manual Removal Instructions
If you cannot obtain the removal tool, or if it does not work in your situation, follow these steps:
- Run LiveUpdate to make sure that you have the most recent virus definitions.
- Do one of the following:
- If you are running Windows NT/2000/XP, skip to step 3.
- If you are running Windows 95/98/Me, edit the System.ini file as follows:
- Click Start, and then click Run.
- Type the following, and then click OK:
The MS-DOS Editor opens.
NOTE: If Windows is installed in a different location, make the appropriate substitution.
- Locate the line that begins with shell=
- Position the cursor immediately to the right of the equal sign.
- Press Shift+End to select all the text to the right of the equal sign, and then press Delete.
- Type the following text:
The line should now look like:
NOTE: Some computers may have an entry other than Explorer.exe after shell=. If this is the case and you are running an alternative Windows shell, change this line to shell=explorer.exe for now. You can change it back to your preferred shell after you have finished this procedure.
- Click File, click Exit, and then click Yes when you are prompted to save the changes.
- Restart the computer.
NOTE: When your computer restarts, it is likely that infected files will be found. We recommend that you attempt to repair the infected file. Quarantine any file that is not repairable.
- Start Norton AntiVirus and make sure that it is configured to scan all the files. For more information, read the document, "How to configure Norton AntiVirus to scan all files."
- Scan your system with Norton AntiVirus. For more information, read the document, "How to run a full system scan with Norton AntiVirus."
- For each file detected as infected by W32.Nimda.A@mm or W32.Nimda.A@mm (html), choose Repair. Quarantine any file that is not repairable.
- For each file detected as infected by W32.Nimda.A@mm (dr), W32.Nimda.enc, or W32.Nimda.A@mm (dll), choose Delete.
- Restore Admin.dll and Riched20.dll from a backup, or from the Microsoft Windows or Office .cab files if necessary.
- Remove unnecessary shares.
- Delete the guest account from the Administrators group, if applicable.
System Restore option in Windows Me/XP
Windows Me and Windows XP users should temporarily turn off System Restore. Windows Me/XP uses this feature, which is enabled by default, to restore files on your computer in case they become damaged. When a computer is infected with a virus, worm, or Trojan, it is possible that the virus, worm, or Trojan could be backed up by System Restore. By default, Windows prevents System Restore from being modified by outside programs. As a result, there is the possibility that you could accidentally restore an infected file, or that online scanners could detect the threat in that location.
For instructions on how to turn off System Restore, read your Windows documentation or one of the following articles:
- How to disable or enable Windows Me System Restore.
- How to disable or enable Windows XP System Restore.
For additional information, and an alternative to disabling Windows Me System Restore, see the Microsoft Knowledge Base article Anti-Virus Tools Cannot Clean Infected Files in the _Restore Folder , Article ID: Q263455.
How to extract the Riched20.dll
If you see errors when you start programs such as Microsoft Word, or the programs will not start, you need to extract the Riched20.dll file. (As an alternative, you can re-install the operating system and the affected programs.)
See the instructions for your operating system.
NOTE: These instructions are provided for your convenience, and will work on most computers. For additional information on extracting files, including other Windows files that may have been damaged, read one of the following:
- If you are using Microsoft Outlook 2002 or Microsoft Office 2002, there is an easier way to do this. These programs have the ability to replace the Riched20.dll file if you first rename it. For instructions on how to do this, read the Microsoft Knowledge Base article, "OL2002: Outlook Stops Responding with Riched20.dll Error Messages," Article ID: Q291651.
- The Microsoft Knowledge Base article, "How to Extract Original Compressed Windows Files," Article ID: Q129605
- "How to extract files in Windows 98 and Windows Me"
You need to use the Extract command at a DOS prompt. Follow these steps to do this, using the instructions for your operating system.
- You will need a Windows 98/Me startup disk. (If you are using Windows 95, you will still need one that was created on a Windows 98/Me computer). For instructions on how to create one, see the document, "How to create a Windows Startup disk."
- Have the Windows installation CD available.
- When typing the command, substitute the appropriate drive letter for your CD-ROM drive for the letter x. For example, if you are using Windows 98, and the CD-ROM drive is the drive D, then you would type
extract /a d:\win98\win98_28.cab riched20.dll /L c:\windows\system
- If Windows is installed in a folder other than C:\Windows, then substitute the appropriate path or folder name in the last part of the command that refers to the \Windows folder.
- For detailed instructions on using the Extract command, see the Microsoft document, "How to Extract Original Compressed Windows Files," Article ID: Q129605.
- As a somewhat easier alternative to the following procedure, if you are using Windows 98, then you can use the System File Checker to restore the file. For information on how to do this, see your Windows documentation.
- Shut down the computer and turn off the power. Once the computer is off, insert the Windows 98/Me Startup disk in the floppy disk drive and turn on the computer again. At the menu, select Start with CD-ROM support.
- Type the command that applies to your operating system:
- If you are using Windows 98, then type the following and press Enter:
extract /a d:\win98\win98_28.cab riched20.dll /L c:\windows\system
- If you are using Windows 95, then type the following and press Enter:
extract /a win95_10.cab riched20.dll /L c:\windows\system
- If you are using Windows 98, then type the following and press Enter:
NOTE: If you see an error message of any kind, then repeat step 2, making sure that you typed the correct command for your operating system and that you typed it exactly as shown. Otherwise, type exit, and then press Enter.
Windows NT 4.0
- Make sure that Windows is configured to show all the files.
- Search for and then delete all the Riched20.dll files.
- Re-apply the most recent service pack. The service pack will replace the file with a new copy.
- If, after replacing the Riched20.dll file, programs such as Microsoft Word or Office no longer run, or you see error messages when they start, you may have to re-install Microsoft Office.
If you are using Windows 2000, a built-in program will find and replace missing or corrupt system files. To replace the corrupted Riched20.dll, follow these steps:
- Make sure System File Checker is enabled:
- Click Start, and then click Run.
- Type cmd, and then click OK.
- Type the following, and then press Enter:
- Type exit, and then press Enter.
- Make sure that Windows is set to show all the files:
- Start Windows Explorer.
- Click the Tools menu, and then click Folder options.
- Click the View tab.
- Uncheck "Hide file extensions for known file types."
- Uncheck "Hide protected operating system files" and under the "Hidden files" folder, click "Show hidden files and folders."
- Click Apply, and then click OK.
- Search for Riched20.dll:
- Click Start, point to Find or Search, and then click Files or Folders.
- Make sure that "Look in" is set to (C) and that Include subfolders is checked.
- In the "Named" or "Search for..." box, type or copy and paste the following filenames:
- Click Find Now or Search Now.
- Delete the displayed files.
- Restart the computer.
- System File Checker will replace any missing Riched20.dll files. If, after replacing the Riched20.dll file, programs such as Microsoft Word or Office no longer run, or you see error messages when they start, you may have to re-install Microsoft Office.
Writeup By: Eric Chien