Discovered: October 29, 2001
Updated: February 13, 2007 11:37:37 AM
Also Known As: I-Worm.Nimda.e [Kaspersky], PE_NIMDA.E [Trend], Win32.Nimda.E [Computer Associ, W32/Nimda-D [Sophos], W32/Nimda.E@mm [Frisk], Win32/Nimda.E worm [ESET], W32/Nimda.gen@MM [McAfee]
Type: Worm, Virus
Systems Affected: Microsoft IIS, Windows
CVE References: CVE-2001-0154 CVE-2000-0884

W32.Nimda.E@mm is a new version of W32.Nimda.A@mm that contains bug fixes and other modifications, which are designed to prevent antivirus programs from detecting this variant.

For additional information, read the Microsoft TechNet article, "Information on the "Nimda" Worm ."

For information on .enc detections, read the article, "What is an .enc detection? "

Antivirus Protection Dates

  • Initial Rapid Release version October 29, 2001
  • Latest Rapid Release version February 07, 2018 revision 032
  • Initial Daily Certified version October 29, 2001
  • Latest Daily Certified version February 08, 2018 revision 007
  • Initial Weekly Certified release date October 29, 2001

Click here for a more detailed description of Rapid Release and Daily Certified virus definitions.

Technical Description

This worm is similar in functionality to W32.Nimda.A@mm, though some differences include the modification of filenames that this worm uses.

  • The received attachment has been changed to: Sample.exe
  • The dropped .dll file is now: Httpodbc.dll
  • The worm now copies itself to the \%Windows% folder as Csrss.exe instead of Mmc.exe

    NOTE: %Windows% is a variable. The worm locates the \Windows folder (by default this is C:\Windows or C:\Winnt) and copies itself to that location.

In addition, the author of this worm has introduced new bugs in the code. Specifically, the worm may re-infect files, producing multiple infected files. Such files can become corrupted and may need to be deleted instead of repaired.

NOTE: Norton AntiVirus already detects infected HTML files as W32.Nimda.A@mm (html).

Virus Definitions
Virus Definitions may be downloaded using LiveUpdate or from the Symantec Security Response Web site.


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.



  • This is a complicated and damaging threat. In some cases, scanning with Norton AntiVirus will not completely remove this virus. If you have any detection of any variant of W32.Nimda.E@mm, we strongly recommend that you obtain the W32.Nimda.E@mm Removal Tool.
  • If you are on a network or have a full-time connection to the Internet, disconnect the computer from the network and the Internet. Disable or password protect file sharing before reconnecting the computers to the network or to the internet, or attempting removal. To ensure that the worm does not re-infect the computer after it has been removed, Symantec suggests sharing with Read Only access or using password protection. For instructions, see your Windows documentation, or the document "How to configure shared Windows folders for maximum network protection."

Manual removal
You should follow this process if you cannot obtain the Removal Tool.
  1. Run LiveUpdate to make sure that you have the most recent virus definitions.
  2. Start Norton AntiVirus and make sure that it is configured to scan all the files. For instructions, read the document, "How to configure Norton AntiVirus to scan all files."
  3. Run a full system scan.
  4. Delete all the files that are detected as W32.Nimda.E@mm (dr) and W32.Nimda.enc and, if necessary, replace them with clean backup copies. For all the files detected as W32.Nimda.E@mm and W32.Nimda.A@mm (html), click Repair.

Writeup By: Eric Chien