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W32/Nimda is an Internet virus that can send itself by email, as a mass mailer. It can be activated on all Microsoft Windows 9x/Me and NT/2000 Platforms. Nimda sends itself as email attachment. These attachments are named README.EXE, the extension being usually unlisted.
The email looks differently: the subject is a random text and the body is usually empty. In Outlook or Outlook Express, the attachment is not in preview mode.In some cases the attachment can have the extension .COM or .WAV.
If the file README.EXE is opened automatically or by double-click, the worm copies itself in Windows Temp folder. It creates a file with the name FormMExxxx.TMP.EXE, where xxxx are random characters. This file is run and then
deleted by the next system start in Windows 9x/Me. Then the worm copies itself in Windows system as:
The files with the same name will be overwritten. The file LOAD.EXE will be inserted in SISTEM.INI. Thus, the worm will be activated by the next system start:
SHELL=exploerer.exe load.exe -dontrunold
After a few minutes, the worm creates various .EML or .NWS files in Windows subfolders. These too contain the worm. If there are any shared folders with writing rights, the worm copies itself in the subfolders of the network drives, as .EML or .NWS files.
Finally, the worm resets all the Windows properties back to the standard values. After this, there are no "hidden" or "system" files declared. The extensions of the known program files are suppressed.
If there is an Internet connection, Nimda tries to download by FTP a file named ADMIN.DLL. In NT the worm tries to log as a guest and to give this account administrator rights. From this moment on, the drive C:\ is shared with read and write properties. The worm deletes then all the keys in the registry:
If the worm is activated on IIS Web-server, it creates the file README.EML. When this file is opened (by accessing a web-site) it installs in Java-Script the following files:
When one of the above sites is accessed, Java-Script is opened. The browser loads the README.EML file on the local computer. Some browsers are set to open and immediately run the attachment file README.EXE.
Description inserted by Crony Walker on Tuesday, June 15, 2004