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Virus Encyclopedia

Welcome to the Virus Encyclopedia of Panda Security.


Threat LevelHigh threatDamageSevereDistributionNot widespread
Common name:Nimda
Technical name:W32/Nimda
Threat level:Medium
Alias:W32/Nimda.A,, W32/Nimda@mm, W32/Nimda.A@mm, W32/Nimda.htm, Nimda.htm

It infects files with an EXE extension. It shares and grants access to all disk drives, allowing the free space on the disk drive to be used up.

Affected platforms:

Windows XP/2000/NT/ME/98/95

First detected on:
Detection updated on:May 14, 2009
Proactive protection:
Yes, using TruPrevent Technologies
Country of origin:CHINA

Brief Description 


Nimda is a dangerous worm, which is highly contagious and difficult to recognize. It spreads extremely quickly using its own methods, through the following means:

  • Web pages. Nimda uses the Web Directory Traversal Exploit in IIS Web servers to modify pages written in the language HTML. It modifies these pages by inserting a script in them. When users access modified pages, they will be infected.
  • E-mail messages with an attachment called README.EXE. Nimda exploits a vulnerability in Internet Explorer (versions 5.01 and 5.5), which allows it to carry out its infection when the message carrying the worm is viewed through the Preview Pane.
  • Networks of Windows computers (versions 2000 Pro, NT, 98, 95), in which Nimda can share the disk drives and spread to them.

The danger of Nimda lies in its capacity to spread and infect, which allows it to access all local disk drives (Windows 2000 Pro and Windows NT) and all the disk drives (Windows 98 and Windows 95) shared in a computer network.

Its aim is to spread to as many computers as possible and infect all the files with an EXE extension that it finds. It activates when the user opens the programs Word and WordPad.

Visible Symptoms 


It is difficult to know if you have received the message carrying Nimda, regardless of the means of transmission used.

The only clear indication that Nimda has reached your computer is a file called README.EXE, which is attached to the e-mail messages that are infected with this virus.