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Nimda worm tails off

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The spread of Nimda, the hybrid worm that affects both email users and Web sites, appears to be subsiding. But the effects of the most sophisticated virus yet unleashed are still been felt across the Internet.

Nimda (Admin spelled backwards) exhibits many of the traits of the Code Red worm combined with the email- spreading characteristics that make viruses like the Love Bug and SirCam such a pain.

The worm threatens Microsoft Internet Information Services Web servers and individual users running Microsoft Outlook or Outlook Express on any Windows platform.

A computer can become infected through a variety of means ranging from simply viewing an infected web page using a browser with no security enabled, to opening a malicious email attachment. It can also spread via network shares.

Infected email arrives with an attachment - readme.exe, which is not always visible and contains a randomly generated subject line and no body message. The worm uses its own SMTP engine to email itself out to all the addresses it collects by searching the user's incoming and outgoing email boxes.

The worm targets vulnerable IIS Web servers, using the Universal Web Traversal exploit, which is similar to the flaw used by the Code Red worm. When users visit a compromised Web site, the server will run a script attempting to download an Outlook file, which contains Nimda.

Nimda also has various side effects, such as increasing network traffic while searching for machines to infect, which may cause network bandwidth problems, particularly for those with low-speed connections.

Numerous Register readers have reported Nimda-generated scanning activity that is an order of magnitude greater that created by the Code Red worm.

Paul Rogers, a network security analyst at MIS Corporate Defence, said that networks attached to infected machines can become flooded with traffic and that this was a more serious than any noticeable slowdown to the Internet as a whole.

Rogers said the worm can create backdoors into systems but the exact effect of the worm was unclear because, as an exe file, it was more difficult to analyse the effect of its payload.

This view is backed up by Symantec, which believes that the worm is a file infector that overwrites .Exe files. It creates guest account with administrator privileges and creates open shares on an infected system. Nasty.

Perhaps the only positive news comes from MessageLabs, a managed service provider which scans its customers email for viruses, which reports that spread of Nimda has subsided. at the time of writing today, Messagelabs has blocked 383 copies of Nimda, down on the comparable period yesterday.

Symantec advises number of steps to block the virus, including updating virus definitions, updating IIS Web Server and client software against security vulnerabilities and filtering emails containing a "readme.exe" attachment. These are explained in much more detail here.

Perhaps the best advice is the simplest though: don't open suspicious emails, bin them instead. ®

External links

FBI advisory on Nimda with links to patches and more information

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