Dozens of firms have been infected by the latest mutation of Nimda.

But the outbreak is much less serious than in September, when the contagious worm made its first outing on the Internet. Even so, anti-virus vendors advise users to beware attachments called: "SAMPLE.EXE" (instead of "README.EXE" which appeared in the original Nimda worm).

Jack Clark, product marketing manager for the McAfee division of Network Associates, said the latest modification to Nimda "was not spreading like variants in the past because people have largely put detection in place".

Antivirus experts have discovered five Nimda variants - full definitions can be seen here. This is unlikely to be the final word from virus writers on the subject.

Nimda spreads through several means, ranging from simply viewing an infected web page using a browser with the security features switched off, to opening a malicious email attachment. It can also spread through network shares.

As well as penetrating workstations, Nimda attacks Web servers running under Microsoft Internet Information Server (IIS), using the Web Server Folder Traversal vulnerability. If Nimda gets onto your network, it creates bandwidth-wasting activity, by scanning the network like mad.

AV vendors are updating signature definition files to pick up the latest Nimda variant ®

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