Ever-popular file sharing networks are coming under attack as antivirus experts detect a "first of its kind" worm crawling through the Kazaa network.

Security watchers warned this morning of the "Benjamin" worm, which attacked users of the peer-to-peer network over the weekend. Because of the popularity of file sharing networks such as Kazaa, Benjamin is not short of victims.

When Benjamin infects a computer, it creates a directory accessible by other members of the Kazaa network. It then regularly and frequently copies itself into this directory under a multitude of different names, duping users searching for similarly named files.

Once the worm is downloaded onto another machine and executed, it repeats the process.

The worm also opens up a web page that displays advertising banners, presumably so its creator can benefit from the increased page impressions. This technique has been used in rogue software a number of times in the last few weeks.

"The first Kazaa virus uses an impressive list of over 1,000 titles - including recent movies, MP3 songs or extremely valuable software kits - to trick users into downloading it," said Sorin Dudea, virus researcher at antivirus firm BitDefender.

Benjamin is the first worm found to infect the Kazaa network, but other file sharing systems - Gnutella, for example - have come under attack in the past.

Denis Zenkin, of antivirus solutions provider Kaspersky Labs, said: "This event once again demonstrates the necessity to filter all incoming files for viruses, regardless of how well protected this or any other network is. Before use all data should be run through a mandatory check for virus code using the latest virus database update.

"After Gnutella being attacked almost a year ago, it was to be expected that other fast-growing P2P networks would be the next victims of virus writers," Dudea added.