File-swapping on the internet hit a sour note on Tuesday with the appearance of a virus that attacks users of the Gnutella file-sharing service. Several antivirus vendors say this is the first virus to affect peer-to-peer communications.

Known as the W32/Gnuman.worm, or by the alias Mandragore, the malicious file poses as an ordinary, requested media file. But this masked file is actually an executable (.exe) file that infects a user's PC once the program is run.

Gnutella is free open-source software that allows people to swap files, including music, over the internet without passing through particular servers, a system known as peer-to-peer. The need for central servers is what has brought Napster to court. With Gnutella and its kin there are no company servers and no company to sue.

Officials at data security firm McAfee discovered the virus on Monday but have yet to identify its origin. McAfee said it is a low-risk threat because the virus does not cause much harm, and only users running Gnutella-compatible software - such as Gnotella, BearShare, LimeWire or ToadNode - will be affected. Confidential information and crucial files should not be affected, vendors said.

"This could be the testing ground for something else to come," said Vincent Gullotto, senior director of McAfee's Avert (Anti-Virus Emergency Response Team) labs. "It highlights the potential vulnerabilities in peer-to-peer computing."

The file is 8,192 bytes in length and should not be opened if offered on the Gnutella network. Most antivirus vendors have already released software updates to take care of the file.