Conventional wisdom claims March comes in like a lion and goes out like a lamb. But with new versions of the Bagle e-mail worm and a virulent new form of Netsky virus, March's arrival is looking more wormy than leonine.

As of Monday, five new versions of Bagle appeared over the weekend as well as a new version of Netsky that is spreading rapidly on the Internet and generating a huge volume of virus-infected e-mail messages. The new virus versions use a variety of so-called "social engineering" techniques to fool users. Some new variants also hide in ZIP files to slip past anti-virus filters and into users' e-mail boxes, said Graham Cluley, a senior technology consultant at Sophos.

Netsky.D, a new version of the Netsky worm, is believed to be the biggest threat in the group. As of Monday, Netsky.D was spreading rapidly on the Internet and flooding e-mail servers with infected messages, according to Cluley.

Some of Sophos' customers were receiving thousands of Netsky.D infected messages each hour. That number could increase on Monday as U.S. workers return to their desks after the weekend, he said.

The original Netsky worm first appeared on Feb. 16. Since then, three more variants have been released on the Internet. Like its predecessors, Netsky.D scans an infected computer's hard drive for files containing e-mail addresses and then sends copies of itself to those addresses, antivirus companies said.

Like its predecessors, Netsky.D affects machines running Microsoft's Windows operating system and arrives in e-mail messages with randomly generated subject lines such as "Re: Document," "Re: Your picture" or "Re:approved." The Netsky.D worm disguises its payload as a Program Information File (PIF) attachment that also has a randomly generated name such as "my_details.pif" "document.pif" or "mp3music.pif."

Unlike its predecessors, NetSky.D doesn't spread on peer-to-peer networks, and doesn't use a ZIP file to conceal its contents, according to anti-virus company Network Associates.

The gaggle of new Bagle worms that appeared in recent days use many of the same tricks as the new Netsky worms, and some new techniques, according to anti-virus companies.

Bagle versions C, D, E, F and G appeared between Saturday and Monday and are variants of the first Bagle worm, which appeared on Jan. 19. All target systems running Windows, harvest e-mail addresses from infected machines and open a TCP port to listen for commands from a remote attacker, according to an alert released by computer security company iDefense.

The IDG News Service is a Network World affiliate.