New versions of the Netsky e-mail worm are spreading on the Internet and may be the work of a different author than previous editions of that worm, according to anti-virus software companies.Netsky.S appeared on Monday and Netsky.T was detected Tuesday. They are the 19th and 20th editions of an e-mail virus that first appeared in February. Unlike earlier variants, the new Netsky strains open "back doors" on machines they infect, prompting at least one anti-virus expert to declare the worm the work of a different virus author.Network Associates' McAfee Antivirus Emergency Response Team (AVERT) rated Netsky.S a "medium" threat. The company has received around 300 samples from customers and from virus-infected machines, said Craig Schmugar, virus research manager for McAfee AVERT.The company has received only a few copies of the Netsky.T virus, he said. Sophos PLC said it received just one copy of the Netsky.T worm, according to an advisory.Like its predecessors, the new Netsky variants target machines running versions of Microsoft's Windows operating system. The viruses arrive as files enclosed in e-mail messages that have faked (or "spoofed") sender addresses and vague subjects such as "Re: My details," "Request" and "Thank You!" according to anti-virus company Symantec.Earlier versions of the Netsky variant abstained from opening communications ports that could be used as so-called "back doors" that remote attackers could use to access the compromised system. They removed copies of the Bagle e-mail worm from infected machines.Some anti-virus experts believe that Netsky's attack on Bagle installations is behind a war of words between the Netsky author or authors and the creators of the Bagle virus family in recent weeks. The two groups have used new worm variants as vehicles for barbs and retorts to previous insults.In those exchanges, Netsky's author or authors positioned themselves as the "good guys" locked in a battle with online criminals and spammers. One recent variant, Netsky.Q, even contained an impassioned defense of the Netsky worms."We don't have any criminal inspirations. Due to many reports, we do not have any backdoors included for spam relaying," read text hidden in Netsky.Q and transcribed by Sophos and other anti-virus companies.However, the latest Netsky variants abandon the high ground, opening a backdoor on TCP port 6789, which could be used to receive instructions or malicious code from the worm author. A message in the new worm tries to make distinctions between opening a back door and installing a remote access Trojan, but does not contain any overt criticisms of the Bagle author, Schmugar said."If you look at the 'purpose' behind Netsky, it was trying to uninstall other viruses. Now we're seeing behavior in the new variants like remote access components and denial-of-service attacks," he said.New variants of Netsky could be linked to a promise by its author, buried in an earlier variant of the worm, that the worm's source code would be released on the Internet.Anti-virus companies have noted differences in the worm's code with variants released since that promise was made in text hidden in the Netsky.K worm, though antivirus companies haven't located a copy of the source code on the Internet yet.Still, despite new buried messages and slight variations in the worm's use of file attachments and subject lines, even the latest Netsky worm variants are very similar to previous versions of the worm, Schmugar said.E-mail users should make sure they have anti-virus software installed on their computer and consider deploying an Internet firewall if they have not already done so, Schmugar said.