The next variant of the Sober worm is set to attack computers already infected by previous versions of the malware at the stroke of midnight GMT as 5 January turns into 6 January, according to European antivirus software vendors. Given that both internet service providers and local police are closely monitoring websites likely to be used in the attack, security experts believe the hacker may choose not to engage in any malicious activity this time around.

"Nothing's posted yet [on the websites]," Carole Theriault, senior security consultant at Sophos in the UK, said today. "It's possible he may stay well clear."

Mikko Hypponen, chief research officer at F-Secure in Finland, agreed with Theriault. "It's more likely he'll lay low than engage in activation," he said in a phone interview on Wednesday. Nevertheless, the companies and their peers around the world are keeping a close eye on the situation in case the hacker does choose to launch an attack.

The last major Sober attack, Sober-Z, occurred in late November. At one point, an estimated one in 14 emails on the internet carried it, according to Sophos.

Previous Sober variants have turned users' computers into 'spam machines', spewing out right-wing German propaganda, according to Theriault. The upcoming attack could be something "that makes a big song and dance on machines or something very subtle", she added. Hypponen warned that it's possible, with all the interest focusing on the likely timing of the attack, that the hacker may opt to delay any malicious activity for a little while until the attention dies down.

The Sober worm variants are written in both German and English; the German propaganda only spreads to email inboxes with a .de address and is "invisible to the rest of the world", Hypponen said. While most hackers produce malware for some kind of monetary benefit, the Sober author, according to Hypponen, appears interested in only two things: working towards his next attack and releasing his propaganda.

Many of the previous Sober variants have spread by appearing to be emails from the FBI, CIA or other law enforcement agencies, or offers of video clips of Paris Hilton and Nicole Richie, stars of US reality TV show 'The Simple Life'. After malicious code in an attachment is executed, the worm spreads by sending itself to other email addresses contained on the infected PC.

The best way for users to protect themselves against any potential attack is to ensure they have antivirus software, according to the experts. "If you don't have antivirus, get some," Theriault said. "If you have some, ensure it's up to date and clean up your computer."

Hypponen stressed that users should double-check that their antivirus software is really running and being regularly updated. He pointed out that many worms, not just Sober, typically switch off both antivirus and firewall protection when they attack computers.

Hypponen doesn't hold out much hope that this time around authorities will catch the hacker, whom he refers to as "a lone gunman", most likely resident in Germany or Austria. During November's Sober-Z attack, authorities had the same kind of information they have this time in terms of the likely websites the hacker would go to, but he escaped detection. "He's been playing a game of cat and mouse [with the authorities] for over two years," Hypponen said. "I really do hope they'll be able to track him down."

Back in December, iDefense broke the encrypted code in a variant of the Sober worm and discovered that 5 January 2006 was the date set for the variant to download unknown pieces of code from various web addresses. The date coincides with the 87th anniversary of the founding of the precursor to the Nazi Party.

Hypponen notes that there were initial conflicting reports about the exact timing of the attack, putting it during 5 January, but he said that F-Secure researchers have double-checked the exact date and, according to the Sober code, activation of any malware is due to occur after 5 January.