Finnish security firm F-Secure has cracked a code used by the Sober worm, potentially allowing the company to block the worm from receiving updates.

Sober has mutated constantly since October 2003, when the first variant was picked up, with more than 20 other variants making the rounds. Last month the latest version, called Sober.Y by F-Secure (or CME-681 using US-CERT's CME naming system), was responsible for the biggest outbreak of the year, and still accounts for about 40 percent of all infections detected by F-Secure.

One of the features that has made Sober so dangerous is its ability to download new variants, instantly infecting large numbers of machines, say security experts. The current variant is expected to re-activate itself on 5 January, according to iDefense.

The downloading pattern stumped anti-virus researchers for a time because the URL used was created by a secret algorithm. "Sober has been using an algorithm to create pseudorandom URLs which will change based on date. These URLs point to free hosting servers typically operating in Germany or in Austria," said Mikko Hypponen, F-Secure's manager of anti-virus research.

"The virus author can precalculate the URL for any date, and when he wants to run something on all the infected machines, he just registers the right URL, uploads his program and bang! It's run globally in hundreds of thousands of machines."

F-Secure said that it has cracked that algorithm, allowing it to figure out the URLs the worm variants will attempt to download from. This should allow the hosting providers involved to block the sites, as well as giving system administrators a list of sites they should block at the corporate firewall, Hypponen said.

The worm uses a list of 15 sites with names that are merely character strings, registered with free website providers. one example used being 5 January, according to F-Secure. Every 14 days the list will change to a different 15 sites, with the first change on 6 January, the Hypponen said.

He said F-Secure first cracked the algorithm in May 2005, but didn't publicise the fact until now in order to keep the virus writer in the dark.