The algorithm used in one of the most debilitating e-mail worm attacks in history has been cracked, allowing virus researchers to accurately predict the dates and URLs that will be used in future mutants.
Researchers at Finnish anti-virus vendor F-Secure Corp. first cracked the code used in the Win32.Sober worm family in May this year but withheld details until this week to avoid tipping off the attacker.
Mikko Hypponen, chief incident officer of F-Secure, said the Sober worm uses an algorithm to create "pseudorandom URLs" that change based on the date. "These URLs point to free hosting servers typically operating in Germany or in Austria," Hypponen explained in a blog entry.
A quick check proved that 99 percent of URLs generated by the algorithm do not exist. However, Hypponen said the virus writer can pre-calculate the URL for any date and simply register the domain to upload a malicious program and unleash another round of attack.
"[W]hen he wants to run something on all the infected machines, he just registers the right URL, uploads his program and BANG! Its run globally on hundreds of thousands of machines," Hypponen explained.
The Sober worm, first detected in October 2003, uses a wide range of social engineering tactics to trick e-mail users into activating a malicious attachment. In the past, the worm has used the lure of free tickets for next years World Cup soccer tournament in Germany. It has also regularly spammed out messages with German nationalistic propaganda.
So far, F-Secure has detected more than 20 different variants of Sober and the evidence from the cracked code show that the attacks will never go away.
"Most of these variants contain a routine that activates the virus at later date. After this the virus will try to periodically download and run a file from several Web sites. This is the way most new Sober variants are distributed: the author uploads a new version and all the infected machines will suddenly get infected with the new variant."
One mutant, Sober.Y, was the biggest e-mail outbreak this year, responsible for more than 40 percent of all worm infections. Sober.Y is programmed to activate on Jan. 5, 2006 and Hypponen said that after that date, all the infected machines will regularly try to download and run a file from a Web site, forever.
"The virus even synchronizes the machines via atom clocks so the activation will not happen before Jan. 5, even if the clock of the computer is incorrect," he explained.
F-Secure has posted a list of the download sites that will be used in the next wave of attacks and suggested that system administrators block the URLS at the firewall.
The URLs are:
Hypponen said the list will change every 14 days, beginning Jan. 6, 2005. After that date the list becomes:
The LURHQ Threat Intelligence Group has also published an analysis of the Sober worm code that shows how the virus writer decides on the time of new attacks.