Cracked: Sober Worm Update Scheme

A security firm said it has cracked the Sober worm, and uncovered the URLs the long-running worm will use in January to update itself to a new variant.

Helsinki-based F-Secure is the second vendor this week to claim that it had dug up new information on the two-year-old Sober clan; on Wednesday, Reston, Va.-based iDefense said its research had confirmed the next release date of Sober as Jan. 5, 2006.

F-Secure, however, has identified the Web sites that Sober will use to pull updates to already-compromised PCs, which will, in turn, then spew spam containing the new version.

"Most of [the Sober] variants contain a routine that activates the virus at a later date," said Mikko Hyppnen, F-Secure's chief research officer, in an entry to the company's research blog. "After this, [Sober] will try to periodically download and run a file from several 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."

But Sober's maker isn't stupid; he knows that a single source for his updates could be easily blocked. Instead, said Hyppnen, the hacker has created an algorithm that uses the current date to generate a number of pseudo-random URLs.

The vast majority of these URLs don't currently exist, but that's not a problem for the hacker.

"The virus author can pre-calculate 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!," said Hyppnen. "It's run globally in hundreds of thousands of machines."

In May F-Secure cracked the algorithm used to generate those URLs. But it didn’t go public until Thursday because "the Sober author must know this by now," said Hyppnen. F-Secure used this information to notify Internet service providers and the German police of at least some of the upcoming attacks and the URLs used to update earlier versions. (In mid-November, the Bavarian state police warned of an attack the next day; F-Secure claims that it was responsible for alerting the agency.)

Jan. 5, Sober will start using one or more of the URLs on the list that F-Secure cranked out after cracking the hacker's algorithm.

"You might want to block [these] at your corporate firewall," advised Hyppnen.

According to F-Secure, the list of possible update URLs changes every 14 days, with the next-scheduled switch to take place on Jan. 6. It also posted a list of those addresses on its Web site.