Sober worm's success tied to antivirus weakness

It uses a new technique to avoid virus software scans

By Matthew Broersma

May 12, 2005 12:00 PM ET

TechWorld.com - The longevity of the current Sober worm may be largely due to a new technique it uses to evade virus scans, according to antivirus firm Kaspersky Lab Ltd.
The worm, variously labeled Sober.P, Sober.S, Sober.O and Sober.V by different companies, continues to circulate in large numbers; it made up 84% of all virus traffic as of Monday, according to Lynnfield, Mass.-based virus lab Sophos PLC.
While researchers have attributed its success to the fact that it circulates in both English and German, and to its use of free World Cup tickets as a lure to users, social engineering is only part of the equation, according to Kaspersky Lab.
The newest variant uses a refined mechanism for blocking input/output access to its files by other programs, Kaspersky senior research engineer Roel Schouwenberg said in an alert this week. Previous variants used a similar technique but didn't succeed in blocking programs running in a computer's System account.
Sober.P does what the others didn't do and blocks the System account as well, Schouwenberg said. That means no other programs, including antivirus scanners, could detect Sober.P while it was resident in memory, he said.
"If something can't be scanned, then malicious code can't be detected. This rules out the chance of Sober being detected while running an on-demand scan," he said in the alert, posted on Kaspersky's "Analyst's Diary" site.
While this mechanism doesn't stop an antivirus program from blocking Sober.P from infecting a computer in the first place, once a computer is infected, it makes it more difficult to fix, Schouwenberg explained, saying, "If you aren't aware of infection, how can you take measures against it?"
Some antivirus products lack the features needed to root out such an infection, namely a memory scanner and the ability to kill the worm's processes, Schouwenberg said.

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