Sasser Worm Spreads Automatically Through Windows Hole

Experts, who say the worm has the potential to spread far and fast, have assigned an elevated threat level to it.

A new worm has been detected by the virus research community that spreads through the LSASS vulnerability in various versions of Windows. Researchers and security companies are alarmed and have assigned an elevated threat level to the worm—named Sasser.A—even though it has not yet spread far.

Unlike the Gaobot variant found several days ago that exploited the same hole in Windows, Sasser can spread and infect automatically through the hole. Gaobot is technically a "bot," meaning that it has to be manually run by the user. The vulnerability was patched weeks ago by Microsoft.

According to Ken Dunham, director of malicious code for network security intelligence firm iDefense Inc., "Sasser.A has the potential to become very widespread in a short period of time." The fact that it has not yet spread far may be due, Dunham said, to aggressive patching by users.

Symantec Corp. has given Sasser an elevated severity level of 2 and issued special definitions to detect it. According to Symantecs limited description of the worm, it spreads by scanning IP addresses for vulnerable systems through FTP port 5554. It creates the value "avserve.exe"=%windows%\avserve.exe in the registry key HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\Run.

Users and administrators should move quickly to apply the patch for this vulnerability, but test it beforehand. Microsoft has admitted that a bug in the patch can leave some systems unbootable.

Dunham is also troubled by what he sees as a new trend toward releasing malware code on Friday nights and Saturday mornings, after many security personnel have gone home. "Its the perfect time to strike, and the bad guys know it. Slammer struck during this same time of week, just over a year ago."

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