Microsoft issued an unusual weekend security warning Saturday that a worm has been unleashed on the Internet taking advantage of a security hole announced publicly last month. Microsoft once again urged users to install its most recent critical Windows updates.
The worm's impact is expected to peak today as millions of workers bring their laptops back to their offices after using them over the weekend to access the Internet from relatively unsecured home locations.
Microsoft rated the vulnerability exploited by the W32/Sasser A and Sasser B worms as critical, and security experts urged all users of vulnerable system to apply patches immediately.
"Microsoft has verified that the worm exploits the Local Security Authority Subsystem Service (LSASS) issue addressed in Microsoft Security Update MS04-011 on April 13, 2004," the company said in an announcement posted yesterday and updated yesterday morning. In its security update, Microsoft included a tool that checks for system infection by the Sasser worm.
Versions of Windows XP and 2000 are vulnerable, although not XP 64-Bit Edition Version 2003, Microsoft said.
The worm has positioned itself as one of the quickest-spreading and most virulent ones around, suggesting that the number of incidents will soar at the beginning of the week, according to Luis Corrons, director of security vendor Panda Software SL's PandaLabs unit.
The Sasser worm works in a similar way to last year's Blaster worm, but has not yet spread so quickly, according to Graham Cluley, senior technology consultant at security vendor Sophos .
Despite the Microsoft warning, some users may be reluctant to apply the needed patches, however, since there have been reports that the fixes create new system problems, such as causing some systems to slow down or stop responding.
Both Symantec and Network Associates 's McAfee antivirus unit rated Sasser as medium risk, while Computer Associates International rated it low risk. Trend Micro said it issued a yellow alert to its customers.
Later yesterday, Symantec upgraded the Sasser threat to 4 on its scale of 1 (very low) to 5 (very severe) because of the rising number of infections.
Sasser spreads by scanning IP addresses for access via TCP Port 445 looking for vulnerable systems, according to Symantec. When it finds an unpatched Windows XP or Windows 2000 computer, Sasser.A adds the file "avserve2.exe"="%Windir%avserve2.exe" in the registry, tries to block attempts to shut down or reboot the infected computer (by using the AbortSystemShutdown application programming interface) and then begins scanning other systems via an FTP server on TCP Port 5554 seeking to spread itself, Symantec said. Infection can cause "significant degradation in performance," Symantec added. No additional information on possible malicious payload was available.
F-Secure, Norman, Panda and Sophos have also posted information on the virus.
In a statement, Sophos senior technology consultant Graham Cluley said that Sasser is a similar propagation technique to last year's Blaster worm but so far is not spreading as quickly. "Computers which are not properly protected with antivirus updates, firewalls and Microsoft's security patch are asking for trouble," Cluley added.
David Legard, of the IDG News Service, contributed to this report.