Washington Anti-virus researchers were fighting a new Internet attacker Tuesday similar to the "Code Red" worm that infected hundreds of thousands of computers several months ago.
The worm, known as "W32.Nimda," had affected "thousands, possibly tens of thousands" of targets by midday Tuesday, according to Vincent Gullotto, head virus fighter at McAfee.com, a software company.
Even when the attack isn't successful, the worm's scanning process can slow down the Internet for many users and can have the effect of knocking Web sites or entire company networks offline.
The FBY is investigating the worm, said spokeswoman Debbie Weierman. The agency has not indicated whether the worm is connected to last week's terrorism attacks.
On security e-mail lists, system administrators nationwide reported unprecedented activity related to the worm, which tries to break into Microsoft's Internet Information Services software. That software was the same targeted by Code Red, and is typically found on computers running Microsoft Windows NT or 2000.
Most home users, including those running Windows 95, 98 or ME, are not affected.
Ken Van Wyk, chief technology officer at ParaProtect, said the worm tries to wriggle in through 16 known vulnerabilities in Microsoft's IIS, including the security hole left in some computers by the "Code Red II" worm, which followed Code Red in August.
Code Red, by comparison, attacked through only one hole, which could be patched by downloading a program from Microsoft's Web site.
"It's causing enormous pain because it is at least an order of magnitude more aggressive than Code Red," said Alan Paller, director of research at the nonprofit Sans Institute. "It's a pretty vigorous attacker."
In addition to direct Internet attacks, the worm can also travel via e-mail. The e-mail message is typically blank, and contains an attachment called "README.EXE." Antivirus experts warn that users shouldn't open unexpected attachments.
Efforts to isolate and track the worm were hampered by the swiftness of the attack. Gullotto said the first report came at about 9 a.m. EDT, from a site in Norway.
"It's taken down entire sites," Gullotto said. "I can't even get to the Internet right now."
On Monday, the FBI's National Infrastructure Protection Center warned that a hacker group called the "Dispatchers" said they would attack "communications and finance infrastructures" on or about Tuesday.
"There is the opportunity for significant collateral damage to any computer network and telecommunications infrastructure that does not have current countermeasures in place," officials said in a warning on the NIPC Web site.
Last week, the FBI warned that there could be an increase in hacking incidents after the twin attacks in New York and Washington. They advised computer users to update their antivirus software, get all possible security updates for their other software, and be extra careful online.