A new worm that exploits a widespread vulnerability in Microsoft's Windows operating system continued its spread on Tuesday, making Monday's outbreak the most serious since the appearance of the SQL Slammer worm in January, according to security experts.
The worm, referred to alternately as W32.Blaster, the DCOM Worm or Lovsan worm, first appeared on the Internet late Monday and spread quickly, infecting machines running the Windows XP and Windows 2000 operating systems.
Blaster takes advantage of a known vulnerability in a Windows component called the DCOM (Distributed Component Object Model) interface, which handles messages sent using the RPC (Remote Procedure Call) protocol.
RPC is a common protocol that software programs use to request services from other programs running on servers in a networked environment.
Vulnerable systems can be compromised without any interaction from a user, according to Johannes Ullrich, chief technology officer at the SANS Internet Storm Center, which monitors threats to the Internet infrastructure.
The Internet Storm Center first detected the new worm around 7:00 p.m. Greenwich Mean Time, and Blaster "took off" within the first hour of appearing, Ullrich said.
On Monday evening, antivirus and computer security firms around the world issued warnings about the new worm and instructed users to patch affected Windows systems and block communications ports used by the worm to spread.
By Tuesday morning, those warnings appear to have had some affect, Ullrich said.
"The worm has pretty much levelled out now. ISPs (Internet service providers) blocked port 135, which the worm used for propagation and we're seeing a limited spread," he said.
A flaw in the worm's code that governs which flavor of exploit to use when compromising a vulnerable machine may also account for the slowdowns, Ullrich said.
That flaw caused machines running Windows XP to crash and reboot, temporarily taking the host offline and tipping off the machine's owner.
Ullrich put the number of machines infected by Blaster at 30,000 worldwide, fewer than the number infected by the Code Red and Nimda worms of 2001, but more than were infected by the recent Slammer worm.
At the University of Florida in Gainesville, network security engineer Jordan Wiens saw a surge in attack traffic from an infected computer on the campus network around 3:00 p.m. Eastern Daylight Time and worked quickly to block the worm's spread by shutting down ports that it uses to copy itself to other vulnerable machines.
The University of Florida uses firewalls and intrusion detection system software to protect the campus network from Internet-borne attacks, but was probably undone by a user who was infected at home, then connected to the campus network using a dial-up modem, Wiens said.
Despite acting quickly to deploy filters to stop Blaster, the university's IT staff was coping with a sizeable number of infected computers by 5:00 p.m. Monday, he said.