Internet service providers should take security matters into their own hands by blocking access to communications ports on their customers' computers which are commonly exploited by Internet worms and other malicious programs, according to a SANS Institute report.

Leaving the ports open offers little to customers, while needlessly exposing them to infection and making it more likely that ISPs will be overwhelmed by future virus outbreaks, the report said.

Entitled "Internet Service Providers: The Little Man's Firewall," the report was written by Johannes Ullrich, chief technology officer of the SANS Internet Storm Center, which uses a worldwide network of sensors to track virus outbreaks and other events on the Internet.

The report identifies four communications ports that are commonly left open on Microsoft Windows machines so that users on an office or home network can share files between two Windows systems. However, those ports were never intended to be used to access files over an insecure public network like the Internet, Ullrich said. At least one of the ports, 135, was used by the recent W32.Blaster worm to locate and infect vulnerable Windows machines on the Internet.

But the four ports were known as handy access points for loosely secured Windows machines long before Blaster appeared in early August, Ullrich said. "These machines are taken out on a regular basis and used in large-scale DDoS (distributed denial of service) attacks," he said. By blocking the ports centrally, ISPs would close an open doorway for attackers without requiring any action by their customers, the report said.

Many ISPs already block some or all of the ports named, while others offer customers free personal firewall software to install on their home computers, according to Ullrich. However, home Internet users often lack the technical knowledge necessary to install and configure a firewall or even install a software patch, he said.

Closing the ports would not protect users from all Internet threats. However, it is a simple step that would remove a common and commonly exploited security hole, Ullrich said. "The idea is to get rid of the bulk of problem, then (ISPs) can deal with the remainder of problems on a case-by-case basis," he said.

The IDG News Service is a Network World affiliate.