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

Leaving the ports open offers little to customers, while needlessly exposing them to infection and making the ISP more susceptible to future virus outbreaks, the report says.

Advice: Shut Ports

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

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

But the four ports were known as handy access points for loosely secured Windows systems long before Blaster appeared in early August, Ullrich says.

"These machines are taken out on a regular basis and used in large-scale [distributed denial of service] attacks," he says. By blocking the ports centrally, ISPs would close an open doorway for attackers without requiring any action by their customers, the report says.

Many ISPs already block some or all of the ports, while others offer free personal firewall software for customers to install, 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 says.

First Step

Closing the ports would not protect users from all Internet threats. However, it is a simple step that would remove a common security hole, Ullrich says.

"The idea is to get rid of the bulk of the problem, then [ISPs] can deal with the remainder of problems on a case-by-case basis," he says.

Windows ports are not required to browse the Web or perform other common Internet activities, so a change would be transparent to most ISP customers, Ullrich says. Customers who want to share files could still do so safely over a private network, rather than the public Internet.

While feasible for ISPs that serve consumers and for universities, the solution is not right for every ISP, Ullrich acknowledges. ISPs that serve corporate customers or larger, Internet backbone providers could disrupt customers' networks using the blanket approach the report advocates, he says.

"I think it's a really good idea," says Richard Smith, an independent security expert in Boston. Plugging holes centrally would keep many Internet users from unwittingly opening their computers, and their private lives, to the Internet, he says.

"Most users don't want to share their hard drive with the whole Internet and they don't even know they're doing it," he says. He complains that many ISPs adhere to an "old school" belief of keeping everything on the Internet open. He points to America Online as one exception. The company is using its firewall feature as a major marketing draw for consumers, Smith says.

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