Should ISPs be the ones who keep hacked PCs off the internet? Microsoft's chief security executive used to think so, but now he's had a change of heart.

Scott Charney, Microsoft's corporate vice-president for Trustworthy Computing, said that he no longer thought it was a good idea for service providers to be the ones on the hook for keeping infected PCs from the rest of the internet.

"Last year at RSA I said, 'You know we need to think about ISPs being the CIO for the public sector, and we need to think about them scanning consumer machines and making sure they're clean and maybe quarantining them from the internet,'" he said.

"But in the course of the last year as I thought a lot more about this I realised that there are many flaws with that model."

Consumers may see security scans as invasive and a violation of privacy, and with more and more people using the internet as their telephone, quarantining a PC could amount to cutting off someone's 999 service, he said.

"You see the scenario, right: a heart attack, I run for my computer, it says you need to install four patches and reboot before you can access the internet. That's not the experience we strive for."

Then there's the biggest problem of all. ISPs would have to bear the cost. "It puts a lot of burden on the ISPs, because they're the ones who are gating access to the internet," Charney said.

ISPs have experimented with different ways to cut down on infected computers. Comcast, for example, has a service called Constant Guard that warns customers when they have a security problem.

But cutting off the service to infected customers is an expensive proposition. "It just takes one phone call from a consumer for you to lose your profit margin for the year [on that user]," said Craig Labovitz, chief scientist with network monitoring firm Arbor Networks.

Labovitz said that technology companies have been coming up with new ways to rid the world of infected machines for about two decades now, without success. "Even if we do force end users to keep their patches updated there are still a huge number of zero days," he said, referring to unpatched software flaws that can be used to take over a fully patched PC.

"It's an arms race that keeps going. There certainly isn't any single bullet."

Still, Charney thinks that there are ways to improve things.

He thinks that internet companies could take a page from organisations such as the World Health Organization and find new ways to keep infected PCs away from the rest of the network - to "enforce goodness", he said.

Maybe the solution is for consumers to share trusted certificates about the health of their personal computer, including data on whether it's running antivirus or is completely patched, Charney suggested. He called this "collective defence". An example? A bank could ask customers to sign up for a program that would scan their PC for signs of infection during online sessions. If there was a problem, the bank could then limit what the customer could do - topping transactions off at $2,000, for example.

That might end up to be a more workable model for the internet, Charney said. "The user remains in control. The user can say I don't want to pass a health certificate," he said "There may be consequences for that decision, but you can do it."

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