Schneier on Security

Internet Quarantines

Last month, Scott Charney of Microsoft proposed that infected computers be quarantined from the Internet. Using a public health model for Internet security, the idea is that infected computers spreading worms and viruses are a risk to the greater community and thus need to be isolated. Internet service providers would administer the quarantine, and would also clean up and update users’ computers so they could rejoin the greater Internet.

This isn’t a new idea. Already there are products that test computers trying to join private networks, and only allow them access if their security patches are up-to-date and their antivirus software certifies them as clean. Computers denied access are sometimes shunned to a limited-capability sub-network where all they can do is download and install the updates they need to regain access. This sort of system has been used with great success at universities and end-user-device-friendly corporate networks. They’re happy to let you log in with any device you want—this is the consumerization trend in action—as long as your security is up to snuff.

Charney’s idea is to do that on a larger scale. To implement it we have to deal with two problems. There’s the technical problem—making the quarantine work in the face of malware designed to evade it, and the social problem—ensuring that people don’t have their computers unduly quarantined. Understanding the problems requires us to understand quarantines in general.

Quarantines have been used to contain disease for millennia. In general several things need to be true for them to work. One, the thing being quarantined needs to be easily recognized. It’s easier to quarantine a disease if it has obvious physical characteristics: fever, boils, etc. If there aren’t any obvious physical effects, or if those effects don’t show up while the disease is contagious, a quarantine is much less effective.

Similarly, it’s easier to quarantine an infected computer if that infection is detectable. As Charney points out, his plan is only effective against worms and viruses that our security products recognize, not against those that are new and still undetectable.

Two, the separation has to be effective. The leper colonies on Molokai and Spinalonga both worked because it was hard for the quarantined to leave. Quarantined medieval cities worked less well because it was too easy to leave, or—when the diseases spread via rats or mosquitoes—because the quarantine was targeted at the wrong thing.

Computer quarantines have been generally effective because the users whose computers are being quarantined aren’t sophisticated enough to break out of the quarantine, and find it easier to update their software and rejoin the network legitimately.

Three, only a small section of the population must need to be quarantined. The solution works only if it’s a minority of the population that’s affected, either with physical diseases or computer diseases. If most people are infected, overall infection rates aren’t going to be slowed much by quarantining. Similarly, a quarantine that tries to isolate most of the Internet simply won’t work.

Fourth, the benefits must outweigh the costs. Medical quarantines are expensive to maintain, especially if people are being quarantined against their will. Determining who to quarantine is either expensive (if it’s done correctly) or arbitrary, authoritative and abuse-prone (if it’s done badly). It could even be both. The value to society must be worth it.

It’s the last point that Charney and others emphasize. If Internet worms were only damaging to the infected, we wouldn’t need a societally imposed quarantine like this. But they’re damaging to everyone else on the Internet, spreading and infecting others. At the same time, we can implement systems that quarantine cheaply. The value to society far outweighs the cost.

That makes sense, but once you move quarantines from isolated private networks to the general Internet, the nature of the threat changes. Imagine an intelligent and malicious infectious disease: That’s what malware is. The current crop of malware ignores quarantines; they’re few and far enough between not to affect their effectiveness.

If we tried to implement Internet-wide—or even countrywide—quarantining, worm-writers would start building in ways to break the quarantine. So instead of nontechnical users not bothering to break quarantines because they don’t know how, we’d have technically sophisticated virus-writers trying to break quarantines. Implementing the quarantine at the ISP level would help, and if the ISP monitored computer behavior, not just specific virus signatures, it would be somewhat effective even in the face of evasion tactics. But evasion would be possible, and we’d be stuck in another computer security arms race. This isn’t a reason to dismiss the proposal outright, but it is something we need to think about when weighing its potential effectiveness.

Additionally, there’s the problem of who gets to decide which computers to quarantine. It’s easy on a corporate or university network: the owners of the network get to decide. But the Internet doesn’t have that sort of hierarchical control, and denying people access without due process is fraught with danger. What are the appeal mechanisms? The audit mechanisms? Charney proposes that ISPs administer the quarantines, but there would have to be some central authority that decided what degree of infection would be sufficient to impose the quarantine. Although this is being presented as a wholly technical solution, it’s these social and political ramifications that are the most difficult to determine and the easiest to abuse.

Once we implement a mechanism for quarantining infected computers, we create the possibility of quarantining them in all sorts of other circumstances. Should we quarantine computers that don’t have their patches up to date, even if they’re uninfected? Might there be a legitimate reason for someone to avoid patching his computer? Should the government be able to quarantine someone for something he said in a chat room, or a series of search queries he made? I’m sure we don’t think it should, but what if that chat and those queries revolved around terrorism? Where’s the line?

Microsoft would certainly like to quarantine any computers it feels are not running legal copies of its operating system or applications software.The music and movie industry will want to quarantine anyone it decides is downloading or sharing pirated media files—they’re already pushing similar proposals.

A security measure designed to keep malicious worms from spreading over the Internet can quickly become an enforcement tool for corporate business models. Charney addresses the need to limit this kind of function creep, but I don’t think it will be easy to prevent; it’s an enforcement mechanism just begging to be used.

Once you start thinking about implementation of quarantine, all sorts of other social issues emerge. What do we do about people who need the Internet? Maybe VoIP is their only phone service. Maybe they have an Internet-enabled medical device. Maybe their business requires the Internet to run. The effects of quarantining these people would be considerable, even potentially life-threatening. Again, where’s the line?

What do we do if people feel they are quarantined unjustly? Or if they are using nonstandard software unfamiliar to the ISP? Is there an appeals process? Who administers it? Surely not a for-profit company.

Public health is the right way to look at this problem. This conversation—between the rights of the individual and the rights of society—is a valid one to have, and this solution is a good possibility to consider.

There are some applicable parallels. We require drivers to be licensed and cars to be inspected not because we worry about the danger of unlicensed drivers and uninspected cars to themselves, but because we worry about their danger to other drivers and pedestrians. The small number of parents who don’t vaccinate their kids have already caused minor outbreaks of whooping cough and measles among the greater population. We all suffer when someone on the Internet allows his computer to get infected. How we balance that with individuals’ rights to maintain their own computers as they see fit is a discussion we need to start having.

This essay previously appeared on Forbes.com.

EDITED TO ADD (11/15): From an anonymous reader:

In your article you mention that for quarantines to work, you must be able to detect infected individuals. It must also be detectable quickly, before the individual has the opportunity to infect many others. Quarantining an individual after they’ve infected most of the people they regularly interact with is of little value. You must quarantine individuals when they have infected, on average, less than one other person.

Just as worm-writers would respond to the technical mechanisms to implement a quarantine by investing in ways to get around them, they would also likely invest in outpacing the quarantine. If a worm is designed to spread fast, even the best quarantine mechanisms may be unable to keep up.

Another concern with quarantining mechanisms is the damage that attackers could do if they were able to compromise the mechanism itself. This is of especially great concern if the mechanism were to include code within end-user’s TCBs to scan computers­ essentially a built-in root kit. Without a scanner in the end-user’s TCB, it’s hard to see how you could reliably detect infections.

Posted on November 15, 2010 at 4:55 AM • View Comments