Consider the Metro subway in DC, pictured above. There are absolutely zero physical barriers between the platform and the trains. If evil attacker Evelyn were so inclined, she could easily push a waiting passenger off the platform into the path of an arriving train, maiming or killing the person instantly.
Why does this not happen (regularly)? Evelyn is presumably a rational actor, and she is deterred by vigilante justice and the power of the legal system. If she killed a Metro passenger in the state of Virginia she would probably be executed herself, or at the very least spend the rest of her life in prison. Hopefully they are few people like Evelyn in the world, but would more Metro passengers be murdered if there were no attribution or apprehension of the killers?
How do you think the Metro board would react to such an incident?
- Build barriers to limit the potential for passengers to land in front of moving trains
- Screen passengers as they enter Metro stations
- Mandate trains to crawl within reach of waiting passengers
- Add Metro police to watch for suspicious individuals
- Add cameras to watch all Metro stations
- Lobby Congress to increase penalties
My ranking is intentional. 1 would never happen; it is simply too costly when weighed against the risks. 2 would be impossible to implement in any meaningful fashion and would provoke a public backlash. 3 might happen for a brief period, but it would be abandoned because it would slow the number of trains carrying passengers. 4 might happen for a brief period as well, but the costs of additional personal make it an unlikely permanent solution; it's also ineffective unless the police is right next to a likely incident. 5 and 6 could happen, but they are only helpful for deterrence -- which is not prevention.
Earlier I said Evelyn is a rational actor, so she could presumably be deterred. She could also be mitigated or eliminated. Imagine if Evelyn's action was a ritual associated with gang membership. Authorities could identify and potentially restrict gang members from entering the Metro. (Difficult? Of course. This is why deterrence is a better option.) Authorities could also infiltrate and/or destroy the gang.
Irrational actors cannot be deterred. They may be mitigated and/or eliminated.
Forces of nature cannot be deterred either. Depending on their scope they may be mitigated, but they probably cannot be eliminated. Evelyn's house cannot be built for a reasonable amount of money to withstand a Category V hurricane. Such a force of nature cannot be deterred or eliminated. Given a large enough budget Evelyn's house could be built to survive such a force, so mitigation is an option. Insurance is usually how threats like hurricanes are mitigated, however.
Everyone approaches this problem via the lens of their experience and capabilities. Coders think they can code their way out of this problem. Architects think they can design their way out. I am mainly an operator and, in some ways, an historian. I have seen in my own work that prevention eventually fails, and by learning about the past I have seen the same. In December 2005 I wrote an article called Engineering Disasters for a magazine, and in the coming weeks a second article with more lessons for digital security engineers will be published in a different venue.
I obviously favor whatever cost-effective, practical trade-offs (not solutions) we can implement to limit the risks facing digital assets. I am not saying we should roll over and die, hoping the authorities will catch the bad guys and prevent future crimes. Nevertheless, the most pressing problem in digital security is attribution and apprehension of those perpetrating crimes involving information resources. Until we take the steps necessary to address that problem, no amount of technical vulnerability remediation is going to matter.