Designing with “harmless failures” in mind

Ed Felten at Freedom to Tinker has some interesting points to add to Bruce Schneier’s piece on “Security Mindset” that I posted about yesterday. Felten focuses on the notion of “harmless failures.” It provides still more reason to approach all systems design problems with an eye firmly fixed on the social context in which your technology will operate.

The Security Mindset and “Harmless Failures”

…Not all “harmless failures” lead to big trouble, but it’s surprising how often a clever adversary can pile up a stack of seemingly harmless failures into a dangerous tower of trouble. Harmless failures are bad hygiene. We try to stamp them out when we can.

To see why, consider the donotreply.com email story that hit the press recently. When companies send out commercial email (e.g., an airline notifying a passenger of a flight delay) and they don’t want the recipient to reply to the email, they often put in a bogus From address like [email protected] A clever guy registered the domain donotreply.com, thereby receiving all email addressed to donotreply.com. This included “bounce” replies to misaddressed emails, some of which contained copies of the original email, with information such as bank account statements, site information about military bases in Iraq, and so on. Misdirected ants might not be too dangerous, but misdirected email can cause no end of trouble.

The people who put donotreply.com email addresses into their outgoing email must have known that they didn’t control the donotreply.com domain, so they must have thought of any reply messages directed there as harmless failures. Having gotten that far, there are two ways to avoid trouble. The first way is to think carefully about the traffic that might go to donotreply.com, and realize that some of it is actually dangerous. The second way is to think, “This looks like a harmless failure, but we should avoid it anyway. No good can come of this.” The first way protects you if you’re clever; the second way always protects you.

Which illustrates yet another part of the security mindset: Don’t rely too much on your own cleverness, because somebody out there is surely more clever and more motivated than you are.

Essay Scam Busters