Most knowledge-based authenticators run the risk of poor uniqueness when they rely on users to generate or select the secret information. The human mind tends to seek order in the knowledge we acquire and store. This means that you, I, and a few billion others generate similar secrets for authentication.
Seeing as how I make a living off applying my knowledge I don't complain about this natural phenomenon. However, I do occasionally wish that more authentication system designers took it into consideration. While we can't expect them to overcome this reality, they do need to anticipate it and avoid design or implementation decisions that magnify problems.
For example, PassPoints is a knowledge-based graphical password system developed a couple of years ago. This system presents users with a picture and authenticates them by analyzing where they click within the image. Users choose and click on their own series of points during the authentication enrollment process. During subsequent authentications, only the legitimate user should know where to click within an image.
Of course, the security of the system relies on the premise that there aren't a limited number of predictable click points within the image. Good old human brains will sometimes look at the same picture and click on the same points. Our eyes may all be drawn to the fire hydrant, building corner, or stunning brunette. With graphical passwords this appears to be dependent on what is depicted in a particular image. The more predictable the click points are within a picture, the less useful that picture is for uniquely authenticating users.
So I was pleased to see a recent paper where the authors not only recognize this challenge, but also attempt to automatically evaluate an image for predictable points. Modeling User Choice in the PassPoints Graphical Password Scheme is an update from several of the original PassPoints researchers explaining how new software could sometimes accurately predict (70% - 80% in their test case) clickable points within an image. In turn, this software could be used to eliminate images that have too few likely-to-be-clicked points.
I hope that this trend of investigating and developing quality controls for authentication systems continues.