Input validation: the Good, the Bad and the Ugly

This talk discusses input validation design choices and recommends practices that provide developers a fighting chance to survive architectural decay as an application matures.

The OWASP 2004 Top Ten adviced never to trust user input. Although fundamentally sound, it led to many maintenance nightmares and insecure web applications. This talk argues that the enthusiasm for input validation must be tempered by a resolve to eliminate code duplication to maintain sanity and security. I will show this is possible, even in the face of apparently conflicting objectives, namely usability and protection against malicious users.

The discussion is illustrated by a case study of a well-intentioned but flawed attempt at implementing meticulous input validation. The application's validation code is scattered throughout the code base. I propose an alternative based on dual requirements: on the one hand, ensuring good usability, and, on the other, protecting against malicious users. Writing elegant validation code is found to be very hard in current mainstream technologies, so I explore some promising alternatives.