Secure Coding: Principles and Practices

Chapter 4. Implementation

Your knowledge is of a meager and unsatisfactory kind; it may be the beginning of knowledge but you have scarcely in your thoughts advanced to the state of science.

—William Thomson, Lord Kelvin, On Measurement, 1894

An implementation flaw is a mistake made while writing the software; most, though not all, implementation flaws are coding flaws per se. In our view, implementation flaws typically arise because the programmer is either unfamiliar with secure coding techniques or unwilling to take the trouble to apply them. (No doubt because we like to believe the best in human nature, we think it's much rarer that someone tries hard and fails to successfully write secure code.)

Looking back to the example of the SYN flood attacks, there were certainly implementation flaws in addition to the principal design flaw that led to the attacks. For example, when the array of TCP sockets became exhausted, some operating systems at the time simply crashed. This was the result of a memory overflow that occurred when the software attempted to store an out-of-bounds array value. At the very least, a carefully implemented TCP stack could have prevented such catastrophic failure of the operating systems.

Source code is the final stage in the translation of a design into something users can use, prior to the software's being subjected to testing and (eventually) production. Flaws in source code, therefore, have a direct link to the user base; because the machine translation ...