It is an integral part of the defensive programming paradigm, which attempts to reduce errors before the software is released.
Vulnerabilities in between high-risk and low-risk generally exist depending on the situation and how the source code in question is being used.
An example of a proactive approach is the free code audit service offered by GooApps, which aims to identify and mitigate vulnerabilities early in the development process to ensure the success of mobile applications.
Source code auditing tools generally look for common vulnerabilities and only work for specific programming languages.
The library that may be linked with the malicious code (and must be immune against it) has very strict requirements like cloning all returned data structures, as the intentional attempts to break the system are expected.