Code Review Introduction

Introduction
Code review is probably the single-most effective technique for identifying security flaws. When used together with automated tools and manual penetration testing, code review can significantly increase the cost effectiveness of an application security verification effort.

This guide does not prescribe a process for performing a security code review. Rather, this guide focuses on the mechanics of reviewing code for certain vulnerabilities, and provides limited guidance on how the effort should be structured and executed. OWASP intends to develop a more detailed process in a future version of this guide.

Manual security code review provides insight into the “real risk” associated with insecure code. This is the single most important value from a manual approach. A human reviewer can understand the context for certain coding practices, and make a serious risk estimate that accounts for both the likelihood of attack and the business impact of a breach.

Why Does Code Have Vulnerabilities?
MITRE has catalogued almost 700 different kinds of software weaknesses in their CWE project. These are all different ways that software developers can make mistakes that lead to insecurity. Every one of these weaknesses is subtle and many are seriously tricky. Software developers are not taught about these weaknesses in school and most do not receive any training on the job about these problems.

These problems have become so important in recent years because we continue to increase connectivity and to add technologies and protocols at a shocking rate. Our ability to invent technology has seriously outstripped our ability to secure it. Many of the technologies in use today simply have not received any security scrutiny.

There are many reasons why businesses are not spending the appropriate amount of time on security. Ultimately, these reasons stem from an underlying problem in the software market. Because software is essentially a black-box, it is extremely difficult to tell the difference between good code and insecure code. Without this visibility, buyers won’t pay more for secure code, and vendors would be foolish to spend extra effort to produce secure code.

One goal for this project is to help software buyers gain visibility into the security of software and start to effect change in the software market.

Nevertheless, we still frequently get pushback when we advocate for security code review. Here are some of the (unjustified) excuses that we hear for not putting more effort into security:

“We never get hacked (that I know of), we don’t need security”

“We have a firewall that protects our applications”

"We trust our employees not to attack our applications" 

Over the last 10 years, the team involved with the OWASP Code Review Project has performed thousands of application reviews, and found that every single application has had serious vulnerabilities. If you haven’t reviewed your code for security holes, the likelihood that your application has problems is virtually 100%.

Still, there are many organizations that choose not to know about the security of their code. To them, we offer Rumsfeld’s cryptic explanation of what we actually know. If you’re making informed decisions to take risk in your enterprise, we fully support you. However, if you don’t even know what risks you are taking, you are being irresponsible both to your shareholders and your customers.

''"...we know, there are known knowns; there are things we know we know. We also know there are known unknowns; that is to say we know there are some things we do not know. But there are also unknown unknowns -- the ones we don't know we don't know." ''    - Donald Rumsfeld

What is Security Code Review?
Security code review is the process of auditing the source code for an application to verify that the proper security controls are present, that they work as intended, and that they have been invoked in all the right places. Code review is a way of ensuring that the application has been developed so as to be “self-defending” in its given environment.

Security code review is a method of assuring secure application developers are following secure development techniques. A general rule of thumb is that a penetration test should not discover any additional application vulnerabilities relating to the developed code after the application has undergone a proper security code review.

All security code reviews are a combination of human effort and technology support. At one end of the spectrum is an inexperienced person with a text editor. At the other end of the scale is a security expert with an advanced static analysis tool. Unfortunately, it takes a fairly serious level of expertise to use the current application security tools effectively.

Tools can be used to perform this task but they always need human verification. Tools do not understand context, which is the keystone of security code review. Tools are good at assessing large amounts of code and pointing out possible issues, but a person needs to verify every single result to determine if it is a real issue, if it is actually exploitable, and calculate the risk to the enterprise.

Human reviewers are also necessary to fill in for the significant blind spots where automated tools simply cannot check.