Tuesday, 29 May 2012

10 facts about Security Code Reviews

1.      What is the scope of a security code review?

A security code review is a form of an assessment and it is all driven by analysis. The objective of this assessment to identify all the security flaws present in a software along with their root causes,  by understanding and analyzing its code. 

2.      How different is security code review from application security testing?

Even the application security testing activity aims at identifying the security vulnerabilities of the application but it differs from security code review in its methodology.
Application security testing is performed by sending different test inputs to the application and analyzing their response as required for identifying different security flaws present in it. However, in security code review as the access to the entire code base of the application is available here is no need to send any test data to the application/software, the flaws can be easily discovered by analyzing the code.

In a security code review, the reviewer gets exposed to entire code layout of the application and many other important sections of the code, which would never get analyzed in an application security test. Some of those crucial areas include:
  •  Entry points for different inputs
  • Internal interfaces and integrations
  • Data handling and validation logic
  • Use of external API’s and frameworks
Code reviews are thus able to uncover security vulnerabilities from all areas of the code can be considered to be a much more authentic form of assessment than application security testing. 

3.      Is application testing included as a part of a security code review?

NO, there is no explicit need to perform application testing in a security code review; all the security flaws can be easily identified through the code. 

However, it is useful to access a running application to understand all its features. This understanding is required for identifying application related test cases for the review. 

There could be a need to perform small amount of testing to verify the flaws discovered at the code level. But, the process of discovering flaws is not dependent on any application testing method. It is only used, if required as an aid to support the analysis. 
4.      Is every line of code being reviewed in a security code review?

NO, I am sure this answer may have come to you as surprise. But, it is very much true. To get you more clarity on it let me ask you a question. 

Do you think security flaws will be present in every line of the code?

Well, if your answer to that one was No, I am sure you would have got the catch!

Certain sections of the code will be more prone to security vulnerabilities than the rest. Security flaws may arise due to improper coding or lack of specific security control at places where they are required in the code.  Both, different kinds of improper coding practices for any platform and the sections of the code that requires necessary security controls are known to a considerable extent today. Thus while reviewing the code looking out for such vulnerable cases first is important. What remains then are application's feature specific flaws, which require a high level understanding of the business logic and not line by line analysis of the code.

Considering the limitation of time for carrying out this activity the better way to do this it is:
  • Understand application features and realize application specific test cases to be reviewed for different features 
  • Understand the code layout of the application. So that tracing the code for any feature is simplified
  • Understand different validation schemes and techniques (data and access control validations) 
  • Identify security flaws in the application design and  underlying platform/framework 
  • Look for all  insecure code instances across the application 
  • Feature wise look for application specific security vulnerabilities (lack of business validation)
However, there must be a robust approach to review the applications so as to blend the above checks in a manner that gives optimum accuracy in review results.
5.      Are functional bugs discovered during a security code review?

NO, functional bugs are not discovered in a security code review. However, nothing would stop someone from analyzing functional defects while performing security code review. But if it is done so, it may hamper the quality of the review. If the reviewer focuses on many things in a single review it may be difficult to achieve optimum results and chances of error will creep in. 

The objective of the security code review is well defined and it is only aimed at discovering all the security flaws in the application/software being reviewed.

Due to above factors the security code reviews must be carried out separately, which will make the process of the review more exhaustive and thorough.
6.      Is design of the application reviewed?

YES, the review of the application design for security flaws is included in the security code review of the application. Understanding the design gives a good insight of the code layout and the interaction of various components of the application

The design related flaws are insightful and if they are left uncaught they may pose a big risk to the application. It requires good deal of understanding and manual analysis to uncover design flaws in an application. And it is unquestionably a part of the security code review process.
7.      When should I perform security code review?

It is needless to say that code must be developed with the sense of security in mind. Application developers must be aware of insecure coding and development practices and must be cautious about avoiding them while developing applications. 

There is no ideal time to carry out a review, but it is advisable to carry security code review after completion of a feature, module or an application. Incomplete code will not give proper sense of the functionality to the reviewer and thus might hamper the reviewer’s understanding of the code.

Review of smaller chunks of code can be done easily if it is a part of the development process. It is good practice though to incorporate security checks at different stages of application development.
8.      I have reviewed my application once is that enough?

Well, good to know that the application has been reviewed once. But before I answer that question let me ask you that will the application’s code base not undergo any code change after it is developed?

Owing to different reasons the code changes are continuously introduced into the application. These code changes will also bring in some security flaws with them if they go without security review into the application. Hence, it is just not enough to review the applications code base once. Either there must be a continuous review process for all the code changes introduced into the application or a periodic reviews of the entire code base of the application.

Also, it is good to follow security best practices while developing applications so that the chances of security flaws introduced into the application keeps decreasing.
9.      Are there tools available for performing security code reviews?

Yes, there are many code scanners available that can perform good secure code analysis. As needed they are useful in identifying insecure code instances and API usage. However, there are many areas of the analysis that require manual efforts once like review of design and application specific cases, etc. It would be good to use scanners as an aid to make the review process comfortable. But they cannot be used as a complete replacement to manual analysis in this activity. (Check this)
10.      Who can perform a security code review?

It takes a good deal of application security and software programming knowledge to be able to understand the code and analyze it thoroughly. There are many application security companies that also offer “security code review” as a service who can be one of the choices to carry out this activity. However, external vendors may have less understanding about the application and internal security teams may lack a defined process to carry out such a review activity. Irrespective of this trade-offs it must be ensured that the best guys are picked up to perform this activity. (Check this to help you further.)


  1. Hi Ashish,

    Could you please help me out on how and what(points need to look in source code) to review the code for all OWASP top 10.

  2. Hi Girish,

    You can start with reading the OWASP CR guide -


    Let me know if you have any specific query.