Code Reviews and Compliance
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The Payment Card Industry Data Security Standard (referred to as PCI from now on) became a mandatory compliance step for companies processing credit card payments in June 2005.
Performing code reviews on custom code has been a requirement since the first version of the standard. This section will discuss what needs to be done with regards to code reviews to be compliant with the relevant PCI requirements.
Code Review Requirements
The PCI standard contains several points relating to secure application development but we will focus solely on the points which mandate code reviews here. All of the points relating to code reviews can be found in requirement 6: Develop and maintain secure systems and applications. Specifically requirement 6.3.7 mandates a code review of custom code:
6.3.7 - Review of custom code prior to release to production or customers in order to identify any potential coding vulnerability.
This requirement could be interpreted to mean that the code review must consider other PCI requirements, namely:
6.3.5 - Removal of custom application accounts, usernames and passwords before applications become active or are released to customers
6.5 - Develop all web applications based on secure coding guidelines such as the Open Web Application Security Project guidelines. Review custom application code to identify coding vulnerabilities. Cover prevention of common coding vulnerabilities in software development processes, to include the following:
6.5.1 Unvalidated input
6.5.2 Broken access control (for example, malicious use of user IDs)
6.5.3 Broken authentication and session management (use of account credentials and session cookies)
6.5.4 Cross-site scripting (XSS) attacks
6.5.5 Buffer overflows
6.5.6 Injection flaws (for example, structured query language (SQL) injection)
6.5.7 Improper error handling
6.5.8 Insecure storage
6.5.9 Denial of service
6.5.10 Insecure configuration management
The standard does not discuss specific methodologies that need to be followed so any of the approaches outlined in this guide could be used.
The current version of the standard (version 1.1 at the time of writing) introduced requirement 6.6. This requirement gave companies two options:
1) Having all custom application code reviewed for common vulnerabilities by an organisation that specialises in application security
2) Installing an application layer firewall in front of web-facing applications
Choosing option one would have meant your code being reviewed by an external company. This of course would come with a high cost.
The PCI Council expanded option one to include internal resources performing code reviews. This added weight to an internal code review and should provide an additional reason to ensure this process is performed correctly.