There are a couple approaches for determining when an application should be undergo a security assessment. First, organizations often require new tests after a fixed period of time. This period of time may vary based on the risk level the organization has attributed to each application or application type. It's common for organizations to conduct security assessments of high and medium risk applications every six months to one year. For low risk applications, the time period is often a year to two years. The risk level of an application is often determined based on the type of data and functionality within the application. For example, an Internet facing application that handles credit card transactions would be considered high risk; while an application that simply provides product information and is not subject to regulatory, compliance, or legal requirements may be low risk. Periodic assessments usually supplement the next two approaches.
The second approach is associated with major changes implemented in the application. There are a variety of changes that should trigger a new application assessment. Any changes to a security mechanism should undergo validation. Security mechanisms usually include things like authentication processes, authorization controls, session management features, and data validation and encoding components (think cross-site scripting, SQL injection, etc.). Changes that add or modify a feature in the application should trigger a retest based on the risk level of the application and the sensitivity of data or functionality that the change effects. So, a new feature that adds an “about” tab to a website probably doesn’t need to undergo rigorous security testing; however, a new feature that collects users’ social security numbers absolutely should undergo testing (including evaluation whether collection of this PII complies with the organization’s policies, that there is a need to know for this sensitive data, that the sensitive data is adequately protected at rest and in transit, and that only authorized, authenticated parties can reach this data). Organization’s typically set up criteria and standards around PII, credit card information, and other sensitive data types that automatically trigger a reassessment when an application change related to those data types occurs.
The third approach is to use application security testing as a security gate within an organization’s development process. In this case, specific application types or risk levels would require an assessment before they can be deployed to production. This is usually one of the final steps in a secure software development process and acts as a sanity check or a process improvement opportunity rather than a catch-all for security issues. These types of tests are often highly targeted and would not encompass an assessment of the whole application; instead the security assessment would focus on high risk, new, or updated components introduced in the application.
Previously, I wrote an ISSA article, Titled “Web Application Security Portfolios”, which covered some of this detail. An expanded version of this article can be found in my post here. The article discusses ideas around managing portfolios for each application within an organization, identifying data types and compliance requirements, and tracking security activities.