SAST vs. DAST: Differences and when to use them

Security threats are becoming commonplace as digital transformation accelerates and more organizations use software solutions to facilitate work operations. Cybercriminals continually develop ways to exploit software application vulnerabilities to enter a target organization’s network. A notable example is the 2017 Equifax data breach, which exposed the personal details of 145 million Americans.

Testing ensures software meets requirements, is free of defects, errors, or bugs, and does not contain security vulnerabilities. The most common software testing method is functional testing, which is comprised of tests such as unit and integration tests.

In unit testing, the developer tests program modules or units to ensure they function as required. The test runs independently of any dependencies and ensures the component works as expected. In integration testing, developers run multiple modules or units to explore how they work together to achieve specific functionality.

Functional testing is helpful, but it is no longer enough. It only checks for problems you thought of in advance, intending to assure the functionality meets business objectives. It does not check for security flaws leading to data breaches.

To better secure their applications, developers should leverage other testing tools such as static application security testing (SAST) and dynamic application security testing (DAST). This article explores each type and discusses when to use SAST or DAST. Then, it highlights other forms of application testing you can implement to increase your application’s security.

What is SAST?

Static application security testing analyzes program source code to identify security vulnerabilities. These vulnerabilities include SQL injection, buffer overflows, XML external entity (XXE) attacks, and other OWASP Top 10 security risks.

SAST is white box testing. It scans the software application from the inside out to discover security vulnerabilities in the code before compilation or execution.

A significant advantage of the SAST methodology is that developers can begin testing their application at early development stages without executing a functional component. This approach discovers source code security flaws early and avoids leaving security issues to later development phases, decreasing the development time and enhancing overall program security.

Various tools perform SAST testing. Two of the most popular are Klocwork, a static code analyzer for C, C++, C#, Java, JavaScript, and Python, and Checkmarx, an SAST tool supporting various programming languages.

To mitigate fatal security errors and produce more secure applications, many developers now incorporate SAST testing into their modern continuous integration and continuous deployment (CI/CD) pipelines. Numerous use cases harness SAST to create more secure applications.

For example, SAST can continually monitor source code vulnerabilities for problematic coding patterns that violate software development security best practices. It can also automate testing your application code for vulnerabilities using popular security industry standards, like OWASP Top 10 and SANS Top 25. SAST also helps comply with various data protection laws, such as the Health Insurance Portability and Accountability Act (HIPAA), and the Payment Card Industry Data Security Standard (PCI DSS).

What is DAST?

Dynamic application security testing evaluates a software application. DAST testing simulates the actions of a black hat hacker trying to break into the intended application remotely. It scans software applications in real-time against leading vulnerability sources, like the OWASP Top 10 or SANS/CWE 25, to find security flaws or open vulnerabilities.

The main difference between DAST and SAST lies in how each performs the security testing. SAST scans the application code at rest to discover faulty code posing a security threat, while DAST tests the running application and has no access to its source code.

DAST testing stimulates an outside attacker’s perspective. It assumes the tester does not know the application’s inner functions. It can detect security vulnerabilities that SAST cannot, such as those only appearing during the program runtime.

These DAST tests occur in a later application development phase because they require a complete working application. For example, testers need to interact with the application, providing inputs, checking outputs, and simulating various actions typical of user interactions. These tests help ensure the application is not susceptible to web attacks such as cross-site scripting (XXS) or SQL injection.

Although most DAST tools are commercial, one open-source tool named Arachni provides rich functionality. Arachni’s Ruby framework supports scanning web applications for vulnerabilities including XSS (with DOM variants), SQL injection, NoSQL injection, code injection, and file inclusion variants. It can be helpful to try this free tool before deciding which commercial DAST tool to purchase later.

DAST use cases include detecting misconfiguration in servers or databases that affect web application security during runtime. It can also catch authentication and encryption issues allowing unauthorized access, while SAST cannot. Also, DAST can test other API or web services your web application connects to, in addition to IT infrastructure resources like networking and data storage. So, DAST is valuable for testing the entire IT environment where your application or web services operate.

SAST vs. DAST: Which Should You Use?

Now that you know the main characteristics and objectives of SAST and DAST testing methodologies, let’s discuss which one is best suited to your application testing environment. Organizations should not choose one or the other, but instead, apply both methods to testing applications.

SAST tests the application’s internal source code in early development phases to ensure developers follow the best security practices when writing code. In contrast, DAST testing begins in later development phases in a working application. It tests the application while running to discover its susceptibility to the most common cyber threats.

SAST testing is technology-dependent. So, your SAST tool should support your programming language (PHP, ASP.NET, Java, Python, or PERL) and development framework to ensure complete testing coverage. On the other hand, DAST is technology-independent because it tests the application in runtime from an external user perspective.

To achieve their software application’s maximum security, developers now use SAST and DAST tooling as part of the app’s CI/CD pipeline. DevSecOps should use both methodologies to integrate security into each development phase. This approach helps development teams integrate security controls into their design process without impacting productivity. It accelerates development time without sacrificing the final product’s safety.

Other Forms of Application Security (IAST, RASP, and HAST)

SAST and DAST are not the only available tests. The development community also uses methods such as IAST, RASP, and HAST.

Interactive Application Security Testing

Interactive application security testing (IAST) is a gray-box testing methodology combining the functions of both SAST and DAST. It uses a monitoring mechanism (sensor or agent) in the application’s backend to gather information during runtime.

This approach lets developers test their application’s behaviors at runtime using DAST testing techniques while still monitoring source code execution, like SAST testing. IAST mitigates a significant limitation of the SAST methodology—its inability to follow and test all dependencies, like libraries and frameworks, that modern web applications use.

Runtime Application Self-Protection (RASP)

Runtime application self-protection (RASP) is another security testing methodology to test and protect applications against common vulnerabilities during execution or runtime. DevOps can use RASP to monitor applications in production and take corrective steps when it detects abnormal activity, such as a cyberattack or other malicious action.

Hybrid Application Security Testing (HAST)

HAST combines SAST and DAST methodologies to discover and fix application security vulnerabilities. Although this approach requires more time and budget, it is optimal for designing secure applications.


Most security incidents result from unfixed security errors. Development teams can use a combination of white and black box testing to produce more secure applications to handle today’s complex IT threat environment.

Automating this security testing saves time and produces more accurate results. Learn more about how CircleCI DevSecOps tooling integrates automatic testing into your CI/CD pipeline for more secure applications.

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