Production code typically includes at least one security issue that prompts DevOps and DevSecOps teams to use application security testing methods. Two of the most widely used test automation approaches today are: Static application security testing (SAST), and dynamic application security testing (DAST). SAST and DAST focus on different aspects of the software development life cycle (SDLC) to find security vulnerabilities.
Static and Dynamic Application Security Testing with SAST and DAST Scans
SAST, aka inside-out testing, and DAST, aka outside-in testing or closed box testing, take different approaches to application security testing used at different SDLC stages.
Static application security testing scans proprietary or custom source code for errors and design issues that could lead to exploitable weaknesses. SAST scans are performed earlier in the software development lifecycle. As the name implies, SAST tests software code at rest, looking for issues like broken access controls or one of the other nine OWASP application security risks.
Dynamic application security testing scans a running application without access to the source code. The dynamic nature of DAST means that testing occurs from the front end and adapts as the volume and type of inputs and outputs modulate. Meant to simulate an outside attacker, DAST is typically carried out in a production-like environment with tools that don’t have a view into the components used.
What Software Components Do SAST and DAST Scans?
Referring to a predefined set of rules to detect issues and vulnerabilities, SAST scans a multitude of static inputs, including:
- Documentation like requirements and design specifications
- Application source code
- Related dependencies like frameworks and libraries
DAST scans dynamic internal and external environments, including:
- API endpoints
- Web services
- Physical infrastructure and host systems
In addition to scanning differing artifacts, code components, and environments, SAST and DAST methodologies have strengths that align with their role in the SDLC. Alternatively, they have particular drawbacks as well.
SAST and DAST Strengths and Drawbacks
One of the most valuable strengths of SAST is when a tool detects issues in the source code; it marks the precise location with the file name and line number. And each detected issue indicates a severity level and description. This approach enables DevOps teams to prioritize vulnerabilities and mitigate problems quickly.
SAST tools easily integrate into a CI/CD pipeline. They automatically scan the code when it is committed to a repository. Since SAST is conducted early in the SDLC, it can accommodate various high-level languages. It’s flexible and integral to fixing vulnerabilities before they reach the DAST stage.
The main drawback of using SAST is that it is programming language-dependent. Applications written in multiple languages require multiple SAST tools. Since SAST tools scan the complete source code and rely on a rule set, they can flag many false positives. The trade-off for comprehensive software scanning is slower scanning.
DAST can overlap with SAST at a certain point in the SDLC. It tests developed applications looking for runtime issues, mimicking approaches to exploit vulnerabilities. DAST tools don’t access source code. They can examine a broader set of components, including libraries, plug-ins, and APIs, while looking for runtime and authentication issues and misconfigurations. An advanced feature, some DAST tools generate a proof of exploit, providing evidence that an identified vulnerability is a genuine risk and not a false positive, as well as mitigation guidance.
DAST’s lack of source code access means that the DevOps team can’t always pinpoint where a vulnerability is in the code. DAST requires a runtime environment, pushing testing and mitigation late in the SDLC. Also, the nature of issues that DAST uncovers can require precious time and budget.
End-to-End Software Security Testing Combines Both
Combining SAST and DAST provides continuity across the CI/CD pipeline. It gives an opportunity to mitigate software errors and vulnerabilities from both perspectives—inside-out and outside-in. The beauty of using both types of tools consistently provides a software quality improvement feedback loop. DAST results can inform the rule set for SAST. This finds issues sooner in the SDLC when it’s less costly and easier to resolve.
SAST and DAST become part of a comprehensive and best-practice approach to application security when used together.
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