DAST vs. SAST: Getting real on static and dynamic application security testing

Getting lost in cybersecurity jargon, AppSec acronyms, and vendor claims? Here’s your guide to what two of the major application security testing technologies can and cannot do—and why you should be worrying more about getting the big picture of your application security risks and less about deciding between acronyms.

What is DAST and what is SAST?

Let’s start by getting the definitions out of the way and clarifying what each testing approach is designed to do.

What is DAST?

Dynamic application security testing (DAST) is a black-box testing methodology where a running application is tested from the outside. While dynamic testing is a broad term that encompasses both manual and automated methods, DAST is usually understood to mean automated vulnerability scanning. 

DAST testing tools

Dynamic application security testing tools (aka vulnerability scanners) analyze applications while they’re running, identifying critical security flaws by simulating attacks in a runtime environment. This provides an attacker’s eye view of your application security posture so you can fix potential vulnerabilities before they’re exploited. DAST tools vary in capabilities, from basic manual scanners to full enterprise-grade security platforms such as offered by Invicti. 

When should I use DAST?

Because dynamic application security testing requires a running application, it is commonly used in staging to detect runtime vulnerabilities that were not present during development as well as other security flaws that weren’t detected earlier. Advanced DAST tools can also be used in production as an operational security tool and even integrated into CI/CD pipelines to test builds as early as possible.

What is SAST?

Static application security testing (SAST) is a security testing method that analyzes the application source code to identify potential security vulnerabilities. Because it requires knowledge of application internals, SAST is classified as a white-box testing approach.

SAST testing tools

Static application security testing tools analyze source code prior to deployment of the app, allowing early detection of security flaws during the development process. SAST tools range from IDE plugins to standalone static analyzers and are nearly always tightly integrated into dev pipelines. 

When can I use SAST?

Because they operate on source code and don’t require a running app, SAST scans are used almost exclusively during development work. Depending on the tool, they can run continuously or be triggered at predefined stages in the pipeline. 

What is IAST, then?

Interactive application security testing (IAST), sometimes called gray-box testing, occupies the middle ground between dynamic and static analysis. Depending on the vendor and product, IAST can be a standalone tool that adds dynamic insights to SAST or a way to add source code insights to DAST. 

 

IAST on the Invicti platform is implemented as a server-side agent that communicates with the core vulnerability scanner during testing to find more than DAST alone could without requiring code instrumentation.

SAST vs. DAST: Which should you use?

Static and dynamic approaches to security testing each have their strengths and limitations. While your overall application security program should ideally include both DAST and SAST to maximize coverage, deciding when to use each method depends on your organization, workflows, and specific tool choices. 

As a rule of thumb, SAST works best in early development. Because they operate on source code and are designed specifically to work in development toolchains, SAST tools are easy to build into CI/CD pipelines and the overall dev process. They are also the natural choice for enforcing secure coding best practices.

DAST requires a running application, so it’s typically used in pre-prod and staging to find runtime vulnerabilities and also test third-party components, dynamic dependencies, and APIs used by the app. Being tech-agnostic, DAST is extremely versatile and can also be used in production to cover many use cases in operational and information security, including real-time security assessments as well as compliance and security audits. It can also serve to partially automate penetration testing.

DAST and SAST are especially powerful when used in tandem. For example, you can automate SAST in CI/CD, scan major builds with DAST internally, and then also run scheduled DAST scans in production. This is especially important in heavily regulated industries like finance, healthcare, and government. 

SAST vs. DAST coverage in web application security testing

Test coverage within a specific app and across your entire web application environment is a fundamental attribute of security testing. To give you an accurate picture, a security testing tool needs to know what to test, how to test it, and how to interpret and present the findings.

SAST works on the application source code, so you need to have that code as well as tools that support a specific programming language and web application framework. If you have multiple technology stacks, you could need multiple SAST tools. In practice, SAST coverage is also limited to apps that are in active internal development since you need both the code and the right testing toolchains. The common argument that only SAST provides full test coverage because it tests all the code is only true for the codebase of a specific application—and the limited subset of vulnerabilities that can be detected statically.

DAST tools, on the other hand, are technology-agnostic because they test applications from the outside and examine their behavior, not their source code. This allows DAST scans to cover any number of applications, regardless of tech stack, development status, or source code availability, testing everything that is externally accessible to a visiting browser. Leading dynamic scanners can identify a wide range of vulnerabilities, including misconfigurations and other runtime issues. They also support modern authentication schemes to access site sections and functionality available only to authenticated users.

API security testing

Application programming interfaces (APIs) are the lifeblood of the cloud and gatekeepers of the data delivered by web services. Doing security testing on API endpoints is now a critical requirement to prevent data breaches—and leading DAST solutions provide an automated way to do this.

 

Get the Invicti white paper on API security testing to learn why API security is now an integral part of AppSec.

Security testing accuracy and efficiency with SAST vs. DAST

False positives have always been problematic in automated security testing, understood both as erroneous results and valid but non-actionable findings. In particular, many SAST tools have a reputation for flooding developers with security issues that, while often technically accurate, are irrelevant in a specific context. At best, this requires tedious fine-tuning—and at worst, developers will routine ignore SAST results or bypass the checks altogether.

The advantage of DAST is the ability to look at the running app and identify actual exploitable vulnerabilities instead of just flagging suspicious code constructs. While basic vulnerability scanners can struggle to deliver fully reliable results, advanced DAST solutions can automatically and safely exploit many classes of vulnerabilities to confirm they are real and high-priority issues. This makes DAST the ideal approach for time-strapped development teams, allowing them to focus remediation on vulnerabilities that really matter.

Learn more about proof-based scanning on the Invicti platform.

Finding vulnerabilities with DAST and SAST

To give a specific example, let’s say an application fetches data from an SQL database and insecurely uses raw user input from a web form in its database query:

• SAST will identify the source code fragment that does this and warn the developer that the SQL query is constructed in a way that could (in theory) allow SQL injection.
• A DAST scan will find the page and web form during crawling and simulate SQL injection attacks against it. If any of the test attacks succeed, the scanner will report an actual SQL injection vulnerability on that page.

The difference between SAST and DAST results is the difference between “we should probably look at this” and “we need to fix this now.” This is especially important for weaknesses such as cross-site scripting (XSS), where many suspicious code constructs will never lead to an actual exploitable vulnerability. Advanced DAST tools can even identify out-of-band vulnerabilities, which are security gaps that don’t cause direct reactions to testing.

Building SAST and DAST into your SDLC

Testing your applications for all types of vulnerabilities as early as possible in the software development lifecycle (SDLC) is crucial to fix security issues before they make it into production. Source code analysis is the most natural way to find and eliminate security defects during early development. SAST is typically easy to integrate with development environments and workflows, whether as an IDE checker or a standalone analysis process. However, because SAST only looks at static code and cannot identify runtime vulnerabilities and misconfigurations, some form of dynamic testing is still needed in the SDLC. 

DAST tools also can and should be integrated into the SDLC. While they do require a runnable application to test, this is less of a hindrance with modern web frameworks that can autogenerate code for prototyping at any stage of development. The big advantage of DAST in the SDLC is that it can run at multiple stages of your pipeline, from partial testing in development to full-scope tests in staging and then production testing by security teams. In fact, because DAST is technology-agnostic and checks the entire application for vulnerabilities, regardless of the implementation details and source code availability, it’s the recommended starting point for adding security testing into the SDLC.

DevSecOps on the Invicti platform: Never mind the acronyms, give me results

It’s all too easy to get drawn into choosing one approach over another or (worse still) ticking boxes to make sure you catch all the AST acronyms. The ultimate goal, though, isn’t to complete a shopping list but to find a way to get your web applications secure and keep them secure. The way to get there is different for each organization and rarely quick or easy. At Invicti, we’ve come up with a fast-track approach that builds on the unique capabilities and features of our DAST-first AppSec platform.

The Invicti platform is built around the industry’s most mature and advanced DAST scanning engine, which uses proof-based scanning to automatically confirm the vast majority of exploitable high-impact vulnerabilities with no risk of false positives. These confirmed results can be sent directly to developers via out-of-the-box integrations with issue trackers and CI/CD pipelines to make sure that application security can keep up with the extensive automation of DevOps development processes. Each vulnerability report includes detailed remediation guidance and each fix can be automatically retested, enabling organizations to set up a hands-off AppSec process that doesn’t interfere with development and leads to more secure code in the long run.

With Invicti’s comprehensive security platform, you can stop counting your AST acronyms and start taking real control of your security posture. Yes, you do get DAST, SAST, IAST, SCA, API security, and much more besides, but instead of focusing on the tools, you can now finally focus on real-life security improvements—with the world’s best DAST engine keeping things honest.