Dynamic Application Security Testing: DAST Basics

What is DAST?

DAST is a security tool that attempts to penetrate an application from the outside by checking its exposed interfaces for vulnerabilities and flaws. Sometimes called a web application vulnerability scanner, it is a type of black-box security test. It looks for security vulnerabilities by simulating external attacks on an application while the application is running. 

The dynamic part of DAST’s name comes from the test being performed in a dynamic environment. Unlike SAST, which scans an application’s code line by line when the application is at rest, DAST testing is executed while the application is running. This is not to say that testing is performed while the application is in production. While DAST can be used in production, testing is usually carried out in a QA environment. 

DAST is extremely good at finding externally visible issues and vulnerabilities. This includes a number of security risks from OWASP’s top ten, such as cross-site scripting, injection errors like SQL injection or command injection, path traversal, and insecure server configuration.

One of DAST’s advantages is its ability to identify runtime problems, which is something SAST can’t do in its static state. DAST is excellent at finding server configuration and authentication problems, as well as flaws that are only visible when a known user logs in.

Why is DAST important?

DAST has become an important part of application security strategies for several reasons:

• Identifies real-world vulnerabilities that attackers could exploit once an application is live.
• Discovers security issues without access to the source code. This makes DAST valuable for testing proprietary or third-party applications where source code is unavailable. It also enables organizations to verify the security of applications built by external vendors.
• Enables early detection of vulnerabilities, helping organizations reduce the risk of breaches, data leaks, and compliance violations. Early detection can also save significant costs, making it possible to fix vulnerabilities earlier in the SDLC.
• Complements other security testing methods like SAST and penetration testing. While each method has its strengths, using DAST as part of a layered security approach ensures a broader coverage of potential risks across the application’s lifecycle.

How does DAST work?

DAST works by implementing automated scans that simulate malicious external attacks on an application to identify outcomes that are not part of an expected result set. One example of this is injecting malicious data to uncover common injection flaws. DAST crawls a web application to identify all HTTP and HTML access points, systematically tests them, and also emulates random actions and user behaviors to find vulnerabilities. 

Because DAST has no access to an application’s source code, it detects security vulnerabilities by attacking the application externally. DAST does not look at code, so it can not point testers to specific lines of code when vulnerabilities are found.

Security experts are heavily relied upon when implementing DAST solutions. For DAST to be useful, security experts often need to write tests or fine-tune the tool. This requires a solid understanding of how the application they are testing works as well as how it is used. Security experts also must have a strong knowledge of web servers, application servers, databases, access control lists, application traffic flow, and more to effectively administer DAST.

Though they may sound similar, DAST differs from penetration testing (or pen testing) in several important ways. DAST offers systematic testing focused on the application in a running state. Pen testing, on the other hand, uses common hacking techniques with the owner’s permission and attempts to exploit vulnerabilities beyond just the application, including firewalls, ports, routers, and servers.

The evolution of DAST technology

Legacy DAST

Legacy DAST tools are traditional web vulnerability scanners designed primarily to test classic web applications built with simple, server-rendered HTML. They operate by crawling application pages and injecting a broad set of predefined attacks to detect vulnerabilities. These tools typically focus on known vulnerabilities such as SQL injection, cross-site scripting (XSS), and outdated server configurations.

However, legacy DAST tools have limitations when testing modern applications. They struggle with dynamic content, complex authentication workflows, API endpoints, and client-side rendering frameworks like React or Angular. Their scanning methods often miss vulnerabilities that are hidden behind multi-step user interactions, dynamic fields, or asynchronous communications (e.g., AJAX requests).

Modern DAST

Modern DAST tools have evolved to address the complexities of contemporary application architectures. They are designed to handle single-page applications (SPAs), APIs, microservices, and applications that rely heavily on JavaScript for client-side rendering. These tools can interact intelligently with dynamic elements, maintain session state across complex authentication flows, and test RESTful and GraphQL APIs.

Modern DAST tools often include features like API scanning, headless browser crawling, machine learning to optimize attack patterns, and integration capabilities with CI/CD pipelines for continuous security testing. They also provide better support for custom authentication mechanisms and can simulate real user behavior, making it possible to uncover deeper and more complex vulnerabilities.

DAST vs. SAST vs. IAST vs. SCA

Let’s see how DAST compares to related application security categories:

• Dynamic application security testing (DAST) is a type of black-box security testing in which tests are performed by attacking an application from the outside.
• Static application security testing (SAST) is white-box testing that analyzes source code from the inside while components are at rest. Interactive application security testing (IAST) works from within an application through instrumentation of the code to detect and report issues while the application is running.
• Software composition analysis (SCA) scans your code base to provide visibility into open source software components, including license compliance and security vulnerabilities.

Each type of AST tool focuses on a slightly different aspect of application security. There are important tradeoffs between the different tools, as illustrated in the following diagram.

DAST pros and cons

DAST is a valuable testing tool that can uncover security vulnerabilities other tools can’t. Though DAST excels in certain areas, it does have its limitations. Let’s look at the top pros and cons of this technology. It is worth noting that some of these limitations are mitigated by next-generation DAST technology.

Pros

#1 Technology independent

Because DAST doesn’t look at source code, it is not language or platform specific. Not being limited to specific languages or technologies allows you to run one DAST tool on all your applications.

#2 Low false positives

Based on OWASP’s Benchmark Project, DAST has a lower false positive rate than other application security testing tools. Testers can zero in on real vulnerabilities while tuning out the noise.

#3 Identifies configuration issues

DAST excels at finding security vulnerabilities that occur only when the application is operational. In addition, DAST attacks an application from the outside in, placing it in the perfect position to find configuration mistakes missed by other AST tools.

Cons

#1 Not highly scalable 

One of the main downsides to DAST is its heavy reliance on security experts to write effective tests, which makes it very difficult to scale.

#2 No code visibility

DAST does not have any visibility into an application’s code base. This means DAST can’t point developers to problematic code for remediation or provide comprehensive security coverage on its own.

#3 Slow scans

DAST is not known for its speed, and many users report scans taking too long. Forrester estimates that DAST scans can last as long as 5-7 days. In addition, DAST scans typically find vulnerabilities later in the software development life cycle (SDLC), when they are more costly and time consuming to fix.

DAST tools: open source vs. commercial

Open source DAST tools provide a cost-effective way for organizations to conduct security testing. Popular examples include OWASP ZAP (Zed Attack Proxy) and w3af (Web Application Attack and Audit Framework). These tools offer strong baseline scanning capabilities and are highly customizable, allowing security teams to write their own scripts, plugins, and integrations.

However, open source DAST tools often require significant manual setup, tuning, and ongoing maintenance. They may lack advanced features like sophisticated authentication handling, API scanning, or deep integration with modern development pipelines out of the box. Support is community-based, which can mean slower resolution times for critical issues.

Commercial DAST tools are typically offered by established security vendors and come with a broader feature set. These tools often support modern application architectures, handle complex authentication mechanisms, scan APIs, and offer integration with ticketing systems, CI/CD pipelines, and security orchestration platforms.

Commercial tools usually come with professional support, regular updates, extensive documentation, and detailed reporting capabilities tailored for compliance needs. Many offer machine learning and AI enhancements to improve vulnerability detection and reduce false positives.

DAST: One piece of your application security puzzle

In a modern DevOps practice, security and developer teams need testing solutions that help secure applications without slowing down development. In this sense, DAST is a powerful tool. In fact, after SAST, DAST is the second largest segment of the AST market. Forrester’s research reports that 35% of organizations surveyed already use DAST and many more plan to adopt it. 

When it comes to application security, however, there is no one tool that can do it all. Though DAST fills an important function in finding potential run-time errors in a dynamic environment, it will never find an error in a line of code. DAST doesn’t provide comprehensive coverage on its own.

For this reason, most organizations need a number of AST tools working in concert to effectively reduce their security risks. DAST excels in looking at external attack methods. SAST finds coding errors by scanning the entire code base. Together with an SCA solution to handle your open source software, they provide the comprehensive testing strategy your organization needs.

*** This is a Security Bloggers Network syndicated blog from Mend authored by Mend.io Communications. Read the original post at: https://www.mend.io/blog/dast-dynamic-application-security-testing/