What is dynamic application security testing (DAST)?

Compared to other types of application security (AppSec) testing, DAST stands out for its outside-in approach. While other tools require source code and internal access to the application to assess security vulnerabilities, DAST tests applications in their runtime environment from the outside, using simulated attacks to mimic malicious actors. For this reason, DAST is sometimes called outside-in testing or black box testing—a method of testing in which systems are examined without the tester accessing, investigating or even knowing about the internal workings.

Developers today work quickly, often updating specific code areas multiple times a day without a comprehensive view of the entire codebase. They rely heavily on third-party and open-source components and often struggle to collaborate effectively with security teams. Most also work on increasingly complex applications, with numerous features, libraries and dependencies, all while managing constantly evolving cybersecurity threats.

The result is a constantly increasing surface area for security vulnerabilities that intensifies the difficulty of writing secure code and protecting sensitive information from data breaches. Developers need ways to test for potential vulnerabilities as they work, without compromising their productivity.

DAST helps make this possible by automating the security testing process. It works by mimicking the actions of real-world hackers, working from the outside to uncover potential vulnerabilities in running applications. DAST allows developers to test their code and see how it impacts overall app security before it goes live and excels at pinpointing security problems, like authentication errors and code vulnerabilities, often missed by other testing methods, like Software Composition Analysis (SCA).

Modern DAST (see below) tools also seamlessly integrate into DevOps and CI/CD pipelines to offer interfaces for all stages of the development, including early in the application development workflow.

Build and deployment integrations are one reason DevOps teams commonly adopt DAST in DevOps/DevSecOps environments as part of a "shift left" approach in which testing occurs early in the software development lifecycle (SDLC) for more cost-effective and less time-consuming remediation. Other DevOps principles DAST tools enhance include prioritizing automation, collaboration and continuous feedback so developers and security teams can remain agile and productive without compromising security.

Since DAST takes a black box approach, it emulates the actions a malicious threat actor might take when trying to breach a web application.

Generally, DAST includes the following five steps:

Though DAST testing tools don't have any official subtypes, security experts often categorize them into two informal groups—modern DAST tools and legacy DAST tools, with the main differences being automation/integration and vulnerability validation.

Legacy DAST tools often lack automation features, though their scanning process is automated. They typically focus on basic testing—sending requests, receiving responses, and making preliminary assessments—and don't offer full vulnerability validation, only lists of potential security issues.

Modern DAST tools have a higher degree of automation and offer a more thorough review of web application vulnerability.

Modern DAST solutions can seamlessly integrate into the SDLC and operate transparently in the background. Additionally, automation servers can trigger modern DAST tools and present scan results as tickets in a developer's issue tracker. Some modern DAST tools even provide proof of exploitation, eliminating the time-consuming need for manual verification by penetration testers or security experts.

DAST is often considered a critical part of web application security testing. Some of its unique advantages include:

• Versatility. Users can deploy DAST at various stages of the software development lifecycle—DAST can test web applications in their running state and applications that have already been deployed without modifications, making it easier to evaluate legacy systems.

• Automation. DAST tools easily integrate into DevOps and CI/CD pipelines, making it possible to run automated security testing early in the development process and significantly reduce the cost of remediation.

• Language agnostic. Since DAST works from the outside in, it doesn't depend on the programming language used in the application and can work on many different frameworks. DAST effectively tests both web interfaces and APIs, emulating how attackers find vulnerabilities.

• Fewer false positives. DAST typically yields lower false positives and negatives when simulating user actions than other methods, like SAST.

• Realistic and repeatable testing. Because it emulates real-world attacker behavior, DAST is a practical solution for identifying vulnerabilities that malicious actors may exploit. DAST also offers the advantage of repeatable testing, allowing for ongoing vulnerability assessment as applications evolve.

• Comprehensive vulnerability discovery. DAST can uncover a wide range of vulnerabilities, including SQL injection, XSS, and misconfigurations.

• Industry standards alignment. Businesses often use DAST to adhere to industry standards and assist with regulatory reporting, like PCI compliance. Many companies use the OWASP Top 10 list as a compliance benchmark for application security risks.

Despite these many benefits, DAST can have limitations. Though DAST is skilled at identifying security flaws in running applications, it may not uncover all vulnerabilities, especially those requiring specific sequences of actions. Combining DAST with other methods—such as static application security testing (SAST—see below), interactive application security testing (IAST), software composition analysis (SCA), and manual penetration testing—can help complement DAST and offer a more comprehensive security program.

Other limitations of DAST can include:

• Focus on executable code. Since DAST mostly tests parts of web applications that have been already deployed (i.e., code that is already runnable), it can miss sections still in development.

• Authentication challenges. DAST may be less effective with non-standard authentication and complex business logic, but professional DAST tools include guidance modules for these situations.

• Operational impact. Without proper tuning, DAST testing can affect normal application operation and possibly introduce sample data or slow down the application. As a result, many run DAST in staging or production clone environments rather than live production.

DAST and SAST, or static application security testing, are two testing methods used to identify security vulnerabilities in web applications. But where DAST assesses applications in their production environment, mimicking malicious user attacks and identifying security issues, SAST delves into their source code, searching for vulnerabilities within the website application.

Cybersecurity experts generally suggest using both SAST and DAST when addressing security risks to have a complete view of potential vulnerabilities. For instance, in examining a program's source code, SAST tools can uncover a wide range of security vulnerabilities that DAST might miss, including SQL injection, buffer overflows, XXE attacks, and other OWASP Top 10 risks.

Using a SAST methodology also encourages early testing during development, reducing the likelihood of security flaws in the application's source code during later phases, leading to shorter development times and improved overall security.