Application Security Testing: SAST, DAST, and Beyond

Finding security vulnerabilities before attackers do is the entire point of application security testing. But the landscape of testing tools and methodologies is confusing, with overlapping acronyms and vendor marketing that makes everything sound essential and nothing sound sufficient.

Here is the practical breakdown. There are distinct testing approaches, each catches different classes of vulnerabilities, and the right strategy combines them based on your risk profile and development workflow.

SAST: Finding Bugs in Your Source Code

Static Application Security Testing analyzes your source code without executing it. The tool reads your codebase, builds a model of data flow and control flow, and identifies patterns that match known vulnerability classes — SQL injection, cross-site scripting, path traversal, insecure deserialization, hardcoded credentials.

SAST tools work early in the development lifecycle. You can run them on every commit, in pull request checks, or as part of your IDE. Catching a SQL injection vulnerability in a pull request costs minutes to fix. Catching it in production costs an incident response, a breach notification, and potentially your customers' data.

The strength of SAST is coverage. It analyzes every code path, including paths that are difficult to reach through normal application testing. A function that is only called during a rare error condition still gets analyzed.

The weakness is false positives. SAST tools lack runtime context. They see that user input flows into a database query, but they may not recognize that the input passes through a parameterized query builder that prevents injection. Tuning false positives is an ongoing investment — expect to spend time configuring rules and suppressing known-safe patterns.

Popular SAST tools include Semgrep, SonarQube, CodeQL, and Snyk Code. For most teams, Semgrep offers the best balance of accuracy, speed, and ease of custom rule creation. If you are already managing dependency vulnerabilities, adding SAST to the same pipeline is a natural extension.

DAST: Finding Bugs in Your Running Application

Dynamic Application Security Testing takes the opposite approach. Instead of reading your code, it attacks your running application like an external adversary would. DAST tools crawl your application, discover endpoints, and send malicious payloads — SQL injection strings, XSS vectors, authentication bypass attempts — then analyze the responses for signs of vulnerability.

DAST catches vulnerabilities that SAST cannot. Server misconfiguration, missing security headers, authentication and session management flaws, and runtime injection vulnerabilities that depend on specific server behavior are all visible to DAST but invisible to static analysis. The OWASP Top 10 includes several vulnerability classes that are most reliably detected through dynamic testing.

The weakness of DAST is coverage. It can only test what it can reach. If your application has endpoints that require complex authentication flows, specific state setup, or unusual input formats, DAST tools may not discover or properly test them. Authenticated scanning — where you provide the tool with valid credentials — improves coverage significantly but adds configuration complexity.

DAST runs later in the development lifecycle, typically against a staging environment that mirrors production. It is slower than SAST because it makes real HTTP requests and waits for responses. A full DAST scan of a moderately complex application can take hours.

Tools like OWASP ZAP, Burp Suite, and Nuclei cover different parts of the DAST spectrum. ZAP is open source and works well for automated pipeline scanning. Burp Suite excels in manual and semi-automated penetration testing. Nuclei specializes in known vulnerability detection using community-maintained templates.

Beyond SAST and DAST

Two additional testing approaches fill gaps that SAST and DAST leave open.

Interactive Application Security Testing (IAST) instruments your application at runtime with an agent that monitors data flow during normal use or testing. When your test suite runs, the IAST agent observes how data moves through the application and identifies vulnerabilities based on actual execution paths. This combines the accuracy of dynamic testing with the coverage benefits of being embedded inside the application. The trade-off is that IAST requires installing an agent in your runtime environment, which adds a deployment dependency.

Software Composition Analysis (SCA) focuses on third-party dependencies rather than your own code. It inventories every library in your dependency tree and checks it against vulnerability databases. Given that most modern applications are more dependency code than application code, SCA catches a significant class of risk that neither SAST nor DAST addresses directly.

# Example CI pipeline combining security testing stages
security-testing:
 stages:
 - name: sast
 tool: semgrep
 config: .semgrep.yml
 fail-on: error
 - name: sca
 tool: snyk
 fail-on: high
 - name: dast
 tool: zap
 target: https://staging.example.com
 fail-on: medium
 authenticated: true

Building a Practical Testing Strategy

The mistake most teams make is treating security testing as a single tool decision. "We use SonarQube" is not a security testing strategy. It is one input among several.

A practical strategy layers these approaches based on when they run and what they catch. SAST and SCA run on every pull request because they are fast and catch issues early. DAST runs nightly or on staging deployments because it is slower but catches runtime and configuration issues. IAST runs during integration test suites when you have an instrumented environment.

Set severity thresholds that match your risk tolerance. Not every finding needs to block a deployment. Critical and high findings from any tool should block. Medium findings should generate tickets. Low findings should be reviewed periodically. This prevents alert fatigue while ensuring that genuinely dangerous vulnerabilities never reach production.

Review findings regularly, not just when they block a pipeline. A weekly triage session where the development team reviews new findings, closes false positives, and prioritizes fixes keeps your security posture improving over time rather than just maintaining a minimum bar. The teams that treat security testing as a continuous improvement process — rather than a gate to get past — are the ones that actually reduce their vulnerability count over time.