IAST vs DAST

Quick comparison

DAST tests your application from the outside. IAST tests from the inside. That single difference shapes everything else: what they find, how accurate they are, how they fit into your pipeline, and what they cost.

How DAST works

DAST treats your application as a black box. The scanner does not know what language you wrote it in, what framework you used, or how your code is structured. It only sees what any external user would see: HTTP requests and responses.

A DAST scan has two phases. First, the crawler discovers endpoints by following links, submitting forms, and navigating the application. Then the active scanner sends attack payloads to each discovered parameter and analyzes the responses for signs of vulnerability.

When testing for SQL injection, DAST sends payloads like ' OR 1=1-- to form fields and URL parameters. If the response contains a database error message, changes in predictable ways, or behaves differently than a normal request, the scanner flags it. For XSS, it injects script tags and checks whether they appear unescaped in the response.

The strength of this approach is simplicity. Point the scanner at a URL and let it work. No agents, no instrumentation, no dependencies on your tech stack. ZAP, Burp Suite, Invicti, and StackHawk all follow this model.

The weakness is that DAST can only infer what happened. It sees the HTTP response but not the code path that produced it. If a SQL injection payload triggers a generic error page instead of a database error, DAST might miss it. If input validation blocks the payload at the edge but a deeper code path is still vulnerable, DAST reports the application as secure when it is not.

DAST also catches things that live outside application code: missing security headers, server misconfigurations, exposed admin panels, TLS issues, and default credentials. These exist at the infrastructure layer, which IAST agents never see.

How IAST works

IAST places an agent inside your running application. The agent hooks into the language runtime and observes how data moves through your code as the application handles requests.

When a request arrives, the IAST agent marks user-controlled input as “tainted.” It follows that tainted data through every function call, variable assignment, and string operation. If tainted data reaches a dangerous function — a SQL query, a file system operation, an OS command — without sanitization, the agent reports a vulnerability.

The report includes the exact file and line number, the full stack trace, and the complete data flow from input source to dangerous sink. This is a different quality of finding than what DAST produces. Instead of “possible SQL injection on /search?q=”, IAST tells you “SQL injection in SearchService.java, line 47: user input from request parameter ‘q’ reaches Statement.executeQuery() without parameterization.”

Contrast Assess, Seeker IAST, and Datadog Application Security are the main IAST products. Each requires a language-specific agent: a Java agent for Java apps, a .NET agent for .NET apps, and so on.

IAST does not scan on its own. It waits for something to exercise the application. That could be your automated test suite, a QA engineer running manual tests, or even a DAST scanner firing requests. The agent reports vulnerabilities only in code paths that actually execute. Code that your tests do not reach stays untested.

Detection comparison

Both DAST and IAST find injection flaws, XSS, and other web application vulnerabilities. But they catch different subsets and find them with different levels of confidence.

What DAST catches that IAST misses

• Server misconfigurations. Missing security headers, directory listing enabled, verbose error pages, default credentials on admin panels. These are infrastructure issues outside the application runtime.
• TLS/SSL issues. Weak cipher suites, expired certificates, protocol downgrade vulnerabilities.
• Network-level exposure. Open ports, exposed internal services, publicly accessible staging environments.
• Authentication flow weaknesses. Session fixation, cookie attributes (HttpOnly, Secure, SameSite), login brute force susceptibility.
• Vulnerabilities in untested code paths. DAST attacks every discovered endpoint regardless of test coverage. IAST only sees paths that execute.

What IAST catches that DAST misses

• Deep data flow vulnerabilities. Injection flaws where the payload does not produce a detectable change in the HTTP response. DAST misses blind SQL injection when the response looks identical. IAST sees the tainted data reaching the query regardless.
• Internal API vulnerabilities. Service-to-service calls within your backend. DAST only reaches the external interface.
• Cryptographic weaknesses. Use of weak algorithms, hardcoded keys, insecure random number generation. The IAST agent sees the actual function calls.
• Unsafe deserialization. Difficult for DAST to detect unless it triggers a visible error. IAST observes the deserialization call directly.
• Framework misuse. Using an ORM in a way that bypasses its built-in protections. IAST sees the actual function calls and knows whether the safe path was used.

Overlap

Both find standard injection flaws (SQL injection, command injection, XSS, SSRF) in code paths that DAST can reach and trigger. For these common cases, IAST provides better detail per finding, while DAST provides broader surface coverage.

False positive rates and accuracy

This is where IAST has the biggest advantage. False positive rates shape how much time your team spends triaging findings versus fixing them.

DAST false positive rates typically range from 20-40% depending on the tool and application. DAST infers vulnerabilities from HTTP response analysis. Timing-based detection (blind SQL injection) is particularly noisy. Proof-based commercial DAST tools like Invicti reduce false positives by safely confirming vulnerabilities, but even they report findings that need manual verification.

IAST false positive rates are typically under 5%. The agent observes actual data flow at runtime, so it only reports vulnerabilities where tainted data genuinely reaches a dangerous sink without sanitization. Contrast Security claims less than 3% false positives in customer environments. NSA testing of Contrast showed 98% accuracy with zero false alarms.

The practical difference: a DAST scan might produce 200 findings that your team needs to review, with 50-80 turning out to be false positives. An IAST scan on the same application might produce 40 findings where 38 are real. That is a different workload for your development team.

The tradeoff is coverage. IAST only reports on code paths that execute during testing. If your test coverage is 60%, roughly 40% of your codebase goes untested. DAST attacks every endpoint it discovers, regardless of your test suite.

CI/CD integration and performance impact

DAST in CI/CD

DAST runs as a separate step after deployment. Deploy your application to a staging environment, point the scanner at it, wait for results. The scan does not affect your application’s performance during the scan (all traffic is external), but scan duration is a concern.

A full DAST scan takes 1-8 hours for medium applications. Most teams handle this by running quick baseline scans on pull requests and scheduling full scans nightly or weekly. ZAP baseline scans finish in 2-5 minutes. StackHawk is designed for CI with scan times under 10 minutes.

DAST needs a running, network-accessible instance of your application. That means maintaining a staging environment and handling test data.

IAST in CI/CD

IAST runs during your existing test execution. Deploy with the agent, run your test suite, collect findings. There is no separate scan step because the agent reports vulnerabilities as tests exercise the application.

The performance overhead is 2-5% during test execution. For most test suites, this adds seconds, not minutes. The findings appear immediately as each test triggers a vulnerability, rather than at the end of a long scan.

IAST requires modifying your deployment to include the agent. In containerized environments, this means updating Docker images or adding init containers. In serverless environments, agent deployment can be more complex.

Integration comparison

Cost comparison

Free options

DAST wins on free availability. ZAP and Nuclei are production-grade free tools that cover most teams’ needs. See our free DAST tools guide for setup details.

IAST has almost no free options. Contrast Assess Community Edition is limited to one Java or .NET Core application. There are no actively maintained open-source IAST tools.

Commercial pricing

Commercial DAST tools range from $5,000 to $50,000+ per year depending on the number of applications and scan frequency. Developer-focused tools like StackHawk start around $5,000/year. Enterprise scanners like Invicti start around $15,000/year.

Commercial IAST tools are typically more expensive. Contrast Assess and Seeker IAST start around $20,000-$40,000/year for small deployments. Per-application pricing means costs scale with the number of applications instrumented. Enterprise deployments run $100,000+/year.

Total cost of ownership

Factor in the time your team spends on false positive triage. If your DAST tool generates 100 false positives per month and each takes 30 minutes to verify, that is 50 hours of developer time. IAST’s lower false positive rate can offset its higher license cost through reduced triage effort.

When to choose each

Choose DAST when

• You need quick, no-setup scanning. Point a scanner at a URL and go. No code changes, no agents, no language dependencies.
• Budget is limited. Free DAST tools are genuinely capable. ZAP + Nuclei costs nothing.
• You scan applications you do not control. Third-party apps, vendor products, acquired applications where you cannot deploy an agent.
• Server configuration matters. DAST catches infrastructure issues that IAST agents cannot see.
• You need broad coverage fast. DAST tests every discovered endpoint without depending on test coverage.

Choose IAST when

• False positives are killing your program. If developers ignore your security findings because too many are wrong, IAST’s accuracy rebuilds trust.
• You need code-level remediation guidance. File, line number, and full data flow make fixing issues faster. Developers do not need to reproduce the finding from a URL and payload.
• You have good test automation. IAST delivers the most value when your test suite exercises most of your code. Low test coverage means low IAST coverage.
• You run continuous testing. IAST adds minimal overhead to existing test runs. You get security results as a byproduct of your normal QA process.
• You already have CI/CD maturity. Teams with established pipelines and containerized deployments can integrate IAST agents without much friction.

Use both when

• Your security program is mature enough to operate multiple tools
• You want IAST’s accuracy for known code paths plus DAST’s coverage of infrastructure and untested endpoints
• You can dedicate time to correlating findings across tools

Many teams start with DAST because the barrier to entry is lower, then add IAST as their testing maturity grows. For a broader comparison that includes static analysis, see the SAST vs DAST vs IAST guide. For understanding where all testing types fit in the software lifecycle, see our application security testing overview.

FAQ

Frequently Asked Questions

Written by

Suphi Cankurt

Suphi Cankurt is an application security enthusiast based in Helsinki, Finland. He reviews and compares 154 AppSec tools across 10 categories on AppSec Santa. Learn more.