Interactive Application Security Testing (IAST) takes a different approach than traditional static or dynamic testing. Instead of analyzing code from the outside (SAST) or probing applications through their interfaces (DAST), IAST instruments the application itself and monitors execution from within.
IAST functions as a sensor inside the application. When code runs, IAST sees exactly which functions are called, what data flows through them, and where that data ends up. This gives us visibility that neither SAST nor DAST can on their own.
How It Works
IAST typically involves adding an agent or library to the application that hooks into the runtime environment. As the application processes requests, the agent monitors data flow, tracking input from users as it moves through the code and identifying where it gets used in sensitive operations like database queries or command execution.
This runtime visibility allows IAST to confirm vulnerabilities rather than just speculating about them. When IAST reports that user input reaches a SQL query unsanitized, it's because the agent watched it happen, not because pattern matching suggested it might.
The OWASP Application Security Verification Standard discusses how different testing approaches like IAST fit into a comprehensive security program.
Tradeoffs
The instrumentation that enables IAST also limits it. You need to deploy the agent with your application, which may not be possible in all environments. There can be performance overhead. And IAST only sees code paths that actually execute during testing. If a vulnerable code path isn't exercised, it won't be found.
IAST also tends to be more complex to set up than SAST (scan some code) or DAST (scan a URL). The additional accuracy comes with operational cost.