Fundamental Differences in Approach

SAST and DAST represent opposite ends of the testing spectrum—white-box versus black-box testing. SAST examines application source code, bytecode, or binaries without executing the program, analyzing the internal structure and logic to identify potential vulnerabilities. This inside-out approach provides complete visibility into how applications process data and make security decisions. DAST, conversely, tests running applications from the outside, sending requests and analyzing responses to identify vulnerabilities that manifest during execution.

The timing of when these tools operate creates another fundamental distinction. SAST integrates early in the development lifecycle, analyzing code as it's written or committed. This shift-left approach enables developers to fix vulnerabilities before they propagate through the development pipeline. DAST requires a running application, typically operating later in the development cycle or against deployed applications. This timing difference significantly impacts how organizations integrate these tools into their workflows.

Coverage patterns differ dramatically between the approaches. SAST analyzes all code paths, including error handlers, administrative functions, and rarely executed code. If code exists in the repository, SAST examines it. DAST only tests code it can reach and execute through external interfaces. Complex business logic, error handling paths, or features behind feature flags might remain untested by DAST if they cannot be triggered externally.