SAST vs DAST

Static application security testing analyses source code or compiled artefacts without executing them, looking for patterns known to be dangerous: unsanitised input reaching a query, a credential committed to a repository, a call into a cryptographic function that should no longer be used. Dynamic application security testing does the opposite. It leaves the code alone and interacts with the deployed application over the network, sending malformed and hostile requests and judging what comes back. One reads the recipe; the other tastes the dish.

They fail in complementary ways, which is the reason the comparison is worth understanding before buying either. Static analysis sees every path through the code including the ones nothing can reach, so it reports a great deal that does not matter and needs triage time budgeted from the first day. Dynamic analysis only ever sees what it can reach, so anything behind a login, a feature flag or an unusual sequence of calls stays invisible unless somebody configures the tool to get there. Static finds a flaw and its line number; dynamic finds the flaws that are genuinely exposed. Neither substitutes for the other, and buying one in the belief that it covers the ground of both is the mistake this comparison exists to prevent.

Where each one belongs

• Static analysis in the pull request. It is fast, it needs no running environment, and a finding costs least at the moment the code is still being written.
• Dynamic analysis against a deployed build. Configuration, security headers, session handling and authorisation only exist once something is actually running.
• Dependency scanning beside both, because neither technique examines the third-party code that makes up most of a modern application.
• A person after both. Neither tool reasons about what your application is supposed to permit, and business logic flaws live in exactly that gap.

Both techniques report weaknesses, and neither answers whether an attacker can reach one, which is the question a report has to settle before anybody can prioritise the work.