SAST vs DAST vs IAST
Three acronyms, one decision: where in the lifecycle you want to be told. SAST reads your source and flags dangerous patterns before anything runs. DAST attacks the deployed application from outside, seeing only what an attacker sees. IAST instruments the running application and watches data flow while your tests exercise it. They overlap less than the vendor grids suggest, and all three share one blind spot that matters more than their differences.
Key Facts
- →SAST runs on code, pre-build, and can point at the exact line. It cannot know what is reachable in production.
- →DAST runs against a deployed application and finds what is exploitable now. It cannot tell you where in the code the flaw is.
- →IAST instruments the running app during your test suite, so it sees real execution paths with low false positives — and only covers what your tests exercise.
- →None of the three tests authorisation between users: no tool can know which records belong to whom.
- →The order that works for most teams: dependency scanning, then SAST on changed code, then DAST against a preview, then authorisation testing.
Quick Comparison
| Aspect | SAST | DAST |
|---|---|---|
| When it runs | Pre-build, on every commitTie | Post-deploy, against a running appTie |
| Needs source code | YesTie | NoTie |
| Points at the exact line | Yes✓ Advantage | No |
| Proves exploitability | No: pattern match | Closer: it attempted it✓ Advantage |
| False positive rate | Typically high | Lower, varies by tool✓ Advantage |
| Finds configuration flaws | No | Yes✓ Advantage |
| Covers code paths never executed | Yes✓ Advantage | No |
| Speed | Seconds to minutes✓ Advantage | Minutes to hours |
| Works on third-party components you cannot see | No | Yes✓ Advantage |
| Authorisation between users | NoTie | NoTie |
| Business logic flaws | NoTie | NoTie |
What is SAST?
Static application security testing. Analyses source, bytecode or binaries without executing them, matching dangerous patterns: injection sinks, hardcoded secrets, unsafe deserialisation, weak crypto. Runs in seconds to minutes on changed files, integrates naturally with pull requests, and points at a line number.
What is DAST?
Dynamic application security testing. Probes a running application from the outside like an attacker: crawls, submits input, observes responses. Finds what is actually exploitable in the deployed configuration — misconfigurations, missing headers, injection that survives your framework, auth bypasses — without seeing any code.
Where IAST Fits, and Why It Is Not a Third Column
IAST instruments the running application — an agent inside the process — and watches how data moves while something exercises the app, usually your existing test suite. Because it sees the real execution path rather than a pattern or a response, its findings come with very low false positives and a line number, which is the combination the other two cannot offer.
The catch is coverage: IAST only observes what gets executed. If your integration tests never touch the admin import endpoint, IAST is silent about it, and silence looks identical to safety. It rewards teams with genuinely good test coverage and disappoints teams who bought it hoping to compensate for not having any.
That is why it belongs alongside rather than instead: SAST for breadth over code, DAST for the deployed reality, IAST to sharpen whatever your tests already cover.
The Blind Spot All Three Share
None of them can decide that a request is a breach rather than a feature. SAST sees a query with a parameter; DAST sees a 200 response; IAST sees data flowing to a database. None of them knows that the invoice ID in that request belongs to a different customer, because that fact exists only in your business rules.
Broken access control is consistently the top web application risk, and it is the category all three miss by construction. Testing it requires holding two accounts, understanding what each should reach, and systematically attempting what they should not — patient human work, or an agent doing what the human would.
The practical consequence: a team can have SAST, DAST and IAST all green and still be one enumerable identifier away from an incident. If your product is multi-tenant, treat authorisation testing as a fourth line item rather than assuming the acronyms cover it.
The Order to Buy Them In
Start with dependency and secret scanning, which is not on this list and is the cheapest risk reduction available: known vulnerable packages and committed credentials cause an unreasonable share of real incidents, and both are near-zero effort to check on every commit.
Then SAST on changed code only. Running it across the whole repository on day one produces a backlog nobody will ever clear, which teaches your team that security findings are noise. Scoped to the diff, it is fast and the feedback lands where the decision was made.
Then DAST against a deployed preview on merge, because it tests the thing you will actually ship, including the configuration. Then authorisation and business-logic testing, continuously if you deploy continuously. IAST is worth adding when your test suite is good enough to make it meaningful — and it is a poor substitute for that suite.
Choosing Between Them on a Constrained Budget
If you can afford exactly one, buy the one that matches how you fail. Teams shipping a monolith with weak test coverage get more from DAST, because configuration and deployment reality is where their surprises live. Teams with many services and strong tests get more from SAST plus IAST, because their surprises are in code paths.
If your application is multi-tenant and holds customer data, neither answer is right and the honest recommendation is to buy authorisation testing first. It is the smallest category and the one whose failures make the news.
When to Choose Each
Choose SAST when…
- →You want feedback in the pull request, at the line that caused it.
- →You need coverage of code paths your tests never execute.
- →Compliance asks for secure code review evidence.
- →You are catching classes early, when a fix is minutes rather than sprints.
Choose DAST when…
- →You want to know what is exploitable in the deployed configuration.
- →You do not have source access, or the surface includes third-party components.
- →Misconfiguration, missing headers and auth bypasses are live concerns.
- →You would rather have fewer, more real findings than complete pattern coverage.
Can You Use Both?
Standard practice, and the layering matters more than the choice: dependency and secret scanning on every commit, SAST on the diff, DAST against a deployed preview on merge, IAST where your tests are strong, and authorisation testing continuously. Buying all of them without that ordering produces a lot of findings and no improvement in how fast things get fixed.
Verdict
They are complements, and the argument about which is better usually means the buyer has not decided where they want to be told. SAST tells you early and imprecisely; DAST tells you late and concretely; IAST tells you precisely about whatever your tests happen to touch. What none of them tells you is whether one customer can read another's data — so if that would be your worst day, budget for it separately and stop expecting the acronyms to cover it.
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