API penetration testing: check yours does not give away too much

An API has no user interface, so authorization flaws are easy to miss, and at the same time the most sensitive data flows through APIs. We test REST, GraphQL and gRPC to the OWASP API Security Top 10, where the top risk is broken object level authorization, the ability to reach another user objects by swapping an identifier.

Unlike a web application we are not constrained by screens and buttons. We work directly on the endpoints, checking each one for permissions, input validation and excessive data exposure. A scanner sees little here, because it does not know the context of who should have access to what.

The two most common and most dangerous classes of API flaw are object-level and function-level authorization. The first lets you read or change a resource belonging to another user, the second lets you invoke an operation reserved for a higher role. Both are invisible from the outside and require testing from the perspective of different accounts.

That is why we agree the roles and test accounts with you, then systematically check every permission boundary. We confirm not only that an endpoint exists, but that it correctly rejects a request the account has no right to make.

An OpenAPI spec or a ready request collection significantly increases coverage in the same amount of time, because we do not have to guess which endpoints and parameters exist. The fuller the picture of the API surface, the more real attack paths we can verify instead of searching for them blindly.

If there is no documentation we will still run the test, mapping the API from application traffic and behavior analysis. It is worth knowing, though, that good documentation is not only a convenience for us but directly more value from the test for you.

The report describes every vulnerability with proof, a CVSS rating and a concrete request that reproduces the problem, so the development team can verify it immediately. We add an executive summary with the real business impact and remediation priorities ordered by risk.

After the fix we run a retest and confirm the gaps are closed. This matters especially for an API, which often sits between a mobile app and the backend, because a single authorization flaw can expose the data of all users at once.

We test an API before exposing it to partners or a client application, and after every contract change: new endpoints, a change to the permission model or a migration to a new version. Every such change can open access that was not there before.

For a publicly exposed API or one handling personal data and payments we recommend a regular testing cycle. Increasingly it is also part of regulatory requirements and a condition of working with larger integrators.