API penetration testing

What it means

API penetration testing is a security test aimed at the interface other software uses to talk to yours. The tester sends requests directly to each endpoint, with and without valid credentials, to see what the API allows, returns or accepts.

It matters because the API is where the rules are enforced. A web page may hide a button, but the API behind it will still answer if someone sends the request by hand. Mobile apps and partner integrations use the same endpoints, so a flaw there reaches them all.

Why it matters

APIs expose data and functions in a form that is easy to automate. If one record can be fetched by changing a number in the request, an attacker can fetch all of them in minutes. If an endpoint returns more fields than the screen shows, the extra data is available to anyone who reads the response. The speed of automation turns a small flaw into a large exposure.

Many serious breaches have involved API access control mistakes, which is why OWASP keeps a separate API security list. Mobile apps, partner integrations and single-page front ends all depend on the same endpoints. Each is reachable by anyone who can read your app's network traffic.

How it shows up

Tests follow the API's own risks: object-level access, function-level access, authentication and token handling, excessive data in responses, missing rate limits, mass assignment, and abuse of business flows. A specification file such as OpenAPI helps the tester cover every endpoint.

The mistakes repeat: forgotten old versions, undocumented endpoints, and checks that exist in the front end but not on the server. Share the specification, accounts in more than one organisation, and a list of known endpoints. Include shadow APIs by comparing the documentation with what the server answers.