HackproofHacks
API security

API Penetration Testing

APIs are where modern applications actually live, and where modern breaches actually happen. Behind every web and mobile front end is an API doing the real work, and an attacker who ignores the polished interface and talks to that API directly often finds it far less guarded than the UI suggests. Testing the API on its own terms is no longer optional.

This engagement tests your REST or GraphQL API against the OWASP API Security Top 10, directly at the endpoint level, and delivers a report focused on the authorization and data-exposure flaws that dominate real API incidents.

Why APIs need dedicated testing

The UI is not a security boundary. Front ends hide buttons and fields, but the API behind them is fully reachable with simple tools, and an attacker uses those tools rather than your interface. Any authorization or validation you enforce only in the browser is effectively absent from the API's point of view, and that gap is exactly what API testing targets.

The dominant API risk is broken object-level authorization, where an endpoint returns a record based on an id without checking that the caller owns it. It tops the OWASP API Security Top 10 because it is both extremely common and directly exposes user data, and it can only be found by a tester manipulating requests across multiple accounts.

APIs also fail in ways specific to how they are built: mass assignment that lets a client set fields it should not, endpoints that return far more data than the UI shows, and missing rate limits on sensitive actions. These require a methodology built for APIs, not a generic web scan pointed at a base URL.

What we test

Broken object-level authorization (BOLA)

The top API risk: testing whether changing an object id returns or modifies data belonging to another user, across every endpoint and object type, authenticated as multiple accounts.

Broken function-level authorization

Whether a lower-privileged caller can invoke administrative or restricted operations by calling the endpoint directly.

Mass assignment and data exposure

Whether a client can set fields it should not by adding them to a request, and whether responses leak internal fields or other users' data the UI never shows.

Authentication and rate limiting

Token handling, authentication flaws, and missing rate limits on sensitive endpoints such as login, transfers, or code entry.

The report you receive

The report is written for API developers and reviewers. It contains:

  • An executive summary conveying the API's real exposure to non-specialists.
  • A methodology statement referencing the OWASP API Security Top 10.
  • Findings rated by severity with the exact requests to reproduce them, impact, and remediation.
  • Clear treatment of authorization findings, the category that dominates API risk.
  • A retest and updated clean report after remediation.

Findings we commonly report in this category

Broken object-level authorization (BOLA/IDOR)

An endpoint returning another user's object when an id is changed, the most common and most damaging API flaw.

Broken function-level authorization

A normal user successfully calling an admin-only endpoint because the server does not check their role.

Mass assignment

A request that sets a protected field such as a role, balance, or tenant id simply by including it in the body.

Excessive data exposure

Responses containing internal or sensitive fields the client never needs, left to the front end to hide.

Frequently asked questions

Why test the API separately from the web app?

Because attackers do. The API is fully reachable without your UI, and any control enforced only in the browser is absent at the API level. The most serious findings, especially broken authorization, are only visible when you test the API directly.

Do you test REST and GraphQL?

Yes. Both are covered, with attention to their specific pitfalls, such as GraphQL query depth and over-fetching alongside the standard authorization and data-exposure risks.

What do you need to test our API?

Access credentials for a few accounts at different privilege levels, and documentation or a collection of endpoints if available. Testing across multiple accounts is what reveals broken authorization.

How long does an API test take?

Usually one to two weeks depending on the number of endpoints and roles, followed by the report. We confirm scope in a short call first.

Related services

Ready to scope your api penetration testing?

Book a free 30-minute scoping call. We agree the scope, timeline, and a fixed price up front — no obligation, and no surprises for your deadline.