API penetration testing

APIs fail differently from the applications in front of them. The serious findings are almost never a missing patch: they are authorization decisions the server never made — an object id that can be changed, a function callable by the wrong role, a field that should never have been writable. Scanners miss them because nothing about the request looks malformed.

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For teams whose product is the API

Platform and product teams shipping API changes weekly, where one version bump can quietly widen what an existing token is allowed to do.

Companies exposing partner or public APIs, where the consumer is another system and there is no interface constraining what gets sent.

Teams with a WAF, a passing DAST scan and a green pipeline, and no answer to what one customer's token can actually reach.

[ WHERE SCANNERS GO BLIND ]

The same request, read three different ways

A DAST scan, a web application test and an API test all send HTTP requests, and they disagree about what counts as a finding. The disagreement matters because the most damaging API issues are perfectly well-formed requests that the server should have refused and did not. Something has to know who is allowed to do what, and only one of these three does.

What each one can actually see

DAST scan

Web application pentest

API penetration test

Knowledge of the API surface

Whatever it can crawl, or the spec you handed it

The endpoints the front end happens to call

Every endpoint in the spec, plus the ones missing from it

Broken object level authorization (API1:2023)

Not found. The request is valid and the response is a 200

Sometimes, where the interface exposes the identifier

Found: change the id with another account's token and read the answer

Testing across roles

One credential, if any at all

The roles the interface offers

Every role against every endpoint, including the ones no interface calls

Object property level abuse (API3:2023)

Not found

Rarely

Found by sending fields the client never sends and reading what came back

Business flow abuse (API6:2023)

Not evaluated at all

Partially, within the flows the interface enforces

Explicitly tested, because the flow is the asset

GraphQL

Weak: one endpoint, and introspection disabled means blind

Depends entirely on the tester

Introspection, query depth, batching and authorization per resolver

Risk identifiers refer to the OWASP API Security Top 10, 2023 edition — the current one at the time of writing. Note that mass assignment and excessive data exposure, separate entries in the 2019 list, were merged into API3:2023 Broken Object Property Level Authorization.

What an API test actually looks for

Broken object level authorization

The first entry on the OWASP API list, and the one that produces the largest incidents. The endpoint accepts an identifier and returns the object without checking that the caller owns it. Testing it is unglamorous — authenticate as one user, request another user's records, see what comes back — and it requires two accounts that a scanner does not have.

Broken function level authorization

An administrative or privileged operation that answers to an ordinary token because the check lives in the interface rather than the server. Frequently found on endpoints no interface calls any more: an old admin route, a bulk operation, an internal maintenance action left routable.

Object property level abuse

Sending fields the client never sends and finding that the server accepted them — role, isAdmin, accountBalance, tenantId. The mirror image is a response returning more than the caller should see, filtered only by the front end. In the 2023 edition both belong to API3, and both are found by looking at what the object model allows rather than at what the interface does.

Authentication and token handling

How tokens are issued, validated, scoped and revoked. Signature verification that accepts an unexpected algorithm, tokens that outlive a password change, refresh flows that can be replayed, scopes granted more broadly than the endpoint needs. Most of this is invisible from the outside and obvious once someone reads the flow.

Unrestricted resource consumption and business flow abuse

Endpoints that can be called faster or larger than intended, and legitimate flows that can be automated against you — bulk enumeration, coupon redemption, reservation holding, account creation. Nothing here is a defect in a single request; the finding is that the sequence has no cost.

Improper inventory management

Old versions still routable, staging hosts serving production data, and endpoints that never made it into the spec. This is where testing overlaps with discovery, and it is the reason a scoped list of endpoints should be treated as a starting point rather than a boundary.

Specs, REST and GraphQL: what changes the scoping

What an OpenAPI spec buys you

A current spec turns discovery time into testing time. It gives parameter types, expected values and the shape of the object model, which is exactly the input needed to construct requests worth sending. Providing one is usually the single highest-leverage thing a team can do before an API engagement starts.

And what it does not tell you

Specs describe intent, not reality. Endpoints get added without documentation, deprecated ones stay routable, and undocumented parameters keep working. Good testing takes the spec as a hypothesis and then goes looking for what is missing from it, because that gap is where the unowned endpoints live.

GraphQL is one endpoint and many questions

A single URL hides the entire surface, so per-endpoint thinking breaks down. Testing focuses on whether introspection is exposed, whether authorization is enforced at every resolver rather than at the query entry point, and whether depth, aliasing and batching let one request do far more work than intended.

Multi-role authenticated flows

This is the point at which a scanner with one credential is structurally blind. Meaningful API testing needs at least two accounts at the same privilege level, to test horizontal access between peers, and accounts at different levels to test vertical access. Provisioning those is the fastest way to raise the value of the engagement.

Frequently asked questions

What is API penetration testing?

Manual, authenticated testing of an API's endpoints against the OWASP API Security Top 10 and against your own business rules. It covers who can read and write which objects, which functions answer to which roles, how tokens are issued and validated, what the responses give away, and whether legitimate flows can be abused at scale. The deliverable is a set of proven requests, each with the account that sent it and what came back.

Why do scanners miss API vulnerabilities?

Because the request is valid. A scanner detects malformed input, known signatures and error conditions. An authorization flaw produces a perfectly ordinary request and a successful response; the only thing wrong with it is that the caller should not have been allowed. Deciding that requires knowing who the caller is, what they own and what the business rule is, and none of those are in the traffic.

Do you need an OpenAPI or Swagger spec?

Not strictly, but it changes the economics of the engagement. With a current spec, the tester spends the time on testing instead of on reconstructing the surface. Without one, discovery has to come first and part of the budget goes there. Either way the spec is treated as a starting point, since undocumented and deprecated endpoints are consistently among the most interesting findings.

How is GraphQL testing different from REST?

In REST the endpoint is the unit of authorization, so testing walks the endpoint list. In GraphQL there is one endpoint and the unit is the resolver, so the questions become whether every resolver enforces authorization independently, whether introspection is exposed, and whether query depth, aliasing and batching let a single request consume disproportionate resources. Testing tools that assume one URL equals one function are not useful here.

How many accounts and roles should we provide?

At minimum two accounts at the same privilege level, which is what makes horizontal authorization testing possible, plus one account per distinct privilege level for vertical testing. In multi-tenant products, add a second tenant. This is the cheapest way to increase the value of an API engagement, and the most common reason a test comes back thinner than expected is that only one credential was supplied.

How often should APIs be tested?

On the cadence at which the API changes, which for most teams means continuously rather than annually. Authorization regressions are introduced by ordinary feature work: a new field on an existing object, a role added for one customer, an endpoint generalized to serve a second use case. A point-in-time test proves a shape the API had on one day, and API surfaces rarely keep that shape for a quarter.

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API penetration testing

For teams whose product is the API