API Penetration Testing
Testing the headless surface — broken object authorization, mass assignment, oversharing
APIs fail differently from web applications. The dominant risk isn't injection — it's authorization: an endpoint that returns any record if you change the ID, an update call that quietly accepts an `isAdmin` field, a response that ships far more data than the UI displays. We test every endpoint version against every user role, in both directions: can a user reach another user's data, and can they reach an administrator's.
What we cover.
- REST, SOAP, and GraphQL endpoints
- Review of OpenAPI/Swagger specs, WSDL files, GraphQL schemas, and Postman collections
- Every endpoint version, tested across every user role
- Vertical and horizontal privilege escalation
- Full OWASP API Security Top 10 coverage, including SSRF and unsafe consumption of third-party APIs
How the engagement runs.
Introduction
API penetration testing assesses application programming interfaces across REST, SOAP, and GraphQL, focusing on the vulnerability classes specific to API communication rather than those of a rendered web front end.
The goal is to bypass the API's controls in order to reach unauthorized data, compromise accounts, or disrupt business logic. The methodology is aligned to the OWASP API Security Top 10 (2023).
Test phases
Planning and scoping
- Documentation gathering.
- Specifications — OpenAPI/Swagger, WSDL for SOAP, or the GraphQL schema
- Postman collections, for valid request examples to test against
- A business logic map of high-value flows such as purchase or funds transfer, which become targets for business-flow abuse
- Scope definition — every endpoint and version including /v1, /v2 and mobile variants, plus distinct user roles (User A, User B, Admin) so both vertical and horizontal privilege escalation can be tested.
Reconnaissance
- Passive discovery — repository dorking for leaked API keys and specifications, and proxying mobile or web traffic to surface undocumented shadow endpoints.
- Active discovery — fuzzing for hidden paths such as /admin, /internal, or /actuator, and verifying which HTTP verbs each endpoint really accepts (if GET /users exists, what does DELETE /users do?).
Vulnerability analysis
Working the full OWASP API Security Top 10, grouped by what actually goes wrong.
- Authorization (API1, API5) — object-level and function-level access control.
- Broken object level authorization — can User A read User B's resource simply by changing an ID, as in GET /invoices/1001 → /invoices/1002?
- Broken function level authorization — can a regular user reach administrative functions, or change a method from GET to DELETE and have it work?
- Authentication (API2) — weak JWT signing secrets, acceptance of the 'none' algorithm, tokens that never expire, weak password recovery, and missing brute-force or credential-stuffing protection.
- Object property level authorization (API3) — the merged mass-assignment and excessive-exposure category.
- Excessive exposure — does the JSON response carry backend fields the UI hides, such as is_admin, password_hash, or identifiers?
- Mass assignment — can unauthorized fields be injected into a POST or PUT, for instance setting a role or a balance during registration?
- Resource and business logic (API4, API6).
- Unrestricted resource consumption — oversized payloads, deeply nested GraphQL queries, or request floods that exhaust CPU, memory, or paid third-party quotas. Are there timeouts and strict rate limits?
- Unrestricted access to sensitive business flows — automation abusing logic that works exactly as designed: scripting a bot to buy the whole ticket allocation, or to create thousands of accounts
- Injection and server risks (API7, API8).
- Server-side request forgery — where an endpoint accepts a URL, can the server be forced to reach internal resources such as a cloud metadata service or localhost?
- Security misconfiguration — verbose stack traces, wildcard CORS origins, or unpatched servers behind the API
- Integrations and inventory (API9, API10).
- Improper inventory management — zombie APIs (a deprecated /v1 still live) and shadow APIs (endpoints nobody documented)
- Unsafe consumption of APIs — the target trusting a third-party response blindly. If it integrates with a payment or identity provider, can that response be spoofed?
Exploitation
Findings are chained to reach maximum realistic impact.
- Object-level plus function-level — use a BOLA flaw to discover an administrator's user ID, then use a BFLA flaw to act on it.
- Mass assignment — register a user injecting an administrative flag, gaining privileged access immediately.
- SSRF to credentials — reach a cloud metadata service, retrieve access keys, and use them to query backing infrastructure directly.
Reporting
- Executive summary framed around business risk and overall posture.
- Technical report for the developers and API owners.
- Attack narrative walking through the significant chains step by step.
- Per-finding detail — description, affected endpoints and parameters, evidence, business impact, risk rating, and prioritized remediation.
Remediation and re-testing
- Debrief meeting to present the findings.
- Targeted re-test after fixes are deployed, validating each vulnerability is genuinely resolved.
What you get at the end.
Who it's for
Teams running production APIs — especially multi-tenant platforms and anything backing a mobile app.
Need a scope for this engagement?
Tell us what's in your environment and we'll come back with a scoped plan.
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