API Penetration Testing
Manual testing of your REST, GraphQL and gRPC APIs by OSCP- and OSWE-certified testers, mapped to the OWASP API Security Top 10.
APIs are where modern applications keep their logic and their data, and they are tested least. Frontends hide parameters that the API accepts; mobile apps call endpoints nobody documented; partner integrations trust callbacks that nobody verified. The result is the most common critical finding we see across clients: broken object-level authorisation, where a changed identifier returns someone else's record. Our API penetration tests go straight to the interface, per role and per tenant, and report in the terms your backend team uses: endpoint, request, response, fix.
What is API penetration testing?
API penetration testing is a hands-on assessment of your application programming interfaces from an attacker's perspective, performed directly against the API rather than through a frontend. We work from your specification or from captured traffic, enumerate every endpoint and parameter, and then attack the things that matter most in APIs: whether one user can read or change another user's objects, whether a function meant for administrators is reachable by anyone with a token, whether the token itself can be forged, replayed or escalated, and whether the business logic behind a sequence of calls can be abused. Every finding is reproduced with the exact request and explained to the engineers who own the service.
Key Capabilities
Object- and Function-Level Authorisation
BOLA, BFLA and tenant isolation: the top of the OWASP API Security Top 10 and the most common critical finding in APIs.
Authentication, Tokens and OAuth
JWT validation, token lifetimes and scopes, refresh flows, OAuth 2.0 and OIDC misconfigurations, service-to-service authentication.
Input Handling and Mass Assignment
Injection, deserialisation, property-level authorisation and the parameters your frontend never sends but your API accepts.
Rate Limiting and Resource Abuse
Brute force, enumeration, expensive queries and the unrestricted resource consumption that turns an API into a cost or availability problem.
GraphQL and gRPC
Introspection and schema exposure, nested query abuse, batching attacks, field-level authorisation, protobuf and reflection handling.
Gateways, Webhooks and Integrations
API gateway configuration, webhook verification, partner callbacks and the third-party integrations that extend your trust boundary.
Who needs API penetration testing?
API-first and SaaS companies, fintech and payment platforms, organisations exposing partner or public APIs, mobile-app backends, and teams whose web application test never looked past the frontend. If your API is the product, or if partners and mobile apps talk to it directly, this is the test that covers what the UI hides.
How an API Penetration Test Works
Scoping and Specification Review
We agree the APIs, roles, tenants, environments and test window, and review the OpenAPI, GraphQL or Postman artefacts you provide.
Endpoint and Parameter Mapping
Every endpoint, method, parameter and object type is enumerated, including undocumented ones found through traffic and discovery.
Authentication and Token Testing
Login and token flows, scopes, lifetimes and service-to-service trust are tested before authorisation work begins.
Authorisation per Role and Tenant
Every object and function is tested across roles and tenants to find where the API trusts the caller too much.
Logic, Abuse and Resource Testing
Call sequences, race conditions, rate limits and expensive operations are abused the way an attacker would.
Report, Debrief and Retest
Findings with exact requests and responses, mapped to the OWASP API Top 10, a debrief with your backend team, and a retest after fixes.
How We Work
Manual first. Our OSCP- and OSWE-certified testers use Burp Suite, custom scripts and schema-aware tooling for enumeration and coverage, but authorisation and logic flaws are found by reading the API and thinking like a caller who should not be trusted. Critical findings are escalated the same day, every finding is reproduced with the exact request, and the report maps each issue to the OWASP API Security Top 10 and to the compliance framework you name.
What you receive
API Penetration Testing FAQ
The questions engineering and security leaders ask us most often before commissioning an API test, answered straight.
What is API penetration testing?
API penetration testing is a manual security assessment of your REST, GraphQL, gRPC or SOAP interfaces, performed directly against the API and independent of any frontend. We work from the specification (OpenAPI, GraphQL schema, Postman collection) or from traffic captures, map every endpoint and parameter, and test authentication, object-level and function-level authorisation, input handling, rate limiting and the business logic behind the calls. The difference from a web application test: the frontend often hides endpoints and parameters the API still accepts, and that is exactly where the flaws live.
Which standards do you follow?
The OWASP API Security Top 10 as the minimum bar, the OWASP Web Security Testing Guide and ASVS for coverage of the underlying application logic, and PTES and NIST SP 800-115 for process. The report states the methodology and maps each finding to its OWASP API category.
Do you need our API documentation?
It helps a great deal. An OpenAPI specification, GraphQL schema or Postman collection plus test accounts per role and per tenant means the testing days go into testing rather than guessing endpoints. Without documentation we work from client traffic captures and discovery, which costs more days and yields less coverage.
Do you test partner APIs and third-party integrations?
Yes, where they fall within your scope and authorisation. Webhooks, callbacks, OAuth integrations and the APIs you expose to partners are often the weakest link, because they were built with different trust assumptions than your own frontend.
How long does an API penetration test take?
An API of average size with dozens of endpoints and two or three roles typically needs two to six testing days, with the report delivered within a week of the last testing day. Large API estates with many services and tenants need more. During scoping we put the day count in writing, including the retest.
Do you also test the API gateway and the identity provider?
Yes. The gateway (rate limiting, routing, JWT validation, WAF rules), the identity provider (OAuth 2.0 and OIDC flows, token lifetimes, scopes, refresh tokens) and the way services authenticate to each other are part of the test, because a flaw there affects every endpoint at once.
How much does an API penetration test cost?
Every test is quoted on scope; there is no fixed price list. The drivers are the number of endpoints and services, the number of roles and tenants, documentation quality, the depth required and whether a retest is included. After a scoping call you receive a written proposal within a few working days with the day count and the named lead tester.
Ready to test your APIs?
Named, certified testers, per-role and per-tenant coverage, and findings your backend team can reproduce in one request.