API Penetration Testing
Beyond the UI.Into the API.
Who this is for
API-first products
Companies whose API IS the product — developer tools, B2B SaaS, fintech infrastructure.
Microservice architectures
Engineering teams with REST/GraphQL/gRPC services that need authorisation and rate-limit validation.
Payment / sensitive data APIs
APIs that handle cardholder data, PHI, PII, or financial transactions where API breaks have regulatory weight.
What is API penetration testing?
The targeted, predominantly manual assessment of API endpoints for exploitable security weaknesses.
Modern APIs use a range of protocols (REST, GraphQL, SOAP, gRPC) and authentication schemes (OAuth 2.0, JWT, API keys, mTLS), each with characteristic weakness patterns testers must understand to assess effectively.
The threat landscape
OWASP API Security Top 10
Broken Object-Level Authorisation
BOLA — the #1 API risk.
Broken Authentication
Weaknesses in how identity is verified and sessions are managed.
Broken Object Property-Level Authorisation
Mass assignment — unintended properties written or exposed.
Unrestricted Resource Consumption
Rate limiting and denial-of-service exposure.
Broken Function-Level Authorisation
Endpoints reachable by roles that shouldn't be able to call them.
Server-Side Request Forgery
APIs tricked into making unintended internal or external requests.
Security Misconfiguration
Insecure defaults, verbose errors, and missing hardening.
Lack of Protection from Automated Threats
Insufficient defence against scripted abuse and scraping.
Improper Inventory Management
Undocumented, deprecated or shadow API versions left exposed.
Unsafe Consumption of APIs
Trusting third-party API data and responses without validation.
What API testing delivers
OWASP Top 10 coverage
OWASP API Security Top 10 coverage across all endpoints.
Authorisation matrix testing
Systematic testing across roles and objects, not spot checks.
Mass assignment & data exposure
Identification of unintended property writes and excessive data exposure.
Rate limiting & abuse testing
Rate limiting and abuse vector testing against scripted attacks.
Authentication & token handling
Validation of authentication flows and token handling end to end.
Business logic testing
Manual testing of API workflows for logic flaws automated tools miss.
API testing has become as important as traditional web application testing — and arguably more important for organisations whose APIs back mobile apps, B2B partners, or microservice architectures.
Why API security testing matters
APIs are now the dominant attack surface for many organisations. Mobile apps, single-page web apps, B2B integrations, and microservice architectures all expose far more API surface than traditional web pages, and developers consistently apply weaker authorisation logic to API endpoints than to UI-fronted equivalents, on the assumption that "no one will call the API directly." They will.
Major breaches in 2024 and 2025 have repeatedly traced back to API-level vulnerabilities: BOLA in mobile app backends exposing customer records, mass assignment in admin APIs enabling privilege escalation, weak rate limiting enabling credential stuffing at scale.
Common consequences of weak API security
- Customer record exposure via BOLA in mobile/SPA backends
- Privilege escalation via mass assignment in admin APIs
- Credential stuffing at scale due to weak rate limiting
- Sensitive data exposure via verbose API responses
- Account takeover via authentication weaknesses
- Compliance failures across PCI DSS and SOC 2
APIs deserve at least the same testing depth as the UI surfaces they back — often more, given they typically have weaker authorisation enforcement.
Who needs API testing?
Any organisation operating APIs — internal microservices, mobile app backends, public APIs, B2B integrations — needs regular testing:
SaaS with mobile & SPA frontends
Mobile-first companies
Open banking & PSD2 APIs
Payment platforms
AI/ML platforms with API surfaces
Microservice architectures
B2B platforms with partner APIs
HealthTech with FHIR APIs
If your API powers a mobile app, a B2B integration, or a microservice architecture handling sensitive data, dedicated API testing is essential — generic web application testing rarely covers API security depth.
How we work
Our API Testing Methodology
CREST-aligned methodology combining OWASP API Security Top 10 with hands-on protocol-specific testing across REST, GraphQL, SOAP, and gRPC.
Scoping & API Documentation Review. We agree the in-scope API endpoints (typically working from OpenAPI/Swagger, GraphQL schema, or Postman collections), user roles, authentication schemes, and test credentials.
Endpoint Enumeration & Mapping. Complete mapping of every endpoint, method, parameter, and response shape, building the full attack surface before testing begins.
Authentication & Token Testing. Detailed testing of authentication mechanisms — JWT validation, OAuth flow integrity, token refresh logic, session fixation, brute force resistance.
Authorisation Matrix Testing. Systematic testing of object-level and function-level authorisation across every user role pair — the highest-value work in any API engagement.
Mass Assignment & Data Exposure. Testing for mass assignment (unintended property writes), excessive data exposure (verbose responses leaking sensitive data), and improper input filtering.
Rate Limiting & Abuse Testing. Testing of rate limiting, account lockout, and abuse vectors that enable credential stuffing or data scraping at scale.
Business Logic & Workflow Testing. Manual exploration of business workflows for parameter tampering, race conditions, and logic flaws that automated tools cannot find.
Reporting & Developer Walk-Through. Detailed findings with reproduction steps via curl/Postman, code-level remediation guidance, and live walk-through with your engineering team.
Typical engagement: 5–10 days for mid-complexity APIs (under 50 endpoints), 10–15 days for larger APIs (50–150 endpoints), longer for major platforms.
What you receive
Every API testing engagement with CYBEROCO includes:
Scoping document
Full endpoint inventory agreed before testing begins.
Executive summary
Written for board and management consumption.
Technical findings
Detailed, with curl/Postman reproduction steps.
Exploitability prioritisation
CVSS scoring plus real-world exploitability context.
Remediation guidance
Code-level, with worked examples for your engineers.
Authorisation matrix report
Full test coverage across every role and object.
OWASP compliance mapping
Findings mapped to the OWASP API Security Top 10.
Remediation retest
Critical and high findings re-verified at no extra cost.
Industries We Serve
We deliver this service across these industries:
Financial Services
Healthcare
SaaS & Technology
E-commerce & Retail
Defence & Government
Cloud & Managed Services
Education
Professional Services
Related services
Explore related CYBEROCO services
Penetration Testing
Manual, methodology-led testing that exploits and chains findings to prove real-world impact.
→Mobile Application Testing
iOS and Android testing covering the on-device app and the API backend it talks to.
→Vulnerability Assessment
Continuous, CREST-aligned scanning and manual validation across your wider estate.
→Ready?